SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0740 AM CDT Sat Sep 25 2021 Valid 251300Z - 261200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Organized severe thunderstorms are not forecast for today and tonight. ...Synopsis/Discussion... In mid/upper levels, a progressive and fairly highly amplified northern stream will continue, led by a substantial field of cyclonic flow shifting eastward from the Great Lakes and Ohio Valley. A closed low was apparent in moisture-channel imagery over northwestern ON, with trough southward over eastern WI then southwestward to the Ozarks. By 00Z, the trough is progged to move to Lake Huron, eastern OH, southwestern VA, and northern GA. By 12Z, it should reach western/central NY, the Delmarva Peninsula, and the NC Outer Banks. The associated surface cold front -- analyzed at 11Z from Lake Huron to northwestern OH, western KY, northern AR, southwestern OK, and south-central NM -- should reach central portions of NY/VA/PA by 00Z. Overnight, it will catch up to a stationary to very slowly moving cold front initially drawn just east and south of Cape Cod to a low off the Outer Banks. Isolated to widely scattered thunderstorms may develop along the western front over OH and into northwestern PA this afternoon, as the eastern edge of the strongest cooling aloft preceding the mid/upper trough overlaps frontal lift and a narrow, weak plume of low-level moisture return. Gusty winds approaching severe limits are possible in the most intense cells, but the unconditional potential appears too low for a categorical outlook area. Mass response (including a strengthening warm-advection plume) to overnight cyclogenesis south of New England may result in sufficient destabilization to support nearly surface-based thunderstorms near the eastern front late tonight over the southeastern MA region. At this time, low/middle-level flow and shear appear too weak to support an organized severe threat. Isolated to widely scattered thunderstorms also are possible across parts of the Southwest, in the northern to eastern sector of a cut-off mid/upper cyclone now centered just west of northern Baja. The associated 500-mb low is forecast to meander between its current location and northwestern Baja for most of the period, with strongly difluent flow over southeastern AZ and southwestern NM. The northern rim of some 50-60-kt flow at 250 mb and 30-40-kt 500-mb winds may extend over that area this afternoon, potentially contributing to enough deep shear for some convective organization in the area from east of TUS to near DMN. Strong gusts and small hail may occur, but lack of both greater DCAPE and colder air aloft indicate that the severe threat is too conditional/isolated for a categorical outlook. ..Edwards/Jewell.. 09/25/2021 Read more LIVE: spc.noaa.gov
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