SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1234 AM CDT Wed Aug 18 2021 Valid 181200Z - 191200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF THE MID-ATLANTIC... ...SUMMARY... A few tornadoes will be possible today across portions of the central Appalachians and Mid-Atlantic. Thunderstorms associated with strong winds and hail may also occur across parts of the Intermountain West, and over portions of the Mid-South/Tennessee Valley. ...Mid-Atlantic... The remnants of Tropical Depression Fred are forecast to move north across the central Appalachians and Mid-Atlantic, with the center of circulation located over West Virginia by the start of the period. Deep tropical moisture will advect northward with low to mid 70s surface dewpoints expected by early afternoon. Some cloud breaks and modest surface heating should be more than enough to support MLCAPE of 500-1000 J/kg through the day. High-resolution and regional guidance are in good agreement that surface wind fields in the northeastern quadrant of the remnant low will remain strong through much of the day. A 40 kt mid-level speed max should also enhance low- and mid-level shear across much of PA, MD, and northern VA, where model soundings show 0-1 km SRH of 200-300 m2/s2. The combination of sufficient buoyancy and strong low-level shear should support multiple supercells with the potential for a few tornadoes and isolated strong wind gusts through the afternoon/early evening. ...Colorado/Utah/Wyoming... Ahead of a deepening upper trough and 50-60 kt mid-level speed max, 12 hour height falls of 60-90 dm are expected across central and eastern Utah. Relatively moist surface conditions with dewpoints in the low to mid 50s will support weak to moderate destabilization by mid morning ahead of a cold front moving east. Model soundings show nearly unidirectional deep-layer shear profiles (25-35 kt), supportive of multicell clusters and perhaps a few supercells from eastern UT, into western CO and southern WY. Isolated damaging wind gusts appear possible with the strongest storms given dry sub-cloud layers and increasing mid-level and surface flow. Some hail risk is also expected with cooler mid-level temperatures (-10 to -12 C) associated with the upper low. Convective debris from early-morning convection may complicate the scenario somewhat, as cloud cover and precipitation could limit diurnal heating. However, given the favorable kinematics and low-level moisture in place, a corridor of higher severe risk may develop if stronger surface heating is realized. ...Mid-South/Tennessee Valley... On the fringes of stronger mid/upper-level flow over the lower OH Valley, scattered thunderstorm development is expected by early afternoon across portions of eastern AR and western TN. Deep moisture with dewpoints in the low to mid 70s will support moderate destabilization (1500-2000 J/kg of MLCAPE) despite poor mid-level lapse rates (less than 7 C/km). A loosely organized cluster may develop and drift east-northeastward this afternoon across the TN Valley with a risk for isolated damaging wind gusts. ..Gleason/Lyons.. 08/18/2021 Read more LIVE: spc.noaa.gov
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