2020

Mesoscale Discussion #0008


Concerning...Outlook upgrade 


Valid 101053Z - 101300Z


SUMMARY...An upgrade to Moderate Risk will occur across central/eastern AR, far northeast LA, northwest/west-central MS, and far southwest TN with the upcoming 13Z Convective Outlook.


DISCUSSION...Based on recent guidance, the probability for significant wind gusts (i.e. gusts at or above 75 mph) to occur late tonight into early Saturday across northeast LA, central/eastern AR, far southwest TN, and northwest/west-central MS has increased. This increase in probability merits an increase to Moderate Risk, which will occur with the upcoming 13Z Convective Outlook. This expanded Moderate Risk will include Little Rock, AR and Memphis, TN. 


More details regarding the meteorological conditions will be available within the outlook.

Mesoscale Discussion #0010


The severe weather threat for Severe Thunderstorm Watch 1 continues.


SUMMARY...The risk for supercells capable of producing tornadoes may increase as early as Noon to 2 PM CST, mainly east of the Interstate 35 corridor of Oklahoma, where trends are being monitored for a new severe weather watch.  Otherwise, scattered vigorous thunderstorm activity may pose a risk for severe hail across northwestern, central and eastern Oklahoma into southeastern Kansas into early afternoon.


DISCUSSION...Mid/upper forcing for ascent, associated with an initial perturbation emerging from larger-scale troughing progressing into the southern Rockies, appears to be contributing to the ongoing increase in thunderstorm activity across Oklahoma and adjacent portions of the southern Plains.  Strongest activity has been initially focused above/to the cool side of a quasi-stationary frontal zone now extending across northwestern Oklahoma into southeastern Kansas and the Missouri Ozarks vicinity.  However, considerable new thunderstorm development is now underway within the warm sector, across central/southern Oklahoma.


Even within the warm sector, storms appear initially rooted above the boundary layer, within a zone of stronger mid-level warm advection (roughly around 700 mb).  South of this zone, warmer and more strongly capping elevated mixed-layer air probably will inhibit convective development in the near term.


With the northeastward progression of the more favorable large-scale ascent, the warm sector thunderstorm development seems likely to become more focused/shift east of the Interstate 35 corridor of Oklahoma by 17-19Z.  As it does, continuing slow surface warming may become increasingly conducive to boundary-layer based storm development, in the presence of strong deep layer shear supportive of supercells.  Models suggest that this will also coincided with strengthening south-southwesterly 850 mb flow (up to 50+ kt along an axis across eastern Oklahoma), and associated enlargement of low-level hodographs may become increasingly conducive to tornado development, in addition to a continuing risk for severe hail.

Mesoscale Discussion #0015


The severe weather threat for Tornado Watch 2 continues.


SUMMARY...Upscale growing and organizing convection will pose increasing potential for damaging wind gusts and a few embedded tornadoes through 4-6 PM CST.  Potential for more discrete supercells, which could pose a risk for a strong tornado or two, may become maximized near Paris TX, and surrounding areas of southeast Oklahoma/northeast Texas.


DISCUSSION...Thunderstorm development has become increasingly widespread, and generally oriented along the 50-55+ kt south-southwesterly 850 mb jet axis across eastern Texas and Oklahoma, which is forecast to continue nosing northward into/across southwestern Missouri through 23-00Z.  The extent to which strongest cells to this point have become rooted within the moistening and gradually destabilizing boundary layer remains unclear, but undoubtedly will increase as boundary layer instability peaks late this afternoon and evening.


Given the strength of the flow and low-level shear, potential for damaging wind gusts and tornadoes should gradually increase in association with "bowing" segments and developing mesovortices embedded within the organizing convective line.  Based on the latest Rapid Refresh forecast soundings, low-level hodographs most conducive to tornadogenesis (including considerable clockwise curvature) in discrete supercells, may become focused around the Paris TX area, by 23-00Z.

Mesoscale Discussion #0025


The severe weather threat for Tornado Watch 5, 8 continues.


SUMMARY...A corridor of increased risk for strong wind gusts and embedded QLCS tornadoes will exist across northern LA for the next hour or two.


DISCUSSION...Recent regional radar imagery depicts increased east-southeastward motion of the convective line moving into northern LA. This forward surge is occurring amidst interaction between the convective line along the front and pre-frontal storms, suggesting these storm interactions are leading to updraft/downdraft augmentation. Surface observations across the region also suggest a meso-low may be developing within this portion of the convective line. The overall environment remains very supportive of organized storms with 0-1 km SRH over 400 m2/s2, MLCAPE over 1500 J/kg, and effective bulk shear around 60-65 kt.


The increased forward speed, stronger updrafts, and meso-low development within this favorable environment all suggest a higher probability for strong wind gusts and embedded QLCS tornadoes exists downstream across northern LA for the next hour or two.

Mesoscale Discussion #0038


The severe weather threat for Tornado Watch 12, 13 continues.


SUMMARY...Greatest potential for tornadoes and damaging straight-line winds will shift from western into parts of northern and central AL over the next couple of hours. Some of the tornadoes could be strong.


DISCUSSION...A squall line has recently consolidated and strengthened over western AL as large-scale ascent associated with a northeastward-ejecting upper trough overspreads the central Gulf Coast states. A couple of embedded low-level circulations have also recently occurred within the line. One such rotational velocity signature earlier produced a TDS over Pickens County AL. Very strong low-level shear exists downstream of the convective line across northern/central AL, with the VWP from KBMX showing southerly low-level winds quickly strengthening to over 60 kt in the 0-1 km layer. Some curvature of the low-level hodograph is supporting effective SRH around 450-650+ m2/s2 given the northeastward motion of individual elements embedded within the line. Coupled with a gradually destabilizing airmass downstream into northern/central AL, this very favorable kinematic environment will be conducive to continued damaging straight-line winds and tornadic circulations embedded within the line. Even with a mainly linear mode, there remains some potential for a strong tornado or two in the short term mainly across parts of west-central AL given the exceptionally strong low-level shear.

Mesoscale Discussion #0075


Probability of Watch Issuance...80 percent


SUMMARY...Thunderstorms are expected to gradually increase in intensity and organization initially from southwest through central Louisiana, and eventually into southwest through central Mississippi into the afternoon. Supercells and bowing segments posing a risk for damaging wind and a few tornadoes will be the main threats. Trends are being monitored for a tornado watch.


DISCUSSION...A stationary front extends from northwest AL to a weak surface low in northern LA where it becomes a cold front and continuing into southeast TX. Forcing for ascent within the exit region of an upper jet rounding the base of an upper trough is beginning to overtake the western part of the warm sector across southwest through central LA. This is manifested by a recent increase in thunderstorms along and behind the front southwest LA through eastern TX. The southerly low-level jet will also respond and become coupled with the exit upper jet region, and is forecast to increase to 50+ kt this afternoon. The 12Z RAOB data from Lake Charles already showed moderate instability with 2500 J/kg MUCAPE in warm sector. Storms should soon become surface based and organize as supercells as temperatures rise through the low 70s F. The storms will subsequently spread northeast into MS this afternoon. The strengthening low and mid-level jets will support 50+ kt effective bulk shear and 200-300 m2/s2 storm relative helicity with largest hodographs expected east of the northeast migrating surface low.

