SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0237 PM CDT Thu Sep 23 2021 Valid 232000Z - 241200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE NORTHERN MID-ATLANTIC INTO SOUTHERN NEW ENGLAND... ...SUMMARY... Isolated strong to severe storms associated with a marginal threat for a brief tornado and occasional wind damage should continue today into tonight across parts of the Mid-Atlantic and southern New England. ...20Z Update... ...Mid-Atlantic into Southern New England... Forecast reasoning from the previous outlook (appended below) remains valid. Only change to the ongoing forecast was to trim up the western edge based on the current position of the front. Otherwise, occasional strong to damaging winds should be the main threat through the rest of the day as the band of convection develops slowly eastward towards the Atlantic Coast. ..Mosier.. 09/23/2021 .PREV DISCUSSION... /ISSUED 1125 AM CDT Thu Sep 23 2021/ ...Mid-Atlantic into Southern New England... A band of showers and storms is ongoing across parts of the Mid-Atlantic along and just ahead of a surface cold front. A zone of large-scale ascent associated with low-level warm advection and modest convergence along the front will continue through tonight, while making slow eastward progress as a closed upper low moves gradually northward across the Great Lakes region into southeastern Ontario. Low-topped convection should continue to train northward this afternoon and evening within the larger belt of precipitation across the Mid-Atlantic. The environment ahead of this activity should remain marginally favorable for isolated strong to severe storms. Visible satellite imagery shows a fair amount of cloud cover ahead of the line, which should slow diurnal heating and steepening of low-level lapse rates though the afternoon. Still, a moist low-level airmass characterized by mid 60s to low 70s surface dewpoints combined with modest diabatic heating should support generally 500-1250 J/kg of MLCAPE, with greater instability across DE/NJ compared to southern NY and vicinity. The strongest and largely meridional winds are expected to remain behind the surface cold front, with only about 30-35 kt of deep-layer shear present across the warm sector. Even so, some enhancement to the low-level wind field is present ahead of the band of storms, and a brief embedded circulation capable of producing a tornado or two remains possible. Otherwise, occasional strong to damaging winds should be the main threat through the rest of the day as the band of convection develops slowly eastward towards the Atlantic Coast. This marginal severe threat may continue through tonight across parts of southern New England where sufficient boundary-layer moisture can support weak instability and the potential surface-based storms. Read more LIVE: spc.noaa.gov
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