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Tuesday, September 28, 2021

SPC Sep 28, 2021 0600 UTC Day 1 Convective Outlook

SPC 1200Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1257 AM CDT Tue Sep 28 2021 Valid 281200Z - 291200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE MID-ATLANTIC REGION... ...SUMMARY... A few strong to severe thunderstorms capable of producing locally damaging gusts and hail are possible this afternoon and evening across parts of the Middle Atlantic vicinity, and across portions of the southern Plains. ...Synopsis... A weak upper low centered over the Four Corners states will gradually become absorbed within broader cyclonic flow, as a trough shifts across the western U.S. through the period. On the southeastern fringe of the diminishing low, a weak trough is forecast to shift into southern Plains during the afternoon and early evening. In the East, gradual amplification of a slow-moving trough is anticipated. This will occur as a lead short-wave feature shifts out of the central Appalachians and across the Mid-Atlantic region early in the period, followed by a mid- and upper-level jet streak that will dig southward out of Canada across the Great Lakes region through the end of the period. At the surface, a cold front will shift across the Intermountain West, while a lee trough persists across the High Plains. In the East, a cold front is forecast to shift off the New England Coast during the day, while sagging southward across the Mid-Atlantic region during the afternoon and evening. ...Mid-Atlantic Region... Scattered showers and thunderstorms should be spreading across the central Appalachians vicinity at the start of the period, ahead of a weak/fast-moving mid-level short-wave trough. This convection -- and associated cloud cover -- casts some uncertainty over northern portions of the risk area with respect to subsequent heating/destabilization. In general however, areas from northern Virginia eastward across the Delmarva Peninsula to southern New Jersey should experience ample pre-frontal heating such that -- given rather steep mid-level lapse rates being advected eastward and ample low-level moisture -- a zone of 1000 to 2000 J/kg CAPE is expected by late afternoon. As the cold front sags southward to the Mason-Dixon line during the afternoon, and a weak lee trough evolves over northern/western Virginia, isolated thunderstorm development is expected. Coverage of convection should remain somewhat limited, in the wake of stronger ascent associated with the earlier passage of short-wave troughing. Any storms which do develop, however, will be capable of producing locally damaging wind gusts -- aided by moderate west-northwesterly flow through the lower and middle troposphere. Steep mid-level lapse rates may also support hail potential with a few stronger updrafts. Risk should continue into the early evening as the front sags southward, diminishing thereafter as much of the convection shifts offshore in tandem with the slow eastward drift of the main upper trough. ...Portions of Texas and adjacent southwestern Oklahoma... Daytime heating of a modestly moist (mainly low 60s dewpoints) boundary layer ahead of an evolving dryline, combined with relatively steep mid-level lapse rates, will support development of 1000 to 2000 J/kg CAPE. As weak short-wave troughing shifts eastward out of New Mexico, scattered afternoon storm development is expected -- initially across the Pecos Valley/Big Bend region, and then expanding northward to southwestern Oklahoma. While mid-level west-southwesterlies averaging 20 to 25 kt should limit storm organization/severe potential in most areas, a rather deep mixed layer will offer potential for sub-cloud evaporation/downdraft enhancement. This suggests potential for locally damaging gusts with a few of the stronger storms, especially where some clustering/cold-pool organization can occur. While a more concentrated corridor or two of wind risk may evolve, at this time the locations of any such areas remain difficult to discern, amidst broad ascent. Therefore, will maintain only 5%/MRGL risk in this forecast update. ..Goss.. 09/28/2021 Read more LIVE: spc.noaa.gov
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