Mesoscale Discussion #0079


The severe weather threat for Tornado Watch 22 continues.


SUMMARY...The threat for tornadoes has undergone a significant increase during the past hour from southwest through central MS, and this threat is expected to persist into the evening including the potential for a couple of strong tornadoes.


DISCUSSION...Storms have undergone a significant increase in organization during the past hour. A couple of storms with a history of producing tornadoes persist from southwest through central MS. Activity is increasing east and southeast of a surface low that will deepen and track northeast along a stationary front this evening. Large low-level hodographs with 300-400 m2/s2 0-1 km storm relative helicity exists along a strengthening low-level jet and within a moderately unstable environment. Additional storms developing in warm sector will likely organize as supercells and bowing segments with an attendant threat for tornadoes and damaging wind.

Mesoscale Discussion #0085


The severe weather threat for Tornado Watch 24 continues.


SUMMARY...Storms have transitioned into supercells, and a new tornado watch will be needed for the area.


DISCUSSION...Strong shear acting for a prolonged period on storms along the cold front has resulted in a tornadic supercell, now over Jasper county, MS. The environment ahead of this activity is favorable for a continued tornado threat, with increasing low-level shear into AL. In addition, boundary layer moisture continues to increase, with PWAT values over 1.75" east of the cold front. Finally, midlevel lapse rates are favorable for robust updrafts, which may aid tornado, and possibly significant tornado threat. Large-scale lift will only increase with time, and additional supercells are possible both along and eventually ahead of the front. As such, an additional tornado watch is likely.

Mesoscale Discussion #0139


The severe weather threat for Tornado Watch 36 continues.


SUMMARY...A tornado threat may persist for a few more counties east and northeast of the Nashville area. Damaging wind gusts are also possible, possibly extending into southern Kentucky.


DISCUSSION...A long-lived supercell, now with a confirmed, potentially intense tornado, continues to move east across the Nashville area. This southern-most supercell is on the southern fringe of the larger-scale area of ascent, and firmly within the local effective SRH maximum where values over 500 m2/s2 exist. 


Surface observations indicate a plume of mid 50s F dewpoints extends east/northeast of this storm, and while instability weakens with eastern extent, strong shear may allow these storms to persist for at least an hour.

Mesoscale Discussion #0193


Probability of Watch Issuance...80 percent


SUMMARY...The initiation of discrete thunderstorm development appears increasingly likely by 6-7 PM CDT, and could commence as early as 4-5 PM CDT across the Big Country near Abilene, in an environment increasingly conducive to the formation of supercells. The primary severe hazards initially are probably large hail and locally strong surface gusts, but potential for tornadoes could begin to increase by early evening.


DISCUSSION...Southwesterly mid/upper flow remains broadly anticyclonic with rising mid-level heights across much of the region.  However, it is and should remain at least weakly difluent, and model output continues to suggest this may transition at mid-levels to broadly cyclonic with weak height falls, as a subtle perturbation overspreads the region by 21-00Z.  As this occurs, there probably will be at least some tempering of increasing lower/mid tropospheric inhibition, which is ongoing in association with the northward advection of warmer elevated mixed-layer air.


Daytime heating of a moist boundary layer (surface dew points in the lower to mid 60s) already appears to be contributing to mixed-layer CAPE of 1000-2000 J/kg, as winds around 500 mb begin to increase to 40-50 kt.  By the time storms initiate, the environment probably will be conducive to supercells.


Low-level forcing to focus and support thunderstorm initiation remains a little unclear, but weak low-level warm advection may contribute to lift.  Convection allowing guidance, particularly the last few runs of the High Resolution Rapid Refresh, suggest convection may initiate across the Big Country, near Abilene, as early as 21-22Z.


Low-level hodographs are initially rather modest in size, and may remain so until after 00-02Z, when near surface wind fields may begin to back to south/southeasterly, while 850 mb flow begins to strengthen to 30-40 kt, as the primary upstream short wave begins to accelerate toward the Southwestern international border area.  However, strongest storms may still pose a risk for large hail, locally damaging wind gusts and perhaps a brief tornado or two into early evening.

Mesoscale Discussion #0195


The severe weather threat for Tornado Watch 54 continues.


SUMMARY...Discrete supercell development being monitored.  Large to very large hail is the initial risk.  Tornado potential will likely increase during the 23-01z (7pm-9pm CDT) period.


DISCUSSION...Radar mosaic shows the initial development stages of supercells beginning to form near the I-20 corridor near Abilene. Surface analysis indicates temperatures are in the upper 70s F with dewpoints in the mid 60s.  RAP forecast soundings show around 2000 J/kg MLCAPE in the warm sector downstream of the developing storms.  The RAP hodographs show veering and strengthening flow with height with little weakness.  The low-level portion of the hodographs are forecast to enlarge further during the next 1-3 hours (0-1km SRH increasing from 100 to 250 m2/s2) which will likely translate to low-level mesocyclone intensification and a subsequent increase in tornado potential during this timeframe.

Mesoscale Discussion #0198


The severe weather threat for Tornado Watch 54 continues.


SUMMARY...Tornado potential over the next 2 hours will likely focus with the discrete supercell ahead of the small bowing segment located farther west.  Marshall/Grayson counties may eventually need a watch extension-in-area (EXA).


DISCUSSION...A discrete supercell with a history of brief tornadoes will likely continue to and east of the I-35 corridor and eventually reach the Red River vicinity between 1030pm-1130pm.  The wind profile per KFWS has shown the enlargement of the low-level hodograph per recent RAP model sounding forecasts.  With moderate buoyancy (1500 J/kg MLCAPE) and 0-1km SRH between 200-300 m2/s2, it seems plausible the discrete supercell will continue uninhibited towards the east-northeast.  CINH should only slowly increase during the next few hours with the rate of CINH increase limited by mid-upper 60s F dewpoints within the inflow of the supercell.

Mesoscale Discussion #199


The severe weather threat for Tornado Watch 55 continues.


SUMMARY...Severe/isolated tornado risk continues across the WW area, including a well-developed supercell now crossing Taylor County.


DISCUSSION...Latest radar loop shows a strongly rotating storm crossing northern Taylor County at this time, within northern portions of WW #55.  KDYX WSR-88D has depicted rotational velocities near 50 kt with this cell at times, along with a CC min/TDS near the Nolan/Taylor County border just after 06Z.  With the KDYX VWP showing substantial veering/increasing of the flow field with height through the lowest 2km, and a thermodynamic environment featuring 1500 J/kg mixed-layer CAPE, expect this risk to continue as storms spread quickly east-northeastward at 45 kt.


Meanwhile farther south, storms continue to increase either side of the Rio Grande, just west/northwest of Del Rio, where a similarly favorable kinematic and thermodynamic environment exists.  A such, severe risk appears likely to increase over southern and central portions of the watch over the next 1-2 hours.

