Severe Storm Outlook Narrative (AC)
Issued by NWS
Issued by NWS
548 ACUS02 KWNS 071732 SWODY2 SPC AC 071730 Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1230 PM CDT Tue May 07 2024 Valid 081200Z - 091200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM NORTHEAST TEXAS INTO THE MID-SOUTH/MID MISSISSIPPI VALLEY AND LOWER OHIO VALLEY... ...SUMMARY... Scattered to numerous severe thunderstorms appear likely Wednesday across parts of the southern Plains into the mid Mississippi, Ohio, and Tennessee Valleys. All severe hazards, including tornadoes, very large hail, and severe/damaging winds should occur. Some of the tornadoes may be strong. ...Synopsis... An upper low/trough, initially over the northern Plains Wednesday morning, will develop east to the Upper Midwest/Great Lakes vicinity by Thursday morning. Meanwhile, a more compact upper shortwave trough over the Lower Great Lakes will shift east/northeast over the Northeastern U.S. This regime will maintain a broad swath of 50-70 kt southwesterly midlevel/upper flow from the central/southern Plains to the Midwest/Mid-South and Northeast. This broad area of enhanced flow aloft will contribute to a strongly sheared environment atop a very moist boundary-layer. While the upper low/trough will progress east through the period, overall height falls across the Midwest/OH-TN Valleys are expected to remain modest until overnight. Large-scale ascent will be somewhat modest/nebulous through the day, with mesoscale features/ascent perhaps more of a driver of storm development/evolution. At the surface, the general expectation is that a surface low over MO early in the period will develop east across the Ohio Valley vicinity. A warm front will extend east from the low across IL/IN/OH, but there is quite a bit of variability in forecast guidance regarding how far north this boundary will retreat during the day. A cold front/dryline also is forecast to extend southwest from the low across western MO/eastern OK/central TX during the afternoon. The cold front will surge east/southeast overnight across the Ozarks/Mid-South. These boundaries will collectively focus thunderstorm development through the period. ...ArkLaTex into the Mid-MS/OH/TN Valleys... A very moist and broad warm sector is anticipated, with surface dewpoints in the upper 60s to mid 70s F already in place across much of the region. Heating into the 80s F and a plume of steep midlevel lapse rates overspreading the region will aid in strong destabilization (MLCAPE 2500-4000 J/kg). With 40-50 kt effective shear magnitudes, organized convection is expected. Convective mode may be somewhat messy, with clusters/semi-discrete supercells and bowing line segments possible. Little inhibition is expected across the warm sector, so thunderstorm coverage could be quite widespread. Initial thunderstorms development is likely during the morning near the triple point in MO. Most guidance suggests this activity will quickly grow upscale into an eastward advancing line/bow across MO. More semi-discrete convection is then anticipated by midday/early afternoon in the vicinity of the warm front across southern IL/western KY, spreading eastward with time across IN/KY/southern OH and TN. Additional storms are expected by mid to late afternoon along the cold front across AR/southeast OK/northeast TX. Large to very large hail (2-3 inch diameter) will be possible with more cellular convection given very large instability, steep midlevel lapse rates and favorable vertical shear. Tornado potential is also apparent, given favorable low-level shear as is evident in enlarged, curved 0-3 km forecast hodographs. Mesovortex tornado potential also will accompany any bows/QLCS structures as well. Furthermore, potentially widespread damaging/severe winds appears possible, especially if strong enough cold pools can be generated within linear segments. Given little inhibition across the region and somewhat nebulous large-scale forcing, convective evolution is somewhat uncertain (widespread convection all at once vs. semi-discrete convection growing upscale with time, etc). A corridor of higher probability severe may develop from southeast MO/southern IL into western KY. However, confidence is too low at this time for a categorical upgrade. ...Iowa and Vicinity... Isolated strong to severe thunderstorms appear possible in associated with a cold-core upper low across parts of IA and vicinity, mainly Wednesday afternoon. Although forecast instability and deep-layer shear appear fairly weak/marginal, a few instances of hail and strong/gusty winds appear possible. A brief tornado or two may also occur along/near the occluded front. ...Southern New England into NY and the Mid-Atlantic/Carolinas... Occasional hail and strong to locally damaging winds appear possible with isolated thunderstorms across parts of NY into southern New England as a surface low and attendant cold front move eastward through the day. Farther south into the Mid-Atlantic/Carolinas, more substantial instability is forecast (up to 2500 J/kg MLCAPE). Large-scale ascent is expected to remain weak over the region. However, strong heating of a moist boundary-layer and the presence of a surface trough extending southward into the Carolinas may be sufficient for greater thunderstorm coverage than previously expected. As a result, a Slight risk has been extended eastward across parts of VA/NC. Large hail and damaging gusts will be possible with thunderstorm activity through the evening. ..Leitman.. 05/07/2024 $$