Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0100 AM CDT Fri Apr 21 2017 Valid 211200Z - 221200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF SOUTHERN OK AND NORTH TX... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE SOUTHERN PLAINS INTO THE OZARKS... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE SOUTHERN PLAINS TO THE TN VALLEY AND MID-ATLANTIC... ...SUMMARY... Several clusters of severe thunderstorms may evolve today and tonight across parts of the southern Plains into the Ozark Plateau, posing a threat for large hail, damaging winds, and perhaps a couple tornadoes. Isolated severe thunderstorms may also impact portions of the Mid-South into the Mid-Atlantic with a threat for hail and gusty winds. ...Synopsis... A stacked cyclone over southern Ontario/Quebec this morning will move only slowly northeastward across Quebec through this evening. A separate trough over the Great Basin/Southwest with attendant mid/upper-level jet streak rounding its base will develop eastward across the southern Plains today, eventually reaching the mid MS Valley late tonight. A southwesterly low-level jet will be present across parts of west/north TX into southern OK at the beginning of the period. This low-level jet is forecast to weaken some through the afternoon across east TX/southeastern OK into the Ozarks before restrengthening by early evening across the same general area. At the surface, an occluded/cold front will extend southward from a low over southern Ontario/Quebec into NY/PA and the Mid-Atlantic, continuing southwestward across the central Appalachians into the TN Valley/Mid-South and the southern Plains. The front should make some slow eastward/southeastward progress through the day from the Mid-Atlantic to Mid-South, while remaining nearly stationary across most of the southern Plains through at least mid afternoon. A secondary weak surface low will likely be present across the Big Country region of TX in the afternoon, with a dryline extending southward from this low across parts of west TX. Glancing large-scale ascent associated with the Great Basin/Southwest upper trough may encourage a northeastward development of this weak low along the cold front into northeast TX by late this evening. The cold front will likely accelerate southward across the southern Plains this evening into tonight behind the surface low. ...Southern Plains into the Ozarks... Substantial uncertainty exists regarding early day convection, its evolution through the morning, and possible redevelopment and re-destabilization along and south of the stalled cold front by afternoon. One or more clusters of elevated thunderstorms will likely be ongoing at the start of the period across parts of the TX Panhandle and much of OK. This activity should spread eastward toward the Ozarks by late morning, and may pose a continued threat mainly for isolated large hail. This morning convection will act to reinforce the front across southern OK/north TX, and little to no northward return of the warm sector is anticipated due to large-scale forcing associated with the mid- to upper-level jet/trough largely remaining to the north of the front. The early day/morning convection may eventually pose a strong to damaging wind risk with eastward extent into the Ozarks and towards the Mid-South by afternoon as some diurnal heating allows for the atmosphere to destabilize along/south of the front. Scattered development along and south of the cold front and east of the dryline in southern OK/north TX may occur in the afternoon and early evening if prior convection has not greatly overturned the boundary layer. Forecast wind profiles would support supercells, with a threat for large hail and damaging winds. In addition, a couple tornadoes may be possible in the early evening along the front/dryline intersection, and perhaps along any outflow boundaries from prior convection where surface winds are locally backed. This convection may grow upscale into a cluster, with perhaps a greater damaging-wind threat through mid evening. ...Mid-South to the Mid Atlantic... Isolated to scattered thunderstorms should develop along and ahead of a cold front by early afternoon from the Mid-South into the TN Valley and extending eastward to the Mid-Atlantic. Most of this region will remain displaced to the south/east from stronger flow associated with the previously mentioned mid/upper-level troughs. But, around 30-40 kt of mid-level flow should contribute to enough effective bulk shear to support mainly multicells, and perhaps a few marginal supercells. Although mid-level lapse rates should remain poor, diurnal heating south of the front and low-level moisture characterized by upper 50s to mid 60s surface dewpoints will allow weak instability to develop by this afternoon, with MLCAPE generally 500-1000 J/kg probable. An isolated hail/wind risk is apparent along the length of the cold front with afternoon/early evening convection. The lack of stronger mid-level flow should act to limit a more organized severe weather threat. ..Gleason/Cook.. 04/21/2017Read more
from SPC Forecast Products http://ift.tt/s7Jkvo
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