Mesoscale Discussion #0200


The severe weather threat for Tornado Watch 55 continues.


SUMMARY...Strong supercell likely produced a significant tornado continues moving east-northeastward within northern portions of WW 55.


DISCUSSION...Latest KDYX radar loop shows a well-organized supercell moving northeastward at 40 kt across southwestern Shackelford County.  A low-level mesocyclone with rotational velocity in excess of 70kt has been observed consistently with this storm over the past 20 minutes, with a pronounced CC min evident.  This suggests high probability that a strong tornado occurred in southwestern portions of the county.  


With the environment featuring mixed-layer CAPE in excess of 1500 J/kg, and 0-1km SRH 600 m2/s2 per latest KDYX WSR-88D VWP, expect this storm to persist in the short term.  While the circulation has weakened in the most recent scans, additional cycling may occur; the current track of the storm would suggest areas in and around Albany TX will be affected in the next half hour.

Mesoscale Discussion #0228


The severe weather threat for Tornado Watch 68, 70 continues.


SUMMARY...Tornado threat will shift east of WWs 68/69 over the next few hours. New tornado watch may be warranted soon.


DISCUSSION...Latest NAM guidance suggests LLJ will strengthen across the western TN Valley this evening as 500mb speed max increases in excess of 100kt from AR into western KY over the next 03-06hr. Earlier trends suggesting a QLCS might be evolving across AR now favor a mix of discrete supercells and clusters. This may be in part to expected strengthening wind fields. Over the next few hours, ongoing corridor of supercells over eastern portions of ww68 should approach, then spread east of the watch. This will necessitate a new tornado watch soon.

Mesoscale Discussion #0250


Probability of Watch Issuance...95 percent


SUMMARY...A gradual increase in severe storm potential is expected through mid-afternoon. A tornado watch will likely be needed by 19Z.


DISCUSSION...Thick cloud cover has limited surface heating across most of southeast Missouri and northeast Arkansas today with more breaks in the clouds and warmer temperatures seen across southern Illinois. Upstream convective activity will continue to keep a cloud shield across most of this region. The only exception will be central Missouri where upper-clouds have started to dissipate and surface heating is increasing.


Areas beneath the cloud shield are slowly destabilizing due to gradual warming temperatures, northward moisture advection, and cooling temperatures aloft. This will continue for the next few hours and eventually erode inhibition enough for surface based storms to develop along and ahead of the surface front this afternoon and into the evening. MLCAPE is expected to peak between 1500 and 2000 J/kg across this region with effective shear around 60 to 70 knots. This will support supercellular structures. 


Considerable storm coverage across southeast Missouri and southern Illinois will likely lead to a messy storm mode and thus limit the overall intensity of the storms. Nonetheless, stronger updrafts will likely be present with a threat for all hazards including a couple of tornadoes. A few discrete storms are also possible ahead of the front which would pose a greater hail/tornado threat. Overall, any tornadoes in this region will likely remain relatively short-lived given the mostly long-straight hodographs and relatively veered surface flow. 


Farther north, along and north of I-44 in Missouri, surface flow is strongly backed and temperatures/dewpoints are increasing. This region, in closer proximity to the surface low, will have a greater threat for longer-track tornadoes given the backed surface flow, and more discrete storm mode.

Mesoscale Discussion #0251


Probability of Watch Issuance...80 percent


SUMMARY...The initiation of scattered supercells appears increasingly probable during the next few hours.  Tornadic potential, including the risk for strong tornadoes, seems likely to increase by 4-6 PM CDT.


DISCUSSION...Rapid deepening of the surface cyclone now centered over southeastern Nebraska is ongoing, with an axis of 2-4 mb 2-hourly surface pressure falls now pivoting northeastward, to the northeast of the lower Missouri Valley.  To the southeast of cyclone center, a dryline continues to sharpen across southwestern Iowa and western Missouri, and may maintain identity into late afternoon, advancing northeastward just ahead of a surging cold front.


Strengthening and backing (to southerly) 850 mb flow ahead of these boundaries is forecast to contribute to enlarging clockwise curved low-level hodographs by late afternoon, beneath a very strong (90-110 kt) 500 mb jet.  As this occurs, the environment appears likely to become increasingly conducive to tornadic supercells given sufficient boundary-layer destabilization.


Mid/high-level cloudiness and precipitation, to the south of the warm front near the Missouri/Iowa border, continues to slow boundary layer destabilization in the warm sector.  However, a tongue of mid 60s+ surface dew points appears to be in the process of advecting north of the Missouri Ozarks, ahead of the dryline, in response to the strong surface pressure falls.


As a vigorous upstream mid-level trough and embedded closed low accelerate northeast of the central Plains, broadly anticyclonic mid-level flow will increasingly transition to cyclonic across the region through 21-23Z.  The leading edge of stronger difluence aloft may already be aiding convective development near the dryline and warm front, southeast of Shenandoah IA.  A further increase in discrete storm development appears possible within the next couple of hours, along/ahead of the dryline to the south.  Further development, intensification and northeastward advection of storms now spreading across the Missouri Ozarks also appears possible within the warm sector.


Regardless, storms interacting with the warm front probably will pose a risk for tornadoes, while strong deep-layer wind fields and shear within the destabilizing open warm sector will contribute to fast storm motions and potential for long-lived/long track supercells with increasing potential to produce strong tornadoes across northeast Missouri into southeast Iowa and adjacent west central Illinois by 21-23Z.

Mesoscale Discussion #0317


Probability of Watch Issuance...95 percent


SUMMARY...Damaging storms, including significant straight-line winds, a few tornadoes and large hail are likely to develop over the next several hours.


DISCUSSION...Steep lapse rates precede a cold front rapidly moving across IL, with a dryline extending into southern MO. Meanwhile, a low-level moisture gradient currently extends east roughly along the OH River, with 60s F into far southern IN. Meanwhile, a residual outflow boundary remains over eastern KY.


Substantial pressure falls currently exist over IL and IN, with storms increasing in coverage over IL and MO. Some of these storms have been severe, despite meager moisture over IL. Recently, a supercell over southeast MO has become much more severe as it encountered the moist air mass.


Large-scale lift will increase through evening as the front moves into IN, KY, and OH. Cold temperatures aloft will maximize updraft strength, with a significant line of severe storms likely, along with some supercells. These storms, whether straight-line winds or with tornadoes, will be capable of damage. The most probable area of tornadoes appears to be near or just ahead of the developing line, along the OH River, where pressure falls will be maximized along the moisture return, and where locally-backed winds will enhance SRH after 03Z.

Mesoscale Discussion #0350


Probability of Watch Issuance...95 percent


SUMMARY...The severe threat will markedly increase by this afternoon, with strong tornadoes, numerous damaging winds, and large hail all likely. A tornado watch will be required across northern/central MS and northeastern LA shortly.


