SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1158 PM CDT Tue Jul 20 2021 Valid 211200Z - 221200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE DELMARVA ACROSS NEW JERSEY...LONG ISLAND...AND INTO SOUTHERN NEW ENGLAND... ...SUMMARY... Scattered strong to severe thunderstorms are possible this afternoon from the Delmarva into southern New England. Hail and damaging winds will be possible. Isolated strong storms are also possible over western Montana. ...Synopsis... An upper trough will move across the Northeast today, with 35 kt midlevel flow, increasing with height to 65 kt at 300 mb, and cool temperatures aloft. A surface trough will extend from coastal ME southwestward into eastern VA, with lift along the front focused close to the Atlantic Coast. A moist and unstable air mass will exist near the front, with several severe storms expected by early afternoon. To the west, an upper high will remain over the Four Corners states, with ridge extending north into SK and MB. Sufficient moisture will remain beneath the anticyclone for scattered daytime storms, with isolated strong storms possible. Farther north into MT, southwest flow aloft will increase late in the day as an upper low moves into BC, with height falls extending south across WA, northern ID and western MT. The increased flow along with steep lapse rates will support a few strong storms forming off the high terrain during the afternoon. ...Delmarva into southern New England... Minimal CIN will quickly erode by late morning with heating as a front pushes to a southern New England to Delmarva line by 18Z. Mid to upper 60s F dewpoints will exist near the front, beneath cooling 500 mb temperatures of -10 to -12 C. Ample heating will thus lead to moderate destabilization with MLCAPE to 2000 J/kg. There is some uncertainty regarding how far inland storms will initiate, especially from northeast PA into southern NY, with some models suggesting development very close to the coast. The greatest threat area at this time appears to be from eastern MA across Long Island, NJ, and into DE, developing between 17Z and 19Z. Other storms may form farther west across central and southeast PA into MD, where low-level flow will be veered but lapse rates steepest. Forecast hodographs are almost entirely straight, with weak winds below 850 mb resulting in very little environmental SRH. However, deep-layer shear of 30-35 kt may favor a mixture of cells or clusters, capable of brief hail and strong to severe gusts. Ample moisture and steep lapse rates for this time of year may yield rapid storm intensification. ...Western MT... Strong heating ahead of the upper trough will erode CIN be early afternoon, with scattered convection forming over the higher terrain of southwest MT and possibly into ID. MUCAPE of 500-1000 J/kg along with increasing deep-layer shear to 40 kt will favor small cells capable of hail or localized severe gusts, possibly into early evening. ..Jewell/Squitieri.. 07/21/2021 Read more LIVE: spc.noaa.gov
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