SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0650 AM CST Sun Nov 15 2020 Valid 151300Z - 161200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE NORTH-CENTRAL APPALACHIANS AND MID-ATLANTIC... ...SUMMARY... Thunderstorms with damaging to marginally severe wind gusts will be possible this afternoon and evening across parts of the north- central Appalachians and Mid-Atlantic. ...Synopsis... The main mid/upper-level feature for this forecast is a strong trough, now apparent in moisture-channel imagery over the mid/upper Mississippi Valley region. This perturbation will move quickly east-northeastward, reaching the lower Great Lakes, upper Ohio Valley and eastern KY by 00Z. By 12Z tomorrow, the trough should reach New England and the coastal Mid-Atlantic region. The 11Z surface chart showed a strong/986-mb low over the central U.P. of MI, with trailing occluded/cold front arching across central/southern Lake Michigan, extreme western portions of KY/TN, southeastern AR, northwestern LA, and south-central TX to the Big Bend region and northern Chihuahua. The surface cyclone will eject northeastward across the James Bay region to northern QC by the end of the period. By 00Z the front should reach eastern portions of NY/PA/VA, central NC, southwestern GA, and the western/central FL Panhandle. By 12Z the front should be offshore all the Atlantic Coast north of central FL. ...Mid-Atlantic region... A relatively shallow, strongly forced band of convection is expected to develop close to the front this afternoon, near the western rim of the outlook area. This low-CAPE/high-shear regime -- much of which may produce no thunder given the truncation of buoyancy below ideal thermal layer for lightning generation -- should sweep eastward toward the Mid-Atlantic coast and southwestern New England, offering strong, locally damaging gusts. Isolated severe/50+ kt gusts are possible as well. This should occur as the zone of strong frontal forcing and cooling aloft (related to the mid/upper trough) impinges on a marginally modified Atlantic-origin boundary layer. Surface dew points should increase to the mid 40s to low 50s north, and low 60s near the southern edge of the outlook. Because of the incomplete airmass modification, and presence of cloud cover, low-level theta-e and lapse rates each will be modest, barely supporting surface-based convection. Forecast soundings generally depict MLCAPE in the 50-250 J/kg range, dropping to near zero north of the outlook area where elevated MUCAPE of similar values is possible. Still, strong large-scale and frontal forcings are expected across the current outlook area (each weakening to the south). Severe deep-tropospheric gradient winds should extend down to levels not far above surface (e.g., 50-70 kt common in the 700-900-mb layer). As such, downdraft momentum transfer is possible, with surface gusts potentially reaching severe limits on at least an isolated basis. For this outlook cycle, the "marginal" area has been expanded northward. This intense wind field conditionally may even support a rogue strong/damaging gust even from elevated cells farther north, penetrating a stable boundary layer. Evidence of any greater degree of destabilization and/or moistening than currently progged may prompt a subset area of larger unconditional probabilities in succeeding outlooks. ..Edwards/Leitman.. 11/15/2020
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