DISCUSSION...The airmass across northeastern LA into central MS downstream of a QLCS in northern LA is rapidly destabilizing. A warm advection wing of elevated convection has developed across this area as a marine warm front continues to lift northward. Surface dewpoints are expected to quickly increase into the upper 60s to lower 70s along/south of the northward-advancing warm front as a strong low-level mass response occurs over the ArkLaTex region this afternoon. With continued diurnal heating, MLCAPE should increase into the 1500-3000 J/kg range by peak afternoon heating. Very strong shear is forecast to be present from low through mid levels, which will support robust storm organization both with the ongoing QLCS and with any cells that can form farther south/east across the open warm sector. Latest VWP from KDGX (Jackson, MS) shows substantial low-level hodograph curvature, with 0-1 km shear approaching 50 kt, and 0-1 SRH around 500 m2/s2. Strong tornadoes may occur given the strength of the low-level flow, along with numerous damaging winds if the storm mode remains mostly linear. There is still some uncertainty regarding supercell potential across northern/central MS this afternoon. If any storms can form farther south/east of the ongoing QLCS, they will encounter a strongly unstable and sheared environment favorable for all severe hazards.

Mesoscale Discussion #0351


The severe weather threat for Tornado Watch 106 continues.


SUMMARY...Strong tornado potential is increasing across northeastern LA into central MS early this afternoon.


DISCUSSION...Multiple tornadic circulations producing TDSs have recently been observed with a QLCS advancing rapidly eastward across north-central into northeastern LA. The VWP from KDGX (Jackson, MS) shows a very strongly sheared low-level wind profile. Over 80 kt of southwesterly flow is being estimated around 3 km AGL, with around 70 kt of 0-3 km shear present. A corridor of greater severe potential, including the possibility of strong tornadoes and widespread damaging winds, will likely be focused through the early afternoon from parts of northeastern LA into central MS ahead of the QLCS and to the south of a northward-developing warm front. Embedded circulations within the line, or a supercell or two developing on its southwestern flank, will pose the best potential for strong tornadoes given the very favorable low-level shear (0-1 km SRH in excess of 500 m2/s2).

Mesoscale Discussion #0355


The severe weather threat for Tornado Watch 106 continues.


SUMMARY...Severe potential should be maximized across parts of central MS this afternoon. This continues to include the possibility of strong tornadoes.


DISCUSSION...A small cluster moving into central MS may be transitioning to a supercell, with echo tops approaching 50,000 ft. Yet another TDS has recently been observed with this convection in Yazoo County, MS. The airmass downstream of this activity remains quite favorable for organized, severe convection. The 18Z sounding from JAN shows a very strong and veered wind profile in the low levels in association with a 60-70+ kt low-level jet. The low-level inversion observed around 850 mb is expected to continue eroding as ascent associated with both the low-level jet and an approaching mid-level trough overspread this region.


A meso-low has also been observed from 18Z surface analysis over far southeastern AR and northwestern MS. Surface winds are locally backed to east-southeasterly along a marine warm front extending eastward from this low across parts of northern/central MS. A relatively confined corridor of greater severe potential is apparent this afternoon across parts of central MS, generally to the east of the ongoing cluster/supercell and south of the northward-advancing warm front. Across this area, a sufficiently unstable airmass coupled with very strong low and mid-level shear will continue to support the potential for tornadoes with any storms that can form/be sustained. Some of these tornadoes could be strong given the ample low-level shear. A threat for damaging winds and perhaps some large hail also exists.

Mesoscale Discussion #0357


The severe weather threat for Tornado Watch 106, 107, 109 continues.


SUMMARY...The potential for damaging winds, large hail, and tornadoes will continue across parts of southern/central MS into west-central AL for the next couple of hours. Strong tornadoes still appear possible.


DISCUSSION...Recent 7 and 9 km CAPPI radar imagery suggests that the storms across southern/central MS have intensified. It appears that the low-level inversion present on the 18Z JAN sounding that may have been hampering robust updraft development has eroded. A surface warm front has continued to lift northward across parts of central into northern AL, with a moderately to strongly unstable airmass present to its south across parts of southern/central MS into AL. The greatest near-term tornado potential appears to be with a supercell currently in Noxubee County MS, and continuing downstream into west-central AL. Low-level flow is backed to southeasterly along the warm front across this area, with latest mesoanalysis estimating effective SRH now exceeds 500 m2/s2. A strong tornado may occur with this supercell given the very strong low-level shear in the presence of a supercell. Otherwise, all severe hazards remain possible for the next couple of hours along and downstream of ongoing convection across southern/central MS. Another strong tornado also appears possible with a supercell moving from Pike into Walthall County in southern MS.

Mesoscale Discussion #0358


The severe weather threat for Tornado Watch 107 continues.


SUMMARY...Based on recent radar and environmental data, a strong to intense tornado with potential peak winds of 115 to 165 mph(EF2-EF3) is likely ongoing.


DISCUSSION...An intense supercell will continue to move through a favorable environment over south-central Mississippi, characterized by 0-1 km SRH around 300 m2/s2 and MLCAPE near 2500 J/kg per latest mesoanalysis data. Recent radar signatures from KLIX as of 2044Z reveal an intense supercell characterized by a 0.4 degree rotational velocity between 60 and 69 kt. A tornadic debris signature has also been noted on recent radar scans. These signatures are occurring in an environment characterized by STP greater than 6. Previous signatures within similar environments produced tornado-damage-estimated wind speeds from 115 to 165 mph and confidence is high for likely strong to intense tornado. A longer-track tornado is ongoing and may continue based on the rotational velocity duration and the storm moving within a favorable downstream environment.


**This MCD covered the Jayess, Mississippi EF4 tornado

Mesoscale Discussion #0360


The severe weather threat for Tornado Watch 106, 107 continues.


SUMMARY...Based on recent radar and environmental data, a violent tornado with potential peak winds of 170 to 205 mph (EF4-EF5) is likely ongoing. THIS IS AN EXCEPTIONALLY RARE EVENT.


DISCUSSION...An intense supercell will continue to move through a favorable environment over south-central Mississippi, characterized by 0-1 km SRH around 400 m2/s2 and MLCAPE near 2000 J/kg per latest mesoanalysis data. Recent radar signatures from KLIX as of 2112Z reveal an intense supercell characterized by a 0.4 degree rotational velocity of at least 70 kt. A tornadic debris signature has also been noted on recent radar scans. These signatures are occurring in an environment characterized by STP greater than 7. Previous signatures within similar environments produced tornado-damage-estimated wind speeds from 170 to 205 mph and confidence is high for a likely violent tornado. A long-track tornado is expected to continue based on the rotational velocity duration and the storm moving within a favorable downstream environment.

Mesoscale Discussion #0362


The severe weather threat for Tornado Watch 106, 107, 110 continues.


SUMMARY...Two tornadic supercells are likely to track northeast across south-central and southeast Mississippi in the next couple of hours. Significant, long-track tornadoes are likely.


DISCUSSION...A tornadic supercell currently moving over northwest Jones County Mississippi has exhibited extremely strong rotation with rotational velocity values around 100 knots on multiple scans. Additionally, multiple scans of TDS have been observed over the last hour. Given the very favorable storm/tornado environment, characterized by STP of 7+, MLCAPE of 2000+ J/kg, and 0-1 km SRH of 400+ m2/s2, this tornadic supercell is likely to continue tracking northeastward for at least the next hour and likely longer. 


Back to the southwest, another tornadic supercell is moving over southern Jefferson Davis County that has exhibited strong rotation with rotational velocity values of 60+ knots. This is following a very similar track to the previous tornadic supercell with the potential for communities to be affected twice in a relatively short amount of time. With no other convection degrading the warm sector south and downstream of these storms, they are likely to continue across Mississippi and into west/southwest Alabama.

Mesoscale Discussion #0364


The severe weather threat for Tornado Watch 107 continues.


SUMMARY...Cells developing near Ascension, Livingston, and Assumption Parishes are expected to become supercellular and potentially tornadic over time.


DISCUSSION...Radar/satellite imagery indicates continued robust growth in convection across Ascension, Livingston, and Assumption Parishes in southeastern Louisiana.  These storms have successfully breached the cap and are exhibiting cellular storm mode in an environment characterized by strong low-level shear (40 kt 0-1km), strong deep shear (60-65 kt 0-6km) and moderate to strong instability (2000+ MLCAPE) amidst 70-75F dewpoints and near 7 deg C lapse rates aloft.


Given the environment, the storms are expected to acquire rotation and eventually pose a risk of all hazards, including strong tornadoes.

Mesoscale Discussion #0371


The severe weather threat for Tornado Watch 109, 110 continues.


SUMMARY...Storms are intensifying as they move into central Alabama, with local enhancement to the tornado risk evident.


DISCUSSION...Latest radar loop shows a couple of storms intensifying within the broader north-northwest to south-southeast band of storms crossing Alabama and southeast Mississippi.  The two strongest storms -- one moving into Cullman County and the other crossing Tuscaloosa county -- appear most intense at this time, with the southern storm likely having produced a tornado recently (70 kt rotational velocity and a small CC minimum have been observed within the past 15 minutes).  As this storm continues moving north-northeastward, through the very favorable thermodynamic and kinematic environment that remains in place, the potential for a strong tornado and/or damaging wind gusts has increased for areas near and northwest of Birmingham.

Mesoscale Discussion #0409


The severe weather threat for Tornado Watch 121 continues.


SUMMARY...The threat for damaging winds, hail and tornadoes continues.


DISCUSSION...Regional radar shows a cluster of storms has entered western portions of WW 121. These storms are undergoing complex mode transformations, with intermittent weak to moderate (30-40 kt) embedded rotational velocity signatures in addition to broader damaging wind potential. Consequently, the short-term evolution of this storm cluster remains somewhat uncertain. However, enhanced mid-level flow is now entering the region, resulting in the appearance of near 50 kt of effective bulk shear. Meanwhile, the environment remains moderately unstable across portions of central Mississippi, characterized by approximately 2000-2500 J/kg of MLCAPE downstream of the ongoing cluster, and thus a damaging wind threat will persist across the watch area for the next few hours as this cluster propagates along the east-west oriented baroclinic zone.


In addition, isolated convection has developed within the warm sector downstream of the aforementioned storm cluster. These storms will pose an initial hail threat. Surface mesoanalysis indicates a modest improvement in low-level shear in proximity to these storms, with near 150 m^2/s^2 of 0-1 SRH. Given the background thermodynamic/kinematic environment, these storms have the best potential for supercell organization, with all modes of severe possible, if they can remain discrete.

Mesoscale Discussion #0412


The severe weather threat for Tornado Watch 121, 122, 123 continues.


SUMMARY...A corridor for higher tornado potential exists across far southern Mississippi into adjacent areas of eastern Louisiana south of an outflow boundary. Storms moving along/south of this boundary will encounter stronger low-level shear.


DISCUSSION...An outflow boundary has moved south from central Mississippi into southern Mississippi where winds have backed to southeasterly along and just south of this boundary. A few supercells developed across southwest Mississippi and eastern Louisiana in the last hour or so, but have now mostly congealed into a cluster due to upscale growth and the southward moving outflow boundary. However, the southern storm is still exhibiting supercell characteristics and has shown MESH values of 1+ inch although convective interference has weakened it somewhat. Surface winds near MCB and eastward have backed to the southeast indicating potentially higher low-level helicity, which could increase the tornado threat. There remains uncertainty due to the amount of potential convective interference and that the outflow boundary could continue sliding southward.

Mesoscale Discussion #0413


The severe weather threat for Tornado Watch 123 continues.


SUMMARY...Based on recent radar and environmental data, a strong to potentially violent tornado with potential peak winds of 120 to 170 mph EF3+ is likely ongoing.


DISCUSSION...Recent radar signatures from KLIX as of 838 PM EDT reveal an intense supercell characterized by a 0.5 degree rotational velocity between 60 and 69 kt. A tornadic debris signature has also been noted on recent radar scans. These signatures are occurring in an environment characterized by STP between 3 and 4. Previous signatures within similar environments produced tornado-damage-estimated wind speeds from 120 to 170 mph and confidence is high for a strong to potentially violent tornado. A potential long-track tornado is ongoing and may continue based on the rotational velocity duration and the storm moving within a favorable downstream environment.

Mesoscale Discussion #0439


Probability of Watch Issuance...80 percent


SUMMARY...Severe storms expected to develop from southwest Oklahoma into northwest TX and spread east this afternoon into the early evening. A tornado watch will likely be needed soon. Very large hail, damaging wind and a few tornadoes are all possible.


DISCUSSION...Early this afternoon a dryline extends from a surface low southward into northwest TX. A quasi-stationary front extends eastward from the low through southern OK. The boundary layer is destabilizing in dry slot region across northwest TX into southwestern OK where some clearing, steep lapse rates, and dewpoints in the upper 60s have contributed to 2000-2500 J/kg MLCAPE. Storms are in the process of developing near triple point across southwestern OK, and additional isolated storms are expected to develop farther south into northwest TX along sharpening dryline. Though low level winds are in the process of veering near the initiation zone, storms may encounter more favorable 0-1 km storm relative helicity for low-level mesocyclones as they move east through the warm sector. Strong (50+ kt) effective bulk shear will support supercells capable of very large hail, damaging wind and a few tornadoes.

Mesoscale Discussion #0440


The severe weather threat for Tornado Watch 133 continues.


SUMMARY...Convective trends suggest a recent uptick in intensity over the past half hour, with rotation noted in several cells.  All modes of severe weather remain possible with this activity.


DISCUSSION...Storms across north/northeastern portions of WW 133 have increased in intensity over the past half hour or so - and are now exhibiting more supercellular characteristics.  A subtle warm front/differential heating zone has developed along an east-west axis in vicinity of these storms - generally from near Tyler, TX to near Fort Polk, Louisiana.  South of this effective boundary, continued low-level advection and limited insolation has maintained a moderately unstable airmass, which is likely allowing for storms to become more surface-based and ingest vorticity along the weak differential heating zone.  This suggests an increase in the tornado and hail threat with this convection.  Again, a strong tornado cannot be completely ruled out given 500+ m2/s2 effective SRH near the differential heating zone.


As storms migrate northeast, spatial extensions of WW 133 may be needed.

Mesoscale Discussion #0442


The severe weather threat for Tornado Watch 134 continues.


SUMMARY...The risk of tornadoes (potentially strong), large hail, and damaging winds will increase eastward within a corridor in WW134.


DISCUSSION...Discrete supercells have developed across the northern portions of WW134 along a cold front dryline combination in central and southern Oklahoma. A tornado has been observed near Springer, Oklahoma already and radar data shows strong low level mesoscylones on all of the storms. These cells will move into an environment supportive of additional tornadoes with strong low level ESRH of 200-400 m2/s2. A strong tornado will also be possible with the more intense low level mesocyclones.

Mesoscale Discussion #0443


The severe weather threat for Tornado Watch 133 continues.


SUMMARY...Threat for all severe hazards, including a strong tornado, has increased across southeast TX.


DISCUSSION...Ongoing supercell entering Walker and Houston Counties has shown trends towards more organization and deviant right motion over the past 15 minutes. This trend suggest maturation into a supercell is near-complete or complete. Given the lack of storms and the moist and unstable air mass downstream, the overall environment appears favorable for continue storm persistence and perhaps additional strengthening. Recent mesoanalysis estimate STP of 6-7 exists in this region. All severe hazards, including a strong tornado, are possible with this supercell and any additional mature development over the region.

Mesoscale Discussion #0445


The severe weather threat for Tornado Watch 135 continues.


SUMMARY...SUMMARY Based on recent radar and environmental data, a strong to intense tornado with potential peak winds of 130 to 165 mph (EF2-EF3) is likely ongoing.


DISCUSSION...DISCUSSION Recent radar signatures from KGRK as of 2250Z reveal an intense supercell characterized by a 0.5 degree rotational velocity of at least 70 kt. A tornadic debris signature has also been noted on recent radar scans. These signatures are occurring in an environment characterized by STP between 5 and 6. Previous signatures within similar environments produced tornado-damage-estimated wind speeds from 130 to 165 mph and confidence is high for a strong to intense tornado. A longer-track tornado is ongoing and may continue based on the rotational velocity duration and the storm moving within a favorable downstream environment.

Mesoscale Discussion #0448


The severe weather threat for Tornado Watch 135 continues.


SUMMARY...The environment remains supportive of all severe hazards, including a strong tornado, downstream of the ongoing storm across Jasper County TX.


DISCUSSION...Long-track supercell moving into Jasper County TX has had a history of producing strong low-level rotation. Recent signatures suggest the storms has undergone recycling of the mesocyclone as it continues to move within a very favorable environment. KPOE VAD recently sampled 0-1 km SRH over 450 m2/s2 and 70 kt of 0-6 km bulk shear. Temperatures ahead of the cell are in the mid 70s with dewpoints in the low 70s. Mesoanalysis estimates MLCAPE around 2000 J/kg. The strong buoyancy expected to persist while the low-level jet gradually increases. The ongoing storm continues to move unimpeded with the warm sector. In all, the environment remains very favorable for storm persistence with a continued threat for all severe hazards, including a strong tornado.

Mesoscale Discussion #0449


The severe weather threat for Tornado Watch 135 continues.


SUMMARY...Based on recent radar and environmental data, a strong to potentially intense tornado with potential peak winds of 120 to 165 mph (EF2-EF3) is likely ongoing.


DISCUSSION...Recent radar signatures from KPOE as of 0037Z reveal an intense supercell characterized by a 0.5 degree rotational velocity between 50 and 59 kt. A tornadic debris signature has also been noted on recent radar scans. These signatures are occurring in an environment characterized by STP greater than 6. Previous signatures within similar environments produced tornado-damage-estimated wind speeds from 120 to 165 mph and confidence is high for likely strong to potentially intense tornado. A tornado is ongoing and may continue based on the rotational velocity duration and the storm moving within a favorable downstream environment.

Mesoscale Discussion #0452


The severe weather threat for Tornado Watch 135 continues.


SUMMARY...Long-track supercell will continue to pose a risk for all severe hazards, including tornadoes, as it moves eastward into far southwest MS.


DISCUSSION...Long-track supercell continues to move east-northeastward across central LA. Northward movement of the warm front continues to be hampered by numerous showers and thunderstorm north of the boundary. However, some slow northward progress has been noted with the dewpoint at HEZ and HZR now 65 and 70 deg F, respectively. Area VAD wind profiles continue to show strong low-level veering favorable for tornadoes. Given the continued threat, a downstream Tornado Watch will likely be needed for portions of far southwest MS and northern southeast LA. 


In the near-term, the highest tornado threat will exist immediately downstream of the ongoing supercell over Rapides Perish LA. Recent radar signatures have shown another increase in organization and the downstream environment is more than supportive of additional tornadogenesis.

Mesoscale Discussion #0453


The severe weather threat for Tornado Watch 135 continues.


SUMMARY...A corridor of increased severe potential, including the threat of a strong tornado, may be developing across central LA.


DISCUSSION...Pair of discrete supercells moving into Natchitoches and Winn Parishes in central LA have recently shown signs of better low-level organization with stronger inflow/outflow structure and a more defined ZDR arc. Recent VAD profile from POE sampled 50 kt near 1 km and over 400 m2/s2 of 0-1 km SRH. Some nocturnal stabilization has taken place but the very moist low-levels and already mature character of the storms suggest limited influence of this stabilization thus far. All of these factor suggest a corridor of increased severe potential, including the threat of a strong tornado, may be developing across central LA where STP is 4-6.

Mesoscale Discussion #0861


The severe weather threat for Severe Thunderstorm Watch 264 continues.


SUMMARY...New Severe thunderstorm watch is warranted for portions of the Dakotas. Fast-moving Derecho continues to produce severe winds in excess of 50kt.


DISCUSSION...Derecho that evolved across eastern UT/CO has progressed across the central Rockies and is now racing across the northern/central High Plains. Latest radar data suggests the strongest surge is near the Black Hills where the leading bow is surging northeast at 65kt. Numerous measured wind gusts between 50-65kt have been noted with this squall line and there is reason to believe damaging winds will persist with this MCS as it surges across the northern Plains in advance of a negative-tilt short-wave trough. New severe thunderstorm watch will be issued around 02z.

Mesoscale Discussion #0869


Probability of Watch Issuance...95 percent


SUMMARY...Thunderstorms are expected to develop rapidly across eastern ND and northwestern MN this afternoon. The environment supports all severe hazards, including tornadoes and very large hail.


DISCUSSION...Air mass over eastern ND has quickly destabilized amid strong diurnal heating and ample low-level moisture. Recent mesoanalysis estimates little to no convective inhibition remains with MLCAPE around 2500 J/kg. This destabilization is confirmed by the building cumulus field over the region. Wave clouds father east and northeast in far northwest MN suggest more stability remains in place over this area.


Based on recent surface analysis, the warm front extends from about 50 miles west-northwest from HCO southwestward through GAF and DTL (roughly along the 82 deg F isotherm). Given this placement, much of eastern ND is already south of the boundary. The warm front is expected to continued northward/northeast throughout the day, result in a warm, moist, and very unstable warm sector. Boundary-layer mixing may result in a veering of the surface winds and modest weakening of the low-level shear. Even so, the low-level flow should remain backed enough to support supercell wind profiles, particularly with northeastern extent (especially along warm front).


Once convective initiation occurs, robust updrafts are expected to develop quickly. All severe hazards are possible, with the tornado and hail threat highest during the first few hours after development. Some questions exist regarding how long storms will stay discrete and cellular, but eventual transition into a more linear system is anticipated.

Mesoscale Discussion #1385


Probability of Watch Issuance...80 percent


SUMMARY...Probability for a few tornadoes is expected to increase and a watch will likely be needed.


DISCUSSION...A very moist air mass is in place across the coastal Carolinas ahead of Tropical Storm Isaias. Recent surface observations had temperatures in the low 80s amid upper 70s dewpoints. These conditions are yielding 1000-1500 J/kg of MLCAPE. As of now, wind fields are relatively modest, with both the LTX and MHX VAD profiles showing weak low-level easterlies/southeasterlies beneath modest mid-level southeasterlies. Mid-level flow is expected to gradually increase as Isaias continues northward. This will result in wind profiles becoming more favorable for tornadogenesis. Main uncertainty will be whether or not updrafts can maintain enough strength/depth to realize the vertical shear. Even with this uncertainty, probability for a tornadoes will likely be high enough to merit a watch and trends will be monitored for most favorable issuance time.

Mesoscale Discussion #1388


The severe weather threat for Tornado Watch 413 continues.


SUMMARY...The tornado threat will increase along the coasts of eastern South Carolina and eastern North Carolina early this evening. As the tornado threat moves northward over the next couple of hours, tornado watch issuance will be needed across parts of eastern North Carolina.


DISCUSSION...The National Hurricane Center currently places Tropical Storm Isaias about 60 statute miles east of Charleston, South Carolina. Isaias is forecast to become a Hurricane and make landfall near Wilmington, North Carolina later evening. A distinct band of heavy rainfall surrounds the center of Isaias and is affecting southern North Carolina and eastern South Carolina at this time. The band in far southern North Carolina consists of several discrete thunderstorms from the Wilmington area extending offshore about 100 statute miles. The latest WSR-88D VWP from Morehead City, North Carolina shows east-southeasterly surface winds veering to the south-southeast at 2 km above ground-level. This is creating strong low-level shear with 0-3 km storm relative helicities around 200 m2/s2. As the strong winds associated with Isaias move northward, low-level shear will increase. This will make conditions increasingly favorable for tornadoes from far eastern South Carolina into eastern North Carolina.

Mesoscale Discussion #1389


The severe weather threat for Tornado Watch 414 continues.


SUMMARY...The tornado threat will likely continue across much of eastern North Carolina over the next few hours. The tornado threat is expected to increase across southeast Virginia where a new tornado watch will likely be needed.


DISCUSSION...The center of Hurricane Isaias is about 20 statute miles southwest of Wilmington, North Carolina. Two bands of heavy rainfall are present around the center. The first and southern most-band is located in southern North Carolina near Wilmington. The second and northern-most band is located in eastern North Carolina near Morehead City, extending northwestward to about 75 miles inland. The Morehead City WSR-88D VWP shows very strong low-level shear with 0-1 km storm relative helicity near 600 m2/s2. This will be favorable to continue a tornado threat with the more discrete cells across eastern North Carolina late this evening into the early overnight period. 


The tornado threat should spread into southeast Virginia as low-level shear and theta-e increases. A new tornado watch will be necessary across parts of southeast Virginia over the next few hours.

Mesoscale Discussion #1390


Probability of Watch Issuance...95 percent


SUMMARY...The threat for tornadoes will develop north across eastern Virginia and the Chesapeake region, and a new watch is likely.


DISCUSSION...Bands of supercells with tornado threat persist over eastern NC northeast of the cyclone center. Objective analysis shows effective SRH 300-500 m2/s2 in this area, where MLCAPE is around 1000 J/kg. A moist air mass is already in place downstream from VA into the Chesapeake. Thus, the rapid increase in low-level shear is likely to support a continued tornado threat tonight and into the early morning hours.

Mesoscale Discussion #1392


The severe weather threat for Tornado Watch 415 continues.


SUMMARY...The threat of tornadoes will continue to spread north out of southeast Virginia and into Maryland and Delaware.


DISCUSSION...Surface analysis shows strong pressure falls moving out of northeast NC and into southeast VA, on the order of 3-4 mb/hr. A band of supercells, tornadic at times, remains largely intact over southeast VA.


To the north, sporadic small storms have been noted over the past few hours, and have increased over the Delmarva recently. This is likely due to pressure falls and low-level warm advection as wind fields increase.


Over the next several hours, 0-1 SRH in excess of 300 m2/s2 will spread north, possibly reaching far southern NJ around 12Z. As this occurs, the environment will become increasingly favorable for tornadoes with both cells ahead of the main arcing line of storms and within the line itself.

Mesoscale Discussion #1450


Probability of Watch Issuance...95 percent


SUMMARY...Widespread damaging winds, some potentially significant (75+ mph), will become increasingly likely as a line of storms moves quickly eastward this afternoon. A Severe Thunderstorm Watch will be needed to address this threat, and an upgrade to Moderate Risk (for significant severe/damaging winds) will be issued with the upcoming 1630Z update to the Day 1 Convective Outlook.


DISCUSSION...A compact MCS moving into central IA as of 1540Z has recently produced numerous severe/damaging wind gusts. Recent radar trends suggest this system has already become very well organized, with the development of an 80-100+ kt rear-inflow jet only a couple thousand feet off the surface per KDMX velocity data. The airmass downstream of this MCS into eastern IA, southern WI, and northern/central IL is already quite unstable, with MLCAPE of 2000-2500 J/kg present per 15Z mesoanalysis estimates. Additional diurnal heating of this airmass is expected to yield very strong to potentially extreme instability by this afternoon, with MLCAPE potentially reaching the 3500-5500 J/kg range by peak heating. Recent VWPs from the KDMX radar show sufficient mid-level flow (around 30-35 kt) to support the continued intensity of the ongoing MCS.


Additional storms have recently formed across parts of north-central into northeastern IA along a weak cold front. Current expectations are for the MCS in central IA to eventually merge with the new development in north-central/northeastern IA. Result of this will likely be a large, very well organized bow echo producing widespread severe and damaging wind gusts across parts of eastern IA into southern WI, and northern/central IL. The forecast combination of very strong to extreme instability with adequate deep-layer shear downstream of the ongoing MCS strongly suggests that a swath of potentially significant severe wind gusts of 75+ mph is becoming increasingly likely this afternoon across parts of these areas. A new Severe Thunderstorm Watch will be needed downstream of the current watch in central IA within the next hour or two. An upgrade to Moderate Risk for numerous significant severe/damaging wind gusts will be issued with the 1630Z update of the Day 1 Convective Outlook.

Mesoscale Discussion #1451


The severe weather threat for Severe Thunderstorm Watch 425 continues.


SUMMARY...A swath of significant severe/damaging wind gusts appears likely across and to the east of the Des Moines metro area.


DISCUSSION...Latest velocity data from the KDMX radar shows an impressive significant wind signature with the ongoing MCS, with 80-115+ kt inbound velocities noted only 500-1000 ft AGL. These very likely severe winds will move across the Des Moines metro area shortly, and they will be capable of producing widespread, destructive damaging winds of 70-80+ mph as the MCS races eastward at 50-60 kt.

Mesoscale Discussion #1452


The severe weather threat for Severe Thunderstorm Watch 425, 426 continues.


SUMMARY...Widespread damaging winds of 70-100 mph are expected to continue eastward across central/eastern Iowa over the next hour or so. These winds will likely impact Cedar Rapids/Iowa City, and eventually the Quad Cities area.


DISCUSSION...A small but potent bow echo will continue rapidly eastward across eastern IA in the next hour or so. The apex of this bow is moving eastward around 55-60 kt, and the potential for widespread damaging winds remains apparent. A recent measured severe wind gust of 99 mph was reported at the Marshalltown ASOS associated with this bow, and a couple other gusts up to 100 mph have also been noted. Current expectations are for this bow echo to maintain its intensity across eastern IA, with 2500-3000 J/kg of MLCAPE present downstream. A swath of 70-100+ mph wind gusts producing destructive damage appears likely to impact the Cedar Rapids/Iowa City area within the next hour, eventually reaching the Quad Cities area around 1800-1830Z.

Mesoscale Discussion #1455


The severe weather threat for Severe Thunderstorm Watch 426 continues.


SUMMARY...The potential for widespread and destructive damaging wind gusts of 70-100+ mph and perhaps a tornado or two will continue as a line of storms moves quickly eastward across northern Illinois. This line will likely impact the Chicago metro area around 2000-2030Z (3 to 3:30 PM CDT).


DISCUSSION...The bow echo over eastern IA has developed a comma head structure on its northern flank and very large rear-inflow jet, with  measured wind gusts of 97 and 112 mph recently reported in the vicinity of Cedar Rapids IA. The 17Z sounding from DVN observed steep 700-500 mb lapse rates of 8.5 C/km. These lapse rates combined with strong heating of a very moist low-level airmass is supporting 3000-3500 J/kg of MLCAPE downstream of the bow. The 25 kt of effective bulk shear also observed on the 17Z sounding is somewhat marginal for storm organization. Regardless, with the bow echo already very well developed/organized, confidence remains high that a swath of widespread, destructive damaging winds of 70-100+ mph will sweep eastward across northern IL and vicinity over the next hour or two. If the current fast eastward motion of the bow (around 60 kt) holds steady, then the widespread damaging wind threat will increase across the Chicago metro area by 2000-2030Z (3 to 3:30 PM CDT). In addition to the substantial straight-line wind threat, some increase in the tornado threat may be developing on the northern flank of the bow, where mid-level rotation has recently been noted. Other cells have also formed ahead of the line across far southern WI and northern IL along a weak warm front, and this convection will pose a threat for isolated large hail, damaging winds, and perhaps a tornado or two.

Mesoscale Discussion #1458


The severe weather threat for Severe Thunderstorm Watch 426, 429 continues.


SUMMARY...A widespread severe/damaging wind threat continues with a line of storms moving quickly eastward. A tornado also remains possible. The greatest threat for 70-80+ mph wind gusts will likely focus over the Chicago metro area over the next hour.


DISCUSSION...Latest velocity data from KLOT shows somewhat less extreme inbound velocities (generally 50-80 kt) compared to earlier. Even so, the overall bow echo remains well organized, and it is moving through a very strongly unstable airmass (3500-4000+ J/kg MLCAPE). A severe wind gust to 54 kt was observed in Aurora IL at 2016Z. The northern portion of the bow will have the best potential to continue producing widespread damaging wind gusts as it moves across the Chicago metro area over the next hour, with some possibly still significant (75-80+ mph). A tornado or two embedded within the line also remains possible across this area, but the primary risk is still widespread damaging straight-line winds. This substantial severe wind threat will likely continue across southern Lake Michigan and into southwestern Lower MI and northwestern IN late this afternoon and into the early evening given a favorable combination of moderate to strong instability present across these areas.


The southern portion of the line into central IL has shown less intensity recently, with outflow beginning to surge southeastward ahead of the updrafts. This is likely due to weaker mid-level flow with southward extent across central IL (see recent VWPs from KILX). Still, isolated damaging winds will remain possible across this area in the short term.

Mesoscale Discussion #1807


Probability of Watch Issuance...95 percent


SUMMARY...The potential for severe thunderstorms and possibly a tornado will likely begin mid-late afternoon in the northeast TX Panhandle and far northwest OK, and then develop eastward this evening towards the I-35 corridor later this evening.


DISCUSSION...Visible-satellite imagery as of 1230pm CST shows the initial stages of a swelling cumulus field near a triple point located over the northeastern part of the TX Panhandle.  Surface analysis indicates a dryline extending south from the low with a cold front arcing from the northwest TX Panhandle east to the triple point and northeast into southwest KS.  Midday water-vapor imagery shows a dryslot wrapping into western OK in wake of elevated morning convection.  Furthermore, a strong mid-level shortwave trough over north-central NM continues to progress eastward towards the southern High Plains where objective analysis indicates -24 deg C 500 mb temperatures over central NM.


Surface observations as of 12pm CST on the West Texas and Oklahoma mesonets show temperatures ranging from 70 deg F around Childress to the lower 60s near I-40 and the upper 50s in northwest OK. Dewpoints are highest near the OK/TX border near I-40 (near 60 F) with mid 50s in northwest OK.


12z HREF and recent time-lagged HRRR guidance indicates convective initiation will likely occur between 130-230 CST.  Given the trends in the swelling cumulus field, this appears to be a plausible scenario.  Forecast soundings this afternoon show a relatively moist and increasingly unstable boundary layer beneath 500 mb temperatures in the -18 to -20 deg C range.  Upwards of 500-1000 J/kg MLCAPE is likely to develop as additional cloud breaks/heating occurs through the mid afternoon.  The belt of stronger 700 mb flow (40-50 kt) is forecast to largely remain on the southern periphery of the discussion area near the I-40 corridor.  Nonetheless, hodographs are forecast to elongate over northwest OK with a moderate enlarging of the hodograph to the immediate north of the belt of stronger 700 mb flow.


A few supercells are possible later this afternoon near the OK/TX border east-northeast into northwest OK.  It is within this focused zone where a tornado or two could develop, in addition to risk for large to very large hail and isolated severe gusts.  This activity will likely evolve into a cluster towards this evening and eventually into a linear band with severe gusts and possibly a hail risk as it moves towards the I-35 corridor.