SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1155 PM CDT Sat Oct 31 2020 Valid 011200Z - 021200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON AND EVENING ACROSS THE NORTH CAROLINA COAST AND OUTER BANKS VICINITY...AS WELL AS PORTIONS OF EAST CENTRAL FLORIDA... ...SUMMARY... Strong storms may impact parts of the North Carolina coast and Outer Banks vicinity, as well as parts of east central Florida, this afternoon and evening, accompanied by at least some risk for severe weather. ...Discussion... Within converging belts of westerlies emanating from the mid-latitude Pacific, broad, amplified mid/upper ridging is forecast to continue to gradually shift eastward across much of western and central North America. With some further building of this ridging possible across and inland of the Pacific coast, significant downstream troughing may also amplify further while progressing into and across the Atlantic Seaboard today through tonight. As it does, an associated 100 kt mid-level jet streak initially digging to the west of the positively tilted trough axis is forecast to shift to the east of the trough and intensify further, with the trough axis taking on more of a neutral to negative tilt across the Atlantic coast by 12Z Monday. This will be accompanied by a significant cold front, and strong surface cyclogenesis from coastal Maine toward the Canadian Maritimes late tonight into early Monday. Preceding the cold front, a cold surface high is now shifting east of the Atlantic coast. Ridging trailing to southwest lingers to the east of much of the Appalachians, but is forecast to weaken early today, with a warm frontal zone advancing inland across the southern Mid Atlantic coast region. Coinciding with low-level moisture return off the Atlantic, and increasing lift ahead of the approaching trough, models suggest that this will be accompanied by an expanding area of stratiform precipitation across the Carolina Piedmont toward the northern Mid Atlantic coast through the day. However, east of deepening lee surface troughing, a narrow plume of low-level warming, aided by daytime heating, may overspread the Carolina coastal plain, before shifting offshore during the 18-22Z time frame. At some point, this appears likely to coincide with substantive boundary-layer moistening, but it remains unclear due to model spread whether this will occur inland, near coastal areas, or mostly offshore. At the same time, after initially shifting northward into parts of northern Florida and the northern Florida Peninsula, the lead front is forecast to return southward, before being overtaken by the stronger trailing cold front by tonight, preceded by a narrow plume of seasonably moist air. ...Southern Mid Atlantic... Marginal severe probabilities will be maintained near coastal areas of North Carolina and southeastern Virginia. Uncertainty associated with continuing model spread precludes increasing severe probabilities, but destabilization depicted by the NAM and Rapid Refresh suggests potential for this across the Outer Banks vicinity of North Carolina. These models suggest that the return of mid 60s surface dew points across this region will contribute to mixed-layer CAPE up to around 1000 J/kg by 20-21Z, despite modest to weak lapse rates. This may coincide with considerable strengthening of lower/mid tropospheric wind fields (40-70+ kt) and shear (including sizable, clockwise-curved low-level hodographs), and large-scale ascent, aided by increasing divergence aloft and low-level warm advection. The environment may become conducive to the development of supercells posing a risk for severe hail, locally strong surface gusts and perhaps a tornado, before spreading offshore late this afternoon. ...East central Florida... Models suggest that locally enhanced convergence, near the intersection of the southward advancing lead front and surface troughing over the interior Peninsula, may become a focus for isolated strong storm development by early this evening. In the presence of a seasonably moist environment with CAPE in excess of 1000 J/kg, deep-layer shear on the southern-most fringe of the westerlies may become sufficient to support supercell structures. This activity could pose a risk for severe hail and wind around the Orlando area into the Space Coast, before spreading offshore later this evening. ..Kerr/Elliott.. 11/01/2020
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