SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0739 AM CDT Thu Oct 29 2020 Valid 291300Z - 301200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM A SMALL PART OF THE FLORIDA PANHANDLE INTO NORTH CAROLINA AND SOUTHERN VIRGINIA... ...SUMMARY... Isolated severe gusts and a tornado or two are possible today from a small part of the Florida Panhandle into North Carolina and southern Virginia. ...Synopsis... In mid/upper levels, a series of northern-stream shortwaves will traverse a more broadly cyclonic flow field over the Upper Midwest and Great Lakes, leading to height falls that will shift eastward across the mid/upper Mississippi then Ohio Valleys. That will contribute to the eastward acceleration and diminution of the initially well-defined, cold-core cyclone centered over the southeastern OK/northeast TX region. By 00Z, the associated 500-mb low should reach the HSV-BNA corridor, with the trough south- southwestward over western AL and southeastern MS. By 12Z tomorrow, the resultant shrunken, open-wave trough should be moving rapidly offshore from the Tidewater NC/VA region. In response to these developments, what now is Tropical Storm Zeta will continue to weaken and race northeastward per the NHC forecast, while entrained in fast, somewhat confluent mid/upper flow on the synoptic scale. The associated surface low -- analyzed on the 11Z chart over extreme southeastern TN -- should race offshore of NJ/Delmarva by 00Z, attaching itself to a frontal zone that will extend southwestward across VA, the Carolinas, GA, and the FL Panhandle. ...Southeast... Fast-moving convection trailing southeast and south of the remnants of Zeta will offer the potential for a tornado or two, as well as damaging to locally/isolated severe gusts, with the threat shifting northeastward over the outlook area from now through afternoon. Relatively backed surface winds and enlarged low-level hodographs will shift northeastward out of the northeastern Gulf coastal region this morning, and over a modestly but favorably destabilizing warm sector in the Carolinas and southern VA. As this occurs, the remaining tornado potential will translate likewise. As Zeta's surface low accelerates across the southern Appalachians, VA and the Chesapeake Bay region through the day, flow to its south in the most-favorable warm sector air will veer, reducing low-level hodograph curvature and rendering low/midlevel wind vectors more parallel. However, diabatic destabilization will remove MLCINH and increase boundary-layer lapse rates and buoyancy over parts of the Carolinas and southern VA, amidst still-strong speed shear. The strengthening instability may be a counterbalancing factor to the weakening flow for a few hours. The tornado threat still appears too conditional and widely spaced for greater than 2% lines. This is related to concerns over weakening of deep-layer lift with time, and related doubts on attainment of enough of the favorable convective modes, despite still-large shear values this morning into early afternoon. Cells embedded in the still-fast low/middle winds also may transfer some momentum down in the form of damaging downdrafts. Severe potential is expected to end by this evening as the low shifts offshore, low-level flow veers further, and already modest low-level instability diminishes. ..Edwards/Mosier.. 10/29/2020
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