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Wednesday, September 22, 2021

SPC Sep 22, 2021 0730 UTC Day 3 Severe Thunderstorm Outlook

SPC 0730Z Day 3 Outlook Day 3 Convective Outlook NWS Storm Prediction Center Norman OK 0201 AM CDT Wed Sep 22 2021 Valid 241200Z - 251200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FRIDAY ACROSS MUCH OF NEW HAMPSHIRE...VERMONT...MASSACHUSETTS...CONNECTICUT...RHODE ISLAND...LONG ISLAND...PARTS OF FAR EASTERN NEW YORK AND FAR WESTERN MAINE... ...SUMMARY... Strong thunderstorms may impact southern and western portions of New England Friday, accompanied by at least some risk for severe weather. ...Synopsis... Mid-level ridging may flatten some across the Canadian Prairies during this period, but models indicate that broad mid-level ridging will prevail across the British Columbia and Pacific Northwest coast through the Canadian Prairies and northern U.S. Rockies. However, downstream flow across eastern North America and the Atlantic appears likely to continue to amplify. This will include digging large-scale mid-level troughing southeast of the northern Great Plains, through much of the Upper Midwest/Great Lakes and middle Mississippi through Ohio Valley region. Within the evolving large-scale troughing, it appears that a couple of initial embedded mid-level lows will pivot around each other, generally near and north of the Great Lakes vicinity. This includes one emerging from the lower Ohio Valley in preceding days, which may continue north of Lake Huron through southern Hudson Bay Friday through Friday night. This may be accompanied by continuing significant surface cyclogenesis near/northwest of James Bay, with a broad, deep, occluded cyclone across southwestern Hudson Bay by 12Z Saturday. The trailing lead cold front is forecast to slowly continue east of the Champlain/Hudson Valleys, and much of the middle/southern Atlantic coast through the central Florida Peninsula. Another reinforcing intrusion of cooler air may surge southeast of the Upper Midwest through the lower Great Lakes and middle Mississippi/lower Ohio Valleys, with yet another advancing into the Upper Midwest and adjacent Great Lakes region by late Friday night. Although mid-level cooling across parts of the Upper Midwest into lower Great Lakes and Ohio Valley may contribute to scattered convection capable of producing lightning, much of the nation, in the wake of the lead cold front, will remain generally dry. Ahead of the lead cold front, a narrow plume of higher moisture content will continue to provide a focus for highest thunderstorm probabilities Friday through Friday night. ...Parts of southern and western New England... Despite a narrow pre-frontal corridor of mid 60s to near 70F surface dew points, weak lower/mid tropospheric lapse and mid/high-level cloud cover may only allow for the development of weak CAPE by peak daytime heating. However, various forecast soundings indicate that this may continue to coincide with moderate southerly flow in the 850-700 mb layer, which could contribute to sizable, clockwise-curved low-level hodographs. While the NAM might be too aggressive concerning this, it is possible that the environment may still become conducive to organized convection, including supercells, with the potential to produce damaging wind gusts and perhaps a tornado or two. ..Kerr.. 09/22/2021 Read more LIVE: spc.noaa.gov
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