Severe Storm Outlook Narrative (AC)
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Issued by NWS
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196 ACUS01 KWNS 270600 SWODY1 SPC AC 270559 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1259 AM CDT Mon May 27 2024 Valid 271200Z - 281200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE SOUTHEAST STATES INTO THE MID-ATLANTIC AND LOWER GREAT LAKES... ...SUMMARY... Severe thunderstorms are expected this afternoon and evening across parts of the lower Great Lakes, Mid-Atlantic, and Southeast. More isolated severe thunderstorms are possible across much of central and eastern Texas, as well as southwest Oklahoma this afternoon and overnight. ...Synopsis... Strong to severe thunderstorm potential today and tonight will be focused along a composite cold frontal/outflow boundary draped from the Great Lakes region into the Mid-Atlantic/Southeast and westward into the southern Plains. 05 UTC surface observations reveal a mature surface low over the upper Great Lakes. A diffuse cold front is noted extending from the mid-MS/lower OH River Valleys (where a severe MCS is ongoing) into the southern Plains, with an additional cold frontal surge analyzed across parts of the Midwest. These features should migrate to the east/southeast over the next 12-24 hours as the synoptic surface low moves into the lower Great Lakes region by late tonight, driven primarily by large scale ascent ahead of a low-amplitude upper wave (observed over the mid-MS/lower OH River Valleys at around 05 UTC). Although only modest deepening of the surface low is anticipated over the forecast period, mid to upper-level flow will likely remain fairly strong (40-60 knots between 700 to 500 mb) ahead of the upper wave. This will support moderate to strong deep-layer shear over much of an expansive warm sector stretching from the Southern Plains into the Southeast/Mid-Atlantic and northward to the lower Great Lakes. Thunderstorms developing along the aforementioned cold front and/or outflow boundary will likely see some degree of organization with an attendant severe risk. ...Pennsylvania and western New York... Northward moisture return is ongoing across PA and NY in the wake of a decaying QLCS. Although lapse rates across the lower Great Lakes into PA will remain fairly marginal, increasing low-level moisture and diurnal heating should erode weak MLCIN by around 18 UTC. Concurrently, a weak mid-level perturbation, most likely associated with the remains of the ongoing MCS across the OH River Valley, will overspread the region around early afternoon resulting in scattered thunderstorm development. While buoyancy will be meager (500-1000 J/kg MLCAPE), 40-45 knot flow above the surface should mix down within stronger downdrafts, supporting a damaging/severe wind threat. A second round of thunderstorms is possible during the late afternoon/evening hours across western PA as a secondary cold frontal surge pushes east. The intensity of this activity will likely be dependent on the degree of overturning and air mass recovery in the wake of the early afternoon thunderstorms. However, a severe wind threat may materialize if sufficient recovery can occur. ...Mid-Atlantic... A moist air mass is currently in place across the Mid-Atlantic with upper 60s/low 70s dewpoints observed from the Carolinas into MD. 6-7 K/km lapse rates are also noted in 00 UTC soundings, which should support 2000-2500 MLCAPE by mid-afternoon across the region. The approach of the upper wave should promote troughing in the lee of the Appalachians. It remains unclear whether convection will primarily be focused along the lee trough or along a residual outflow boundary emanating from the ongoing MCS to the west. Regardless, sufficient buoyancy and deep-layer shear will support organized convection, including the potential for supercells with an attendant risk for large hail, severe wind, and a few tornadoes if sufficient low-level SRH along and ahead of the boundary can be realized, as hinted by some model solutions. ...Southeast... The ongoing MCS is forecast to gradually decay as it moves into central/southern MS/AL/GA by 12 UTC, though embedded segments within the line may pose a severe threat at the start of the forecast period. Regardless, an outflow boundary should become apparent somewhere across the Southeastern states by mid-morning, though guidance varies on the exact placement. Weak capping is noted in 00 UTC soundings from across the region, suggesting that diurnal heating, aided by weak ascent along the boundary, should foster thunderstorm development by early afternoon. MLCAPE upwards of 2000 J/kg combined with around 30 knots of deep-layer shear should allow for some initial storm organization, including the potential for a few supercells. Ambient low-level vorticity along the boundary may support a brief tornado threat with initial convection. However, mean west/southwesterly winds should yield poor boundary-normal deep-layer shear, which suggests storm interactions along the boundary may promote upscale growth by late afternoon with an increase in damaging to severe wind potential. ...Midwest... A mid-level perturbation is noted in water-vapor imagery across northern MT. This feature is forecast to reach the Midwest region by peak heating this afternoon. Cool temperatures aloft over a somewhat dry air mass should promote modest (500-1000 J/kg) MLCAPE. Deep-layer wind shear is expected to be marginal, but lift ahead of the vorticity maximum should promote isolated to scattered thunderstorm development with the potential for strong to severe winds - especially if more consolidated/organized clusters can become established. ...Texas into southwest Oklahoma... Surface observations show the cold front slowly moving south across central TX, but this boundary should become increasingly diffuse through the day amid strong diurnal heating across much of TX. Thermally-induced surface pressure falls to the west of a dryline (noted from west-central TX to the Big Bend region) will maintain east/southeasterly low-level winds through the day. This will not only maintain an influx of rich Gulf moisture (dewpoints in the mid/upper 70s), but will support hodograph elongation given 40-50 knot westerly flow aloft. Forecast soundings suggest sufficient heating will erode MLCIN by early afternoon, but weak confluence along the dryline and neutral to rising heights aloft may limit convective coverage. Consequently, confidence on storm coverage and location is low with considerable spread noted in 00z CAM solutions. However, this appears to be a low-probability, but potentially high impact scenario with the potential for significant hail (2 inches in diameter or larger), severe wind, and perhaps a tornado where storms can develop. Elevated convection appears possible well into the overnight hours across northern TX into southwest OK as isentropic ascent atop the stalled frontal boundary increases with the onset of the nocturnal low-level jet. This activity should primarily pose a severe hail/wind threat that may persist into the early morning hours Tuesday. ..Moore.. 05/27/2024 $$