SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0242 PM CDT Tue Sep 29 2020 Valid 292000Z - 301200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM EASTERN VIRGINIA TO NORTHERN FLORIDA... ...SUMMARY... Isolated damaging winds will be possible through this evening from north Florida across the Carolinas to eastern Virginia. A brief tornado is also possible, mainly centered on eastern North Carolina tonight. ...20z Update... Only minor changes have been made to the ongoing outlook, mainly to adjust/trim the 10% General Thunder line from NY southward through FL based on the current position of the cold front. Elsewhere, 10% General Thunder was also removed from parts of the Upper Midwest and Great Lakes based on latest observational trends and calibrated HREF guidance. While a thunderstorm or two cannot be completely ruled out in these areas, coverage of any lighting flashes should be minimal/sub-10%. ...Southeast and Mid-Atlantic vicinity... The Marginal risk area has been trimmed on the western flank in NC/SC/GA as convection continues to shift east/northeast along/just ahead of the cold front. Otherwise, expect a few storms capable of gusty/locally strong winds will continue into the early evening period. For more details, reference MCD 1723. ..Leitman.. 09/29/2020 .PREV DISCUSSION... /ISSUED 1111 AM CDT Tue Sep 29 2020/ ...Delmarva to north FL... Within an amplified large-scale trough from the Great Lakes to the central Gulf Coast, a primary embedded impulse near the IL/IN border will eject east to the northern Appalachians through early Wednesