2017

Mesoscale Discussion #0085


The severe weather threat for Tornado Watch 10 continues.


SUMMARY...Severe weather including a tornado threat will persist across southeast Louisiana and southern Mississippi during the overnight.  The greatest threat for tornadoes in the short term is expected to be from the vicinity of BTR and eastward from Hammond, LA to Hattiesburg, MS with the increase in supercell development.


DISCUSSION...Recent surface analyses /06-08Z/ indicated a developing mesolow, now located near BTR, with the strongest pressure falls from BTR to MCB, while this region and into southwest and south-central MS also has the coldest cloud tops and greatest lightning coverage/density.  The mesolow is expected to track to the east-northeast along a northward-developing warm front through southern MS to central AL (near or just south of a TCL-BHM line) per trends in mesoanalyses and HRRR forecasts.  Forcing for ascent associated with a progressive and compact shortwave trough moving into Louisiana at this time will sustain a low-level mass response across and downstream from WW 10.  This response is being observed per backed southerly low-level winds with an associated increase in low-level moistening and warm air advection.  These factors will further aid in the development of additional thunderstorms east of the main BTR-MCB storm complex into southeast MS and southwest AL through the overnight.  


Further strengthening of vertically veering wind profiles and increasing deep-layer ascent per an apparent coupled jet structure with the progressive trough suggest supercells should increase in coverage and intensity across WW 10.

Mesoscale Discussion #0096


The severe weather threat for Tornado Watch 16 continues.


SUMMARY...The threat for tornadoes, very large hail, and damaging wind gusts continues across much of Watch 16. Additionally, downstream watch issuance will likely be needed across northeastern LA, southeastern AR, and western MS within 2-3 hours.


DISCUSSION...Multiple supercells have formed late this afternoon across west-central portions of Watch 16, in response to low-level confluence near a surface trough/warm front and mid/upper-level ascent on the nose of an impulse over eastern TX. A surface low was analyzed near the TX/LA border at 23Z, with a warm front extending to its southeast towards New Orleans. Near this boundary, backed surface winds are enhancing storm-relative helicity sufficiently for low-level mesocyclogenesis. Indeed, a fairly prominent tornadic debris signature was noted across far northeast Marion County / far southeast Cass County in TX over the last 20-30 minutes and another recently observed over northeast Bossier Parish.


More supercells are likely to form farther south, with the potential for tornadoes continuing as they progress northeast, especially when these cells approach the aforementioned surface front. Moreover, steep mid-level lapse rates noted in 18/20Z regional soundings and ample effective shear will maintain the potential for robust organized updrafts capable of very large hail. Indeed, recent KSHV data have depicted several broad/deep ZDR column and ring structures aloft, suggesting hailstone growth should be quite efficient.

Mesoscale Discussion #0099


The severe weather threat for Tornado Watch 17, 18 continues.


SUMMARY...Severe thunderstorms will continue to develop into Watch 18 over the next hour. Across northern portions of the watch, the main severe hazard will likely transition to damaging winds (with a tornado or two possible). Farther southeast in Mississippi, a greater tornado/large-hail potential will exist, with a strong tornado possible.


DISCUSSION...In response to increasing warm advection related to the low-level jet, severe thunderstorms near the LA/AR border have begun to grow upscale into a broken band, with embedded mesocyclones still noted. This evolution should continue as the storms progress into Watch 18, resulting in damaging winds likely becoming the dominant threat across northern portions of the watch. A tornado or two will still be possible, however, considering the amplifying low-level flow and notable veering with height of 0-3-km winds.


Farther southeast over south-central Mississippi, dew points in the lower 60s will favor slightly greater surface-based buoyancy. KDGX VWP data suggest around 250-300 m2/s2 of storm-relative helicity is present for low-level mesocyclone organization. With increasing semi-discrete convection over the next several hours, the tornado potential should rise accordingly, and a strong tornado (primarily closer to the MS/AL border) cannot be ruled out.

Mesoscale Discussion #0106


Concerning...Outlook upgrade 


Valid 221134Z - 221300Z


SUMMARY...The 13Z Day-1 Convective Outlook will be upgraded to High Risk for tornadoes -- some strong and long-track -- expected later today.


DISCUSSION...Anomalously rich low-level moisture observed along the eastern/central Gulf Coast, combined with a maintained influx of steep midlevel lapse rates based on the 06Z Slidell LA sounding, and the approach of a powerful midlevel cyclone from the southern Great Plains, all suggest increasing confidence that severe thunderstorms capable of producing strong, long-track tornadoes will be possible with discrete supercell thunderstorms later today. Following coordination with the affected National Weather Service Weather Forecast Offices, an upgrade to High Risk will occur for the 13Z Day-1 Convective Outlook. Additional meteorological details will be provided within the forthcoming outlook discussion.

Mesoscale Discussion #0110


The severe weather threat for Tornado Watch 20 continues.


SUMMARY...Through 16Z, the severe risk will remain concentrated within a zone extending around 90 miles north of the FL/AL Gulf coast eastward to the Atlantic.


DISCUSSION...Through 16Z, bands and clusters of sustained storms will continue within the aforementioned corridor. An MCS/storm cluster across far south-central/southeastern GA will continue to edge toward northeast FL -- affecting areas generally north of Jacksonville. Damaging winds will be the primary concern with this more linear-oriented convection. Warm-advection-related convection with large hail and perhaps isolated wind-damage potential will continue westward through parts of the central FL Panhandle. Then, farther west, areas across the western FL Panhandle and adjacent southwest AL are being monitored for increasing potential for supercell storms to develop onshore from the Gulf and spread east-northeastward. Richer Gulf moisture will continue to be advected inland promoting increasing potential for surface-based storms, and the tornado potential will be increasing.

Mesoscale Discussion #0112


Probability of Watch Issuance...95 percent


SUMMARY...Particularly Dangerous Situation (PDS) Tornado Watch issuance is forthcoming -- around 1730Z.


DISCUSSION...A dangerous outbreak of tornadoes is expected to occur across parts of north FL, southern GA, and far southeast AL this afternoon. Air mass destabilization continues across the eastern Gulf Coast vicinity ahead of deepening surface low pressure near Mobile Bay. As the surface low undergoes rapid deepening along its east-northeastward/northeastward track, markedly strengthening low-level southerly mass fluxes will occur -- encouraging the development of moderate buoyancy south of a warm front branching east of the low.


Present indications are that the ongoing warm-advection shield of convection from southern AL to GA will continue moving northward/northeastward, with the precipitation-flanking warm front to the south advancing northward. As this occurs, surface-based discrete and semi-discrete supercells will continue to spread northeastward and east-northeastward into the warm sector -- initially developing along a surface trough/front extending south of the developing low into the Gulf. The related supercell-tornado risk will spread inland after 18Z. Very long, curved hodographs yielding 400-600 m2/s2 of effective SRH, combined with MLCAPE around  500-1250 J/kg, will support the potential for long-track, fast-moving, strong tornadoes.


The issuance of a long-lead-time Particularly Dangerous Situation Tornado Watch is planned for around 1730Z.

Mesoscale Discussion #0117


The severe weather threat for Tornado Watch 21 continues.


SUMMARY...The onset of a dangerous tornado outbreak is ongoing. A line of potentially tornadic supercells is spreading eastward around 45-50 kt. Long-track, fast-moving, strong tornadoes will be possible.


DISCUSSION...A line of discrete/semi-discrete supercells extending from far southwest GA to central/eastern portions of the FL Panhandle will continue spreading eastward around 45-50 kt. Approximate times of arrival of this line are depicted in the accompanying graphical product. Storm-relative motion from radar suggests tightening/strengthening low-level mesocyclones with this activity. This activity now lies east of a front that arcs northward to surface low pressure over central AL featuring a deepening rate of over 1 mb per hour. Preceding pressure falls of 7-9 mb per 2 hours have been noted, with open-warm-sector pressure falls of 3-6 mb per 2 hours. As a plume of high theta-e air continues to stream northward to a warm front advancing north across south GA (serving as convective inflow), and very strong low-level SRH (around 550 m2/s2 in the lowest 1 km above ground per Tallahassee VAD wind profile) continues given the strong surface cyclogenesis, the potential for significant, long-track, fast-moving tornadoes will exist.

Mesoscale Discussion #0121


The severe weather threat for Tornado Watch 21 continues.


SUMMARY...A line of potentially tornadic supercells will continue tracking eastward around 40-50 kt into the late afternoon hours. Long-track, fast-moving, strong tornadoes will be possible.


DISCUSSION...A line of discrete/semi-discrete supercells and supercell clusters extending from southern GA to eastern portions of the FL Panhandle will continue spreading eastward around 40-50 kt. Approximate times of arrival of this line are depicted in the accompanying graphical product. Very strong wind fields accompanying a rapidly deepening cyclone over east-central AL will continue to support long, curved hodographs. As southerly mass fluxes east of the cyclone continue to support the northward advance of a precipitation-enhanced boundary across southern GA -- with moderate instability south of this boundary -- the supercell-tornado risk should continue to develop northward across parts of southern GA. The potential for strong, long-track, fast-moving tornadoes will continue.

Mesoscale Discussion #0399


Probability of Watch Issuance...95 percent


SUMMARY...The threat for tornadoes is expected to increase from late morning into the afternoon over a portion of east TX. A tornado watch will likely be issued for parts of east TX into western LA by 16Z, and a PDS tornado watch is being considered. The new watch might also replace the eastern portion of tornado watch 108.


DISCUSSION...Early this morning numerous discrete to semi-discrete storms including supercells are developing within a zone of warm advection and isentropic ascent in the vicinity of a warm front located from southern LA through eastern TX. The storms north of the warm front are likely slightly elevated, while activity along and just south of this boundary are probably ingesting near-surface-based inflow parcels. 


Current expectations are for the storms well north of the warm front to weaken as they move north into the more stable boundary layer across northeast TX, but may still pose a short term large hail threat. Visible imagery indicates some diabatic warming may commence by mid morning over southeast TX, contributing to further boundary layer destabilization near the warm front. The front should move slowly north as the low-level jet strengthens across eastern TX into far western LA in response to forcing for ascent within exit region of an upper jet rotating through base of synoptic trough. Increasing 0-1 km hodograph size resulting from the strengthening LLJ and destabilization of the boundary layer should contribute to an increasing threat for supercells with strong low-level mesocyclones and tornadoes as storms continue developing in vicinity of the warm front.

Mesoscale Discussion #0402


The severe weather threat for Tornado Watch 109 continues.


SUMMARY...Through 1830Z, the greatest tornado potential will exist for locations near and surrounding the Lufkin area northeastward toward areas around Shreveport and Natchitoches.


DISCUSSION...Surface pressure falls on the cooler side of an outflow-reinforced warm front analyzed from Nacogdoches County TX to Natchitoches Parish LA are associated with open-warm-sector confluence from the Upper TX Coast northward to the warm front. Convection continues developing along related convergence bands in the weakly capped and strongly unstable warm sector, and will continue spreading toward the warm frontal zone. As this activity matures ahead of a remnant squall line crossing eastern portions of central TX, tornado potential should increase given effective SRH around 400 m2/s2 near the warm front. Significant tornadoes will be possible.

Mesoscale Discussion #0404


The severe weather threat for Tornado Watch 110 continues.


SUMMARY...The risk for significant tornadoes is increasing across Particularly Dangerous Situation Tornado Watch 110.


DISCUSSION...Echo tops in association with supercell clusters and quasi-linear convective segments containing embedded supercells have exhibited notable increasing trends per radar imagery. This activity is intensifying within a warm advection plume preceding a midlevel shortwave trough spreading across Texas. Pressure falls have been increasing in magnitude within the open warm sector from parts of the lower Sabine Valley eastward through central LA, south of a warm front analyzed from south of Shreveport to north of Alexandria LA. This suggests that increasing deep ascent will continue to encourage convective sustenance/intensification. The VAD wind profile at POE indicates a substantially curved hodograph in the lowest 3 km above ground, with around 400 m2/s2 of 0-1-km SRH. Rotating updrafts will remain likely as convection spreads eastward/east-northeastward across the region. The risk for significant tornadoes is increasing, along with damaging winds and severe hail.

Mesoscale Discussion #0406


The severe weather threat for Tornado Watch 110 continues.


SUMMARY...Threat for a few strong tornadoes will persist next couple hours with discrete storms developing through central LA. In addition, the squall line may pose a risk for a few QLCS tornadoes as well as significant wind damage and hail as it advances east through LA.


DISCUSSION...Discrete supercells continue to mature as they develop north and interact with warm front across central LA where VWP data show large low-level hodographs with 0-1 km storm relative helicity from 300-400 m2/s2. Tornadoes, a few of which will be strong, will remain likely with these storms this afternoon. Farther west a squall line is advancing east at around 40 kt. Organized structures continue to be observed within the line including bowing segments, embedded supercells and mesovortices. In addition to damaging wind, QLCS tornadoes are likely as the squall line continues east and especially where the line intersects the warm front across central LA.

Mesoscale Discussion #0440


Concerning...Outlook upgrade 


Valid 051443Z - 051645Z


SUMMARY...Portions of the states of AL/GA/SC are being considered for an Outlook upgrade -- though the details are still being analyzed.  A very notable dry bias is apparent in the latest model guidance, and observational data and previous model runs are being heavily referenced in Outlook preparation.


DISCUSSION...GPS PW data are exhibiting a low bias by at least 0.5 inch from parts of AL into GA, which has influenced initialization of the latest numerical weather prediction guidance.  Per coordination with the Weather Prediction Center, it appears that related initialization is causing substantial dry biases in model guidance -- extended through the Day 1 forecast period.  With an inland influx of rich moisture feeding ongoing intense supercells in southern AL (with dewpoints into the middle 70s), and the around-16.6-g/kg mean mixing ratio per LIX 12Z sounding, and around 1.5-inch PW per Atlanta and Birmingham 12Z soundings, it is evident that rich moisture and related strong buoyancy does indeed extend inland. As such, observational data and previous model runs will play a major role in the consideration for a High Risk upgrade, as the dry bias has influenced the latest model guidance. While an Outlook upgrade is possible, it is not certain.

Mesoscale Discussion #0441


Probability of Watch Issuance...95 percent


SUMMARY...Tornado Watch will be issued by 17z to address increasing severe threat from the eastern FL Panhandle into SC.


DISCUSSION...Boundary-layer cu field is beginning to deepen from the eastern FL Panhandle into southeast GA as surface temperatures rise to near 80F.  Latest high-res Vis imagery depicts clustering/towering CU along a corridor from near MAI to near VDI. Substantial breaks in cloud cover suggest additional heating can be expected and thunderstorms will likely evolve over the next few hours.  Given the strength of the wind fields and rising instability there is increasing concern for discrete supercell formation by early afternoon.  Environmental parameters appear favorable for a few strong tornadoes across this region, in addition to large hail and damaging winds.  Tornado watch will be issued for parts of this area by 17z.

Mesoscale Discussion #0448


The severe weather threat for Tornado Watch 127 continues.


SUMMARY...The severe-thunderstorm and tornado risks continue across remaining-valid portions Tornado Watch 127.


DISCUSSION...Scattered thunderstorms continue developing along a north-south corridor from west-central KY into central AL. This convection is already exhibiting supercellular characteristics, and will continue tracking eastward and northeastward during the next several hours, capable of producing very large hail, damaging winds, and tornadoes.


Some strong-tornado potential during the next few hours will extend from parts of central AL to middle TN and points eastward. This is where the influx of richest boundary-layer moisture exists (surface dewpoints into the lower/middle 60s) supporting around 1500-2500 J/kg per modifications to the 17Z Birmingham sounding -- amid effective SRH increasing to 200-400 m2/s2. This activity could eventually reach into parts of western GA where air-mass recovery continues.

Mesoscale Discussion #0736


The severe weather threat for Tornado Watch 220 continues.


SUMMARY...The risk for very large to giant hail (2-3.5 inches in diameter) and tornadoes will increase nonlinearly (22-00Z).  A large/intense (EF3+ capable) tornado is possible (23-02Z) given the strengthening low-level shear, very large buoyancy, and the mature phase of a quasi-discrete supercell.


DISCUSSION...Radar imagery shows a north-south cluster of supercells over the eastern TX Panhandle which have moved east of the dryline.  OK and West TX mesonet observations show dewpoints in the 66-69 degrees F range and temperatures in the middle 70s to around 80.  Very strong backing of low-level flow, perhaps partially induced by parent storms to the west, has enlarged the low-level hodograph size during the past 30 minutes from Childress County to Wheeler County with 2-4 mb/hour falls in the past 2 hours.  RAP forecast soundings show 0-1 km SRH increasing from 150 m2/s2 to 350 m2/s2 (locally higher and augmented by storms) during the 22-00Z period.  As the low-level shear continues to increase over the next couple of hours, expect with medium to higher confidence of a large/intense tornado ---potentially capable of EF3+ intensity--- evolving from supercell cluster in Wheeler/Gray/Collingsworth counties.

Mesoscale Discussion #0739


The severe weather threat for Tornado Watch 222 continues.


SUMMARY...A considerable tornado threat continues across the discussion area, including significant tornado potential with a cell currently over Rusk County, WI.


DISCUSSION...A cluster of intense thunderstorms with a history of strong rotation, significant hail, and tornadoes, has evolved over the past couple of hours in northwestern and north-central Wisconsin.  Additional activity has matured west of this cluster from near DLH southward to about 25 SSE of MSP.  These storms are moving into a favorable environment for continued significant severe potential (including tornadoes) over the next 1-2 hours, where recent objective analyses indicate 2000+ J/kg MUCAPE and 300-400 m2/s2 effective SRH downstream of this complex of storms within an undisturbed preconvective airmass.  Based on the evolving severe threat, a new tornado watch and/or spatial extensions to the existing watch may be needed around/just after 00Z.

Mesoscale Discussion #0756


Probability of Watch Issuance...95 percent


SUMMARY...Severe thunderstorms will develop across portions of western Oklahoma this afternoon. Tornadoes, a few of which may be strong, very large hail, and damaging winds will be possible. A PDS (Particularly Dangerous Situation) tornado watch will be issued within the next hour.


DISCUSSION...As mid-level forcing for ascent gradually increases this afternoon, thunderstorms will increase in coverage across much of the region over the next 1-3 hours. Surface observations indicate dew points have risen into the mid/upper 60s over much of the discussion area. Wave/billow structures on visible satellite suggest convective inhibition remains over parts of Oklahoma and Kansas this afternoon, but continued boundary-layer heating/moistening and cooling aloft are forecast to remove any remaining inhibition. In turn, as the dry line focuses near the Oklahoma/Texas border, thunderstorms will develop and likely become severe quite quickly. Moreover, open warm sector development is also possible across portions of southwest Oklahoma, within zones of confluence and differential heating.


With the presence of moderate/strong buoyancy (around 2500-3500 J/kg of MLCAPE) and effective shear upwards of 45-50 kt, initial supercellular modes appear probable. Some forecast soundings indicate a deep-layer wind profile marginally supportive of a few left splits with initial warm sector development east of the dry line this afternoon. Cell mergers/interference would preclude a higher-end severe threat. However, as low-level hodographs enlargen (with greater clockwise looping) late this afternoon into this evening, cyclonically rotating supercells may become favored, with a greater eastward component of movement. Any such discrete cell would possess a threat for tornadoes (which could be strong), very large hail, and damaging winds.


Farther west along the dry line, severe thunderstorms will likely organize near the Oklahoma/Texas Panhandle border. Some potential exists for northward-moving cells (from initial convection to the south) to interfere with these cells, as well as cause convective overturning, which would limit the severe threat some. However, the genesis region of the southern cells, as well as their expected motion, will likely keep them east of cells generating along the dry line. In turn, a relatively pristine air mass should exist for supercellular development by mid/late afternoon. Backed surface flow and increasing boundary-layer moisture ahead of the dry line will favor a threat of tornadoes, a few of which could be strong. Very large hail and damaging winds will also be possible. A PDS (Particularly Dangerous Situation) tornado watch will be issued within the next hour.

Mesoscale Discussion #0758


Probability of Watch Issuance...95 percent


SUMMARY...Severe thunderstorms will develop across parts of southern and central Kansas this afternoon and evening, with an attendant threat of tornadoes (some of which may be strong), very large hail, and damaging winds. A PDS Tornado Watch will likely be issued within the next hour or two.


DISCUSSION...Surface analyses indicate a warm front is slowly advancing northward across southern Kansas this afternoon. To the south of this front, ample boundary-layer heating has resulted in considerable mixed-layer buoyancy, with CAPE values upwards of 2500-3000 J/kg. As forcing for ascent continues to increase over the region, low-level moisture will continue to stream north/northwest, promoting a further increase in buoyancy over southern Kansas. With 850-700mb southerly flow strengthening during the late afternoon/evening, warm advection near/south of the warm front will lead to a blossoming of convection over the region. Indeed, current visible satellite data show a developing cumulus field beneath a higher-level canopy. Severe thunderstorms will likely develop out of this cumulus field as it continues to advance north/northwest ahead of the dry line and south of the warm front.


Cells near the warm front will interact with more backed low-level flow, enhancing storm-relative helicity. In turn, with continued north/northwestward transport of rich boundary-layer moisture, the potential exists for a few discrete cells with organized/strong low-level mesocyclones. A tornado threat would likely evolve, with the potential for a few stronger tornadoes, considering the ample low-level helicity. Steep mid-level lapse rates and considerable effective shear will encourage a threat of very large hail. Moreover, somewhat straight mid-level hodographs could yield upscale growth through the evening. While this would reduce the tornado threat some, merging cold pools (aided by dry air aloft) would offer the potential for a few significant severe gusts. In turn, with the potential for all higher-end severe hazards, a PDS Tornado Watch will likely be issued within the next hour or two.

Mesoscale Discussion #0760


The severe weather threat for Tornado Watch 235 continues.


SUMMARY...Potential for tornadoes likely maximized over northwest Texas and southwest Oklahoma over the next couple of hours.


DISCUSSION...KFDR radar data indicate several strong supercells across the discussion area this afternoon, with at least two tornadoes already reported over southwest Oklahoma. Ahead of these cells, surface observations indicate pressure falls around 1-1.5 mb/hour, with a notable backing of low-level flow. In turn, any semi-discrete convection with a deviant eastward component of motion will exist within an environment characterized by strong low-level storm-relative flow. Radar data confirm such a regime, as ZDR arc signatures continue to be noted on these supercells, indicating vigorous storm-relative flow veering with height. Therefore, cyclic low-level mesocylogenesis, with occasional tornadogenesis, will likely continue with these cells as they spread northeast. Very large hail and damaging winds will also remain a threat.

Mesoscale Discussion #0765


The severe weather threat for Tornado Watch 235, 239 continues.


SUMMARY...A heightened tornado risk may evolve northwest of the OKC metro---dependent on slightly less supportive storm-scale interactions resolving in the near term.


DISCUSSION...Radar mosaic shows as cluster of tornadic supercells over northwest OK and supercells with a history of tornadoes over central OK near the I-40 corridor in west-central OK.  The near-storm environment is forecast to become more favorable for strong tornadoes over the next 2 hours as backed east-southeasterly low level flow strengthens in the inflow regions of the established supercells.  Forecast soundings show 0-1 km SRH over central and north-central OK in the 50-75 m2/s2 range (NAM/RAP).  This is in contrast to observed KTLX VAD data showing around 150 m2/s2 0-1 km SRH with weaker SRH near KVNX.  The short-term models indicate SRH will increase and the expectation is for 0-1 km SRH to increase into the 150-300 m2/s2 range.  This will likely prove favorable for intensifying low-level mesocyclones/increased tornado potential.  As storms move farther east, a reservoir of richer low-level moisture is located over north-central OK (likely a partial function of evapotranspiration due to excessive rains earlier this month).  As storms encounter the slightly higher moisture (into the upper 60s degrees F) and strengthening low-level shear, an intense longer track tornado may develop over central/north-central OK this evening.

Mesoscale Discussion #1001


The severe weather threat for Tornado Watch 317 continues.


SUMMARY...All severe hazards, including strong tornadoes, large to giant hail, and damaging winds will continue with multiple supercells across Particularly Dangerous Situation Tornado Watch 317. A downstream watch into western/central NE and southern SD will be needed within the next several hours.


DISCUSSION...At least 3 supercells are ongoing as of 23Z across far southeastern WY/northeastern CO into far western NE. With a very favorable thermodynamic and kinematic environment characterized by MLCAPE of 2000-3500 J/kg and effective bulk shear of 50-60 kt, these supercells have produced over a dozen reports of significant hail (2 inch in diameter or greater) over the past few hours, with several reports of hail greater than 4 inches. At least 2 tornadoes have also occurred. The airmass downstream of ongoing convection will easily support continued significant severe hail, along with a risk for tornadoes and damaging winds through this evening. Strong tornadoes will continue to be a possibility with any discrete supercell across far southeastern WY, northeastern CO, and western NE for the next few hours given a strongly veering and strengthening wind profile with height. Of note, 0-1 km SRH is around 300 m2/s2 per recent VWP from KCYS, and 0-3 SRH is around 500 m2/s2.


A southerly low-level jet will strengthen to around 40-50 kt across western/central NE into southern SD this evening/tonight, and all severe hazards will remain possible into this evening across these areas. Evolution from discrete supercells into one or more bowing line segments appears likely to occur at some point later this evening in tandem with the increasing low-level jet, which would likely increase the threat for damaging winds with eastward extent. In addition, large hail and a few tornadoes should continue to be a threat with any supercell embedded within the line(s). A downstream watch in western/central NE and southern SD will be needed within the next several hours.

Mesoscale Discussion #1004


The severe weather threat for Tornado Watch 317, 320 continues.


SUMMARY...A threat for a few tornadoes and scattered large hail/damaging winds continues across valid portions of WW 317/320. Parts of WW 317 have been locally extended in time until 03Z based on latest radar trends.


DISCUSSION...Two semi-discrete supercells continue across western NE as of 02Z. The southernmost storm has had multiple tornado and large hail reports associated with it over the past several hours. A strongly unstable airmass into central NE/southern SD, where MLCAPE ranges from 2500-3500 J/kg per 01Z mesoanalysis, will very likely support continued severe-caliber storms into the late evening. Strong effective bulk shear values of 50-60+ kt owing to a northeastward- moving upper low over the northern Great Basin will also contribute to robust updraft organization, with all severe hazards remaining possible. A short-term (next hour or two) tornado threat should be maximized across parts of western NE into far southwestern SD with the previously mentioned semi-discrete supercells. Across this area, strongly veering/strengthening low-level winds are contributing to a relative maximum in effective SRH values (300-500+ m2/s2).


Additional convection has recently formed along northern portions of the Laramie Mountains in east-central WY, and this activity may also pose some severe risk, although instability is weaker with northward extent. Current expectations are still for ongoing storms to grow upscale into one or more bowing line segments as they move into north-central NE and western/central SD later this evening. If this were to occur, then damaging winds would likely become the primary severe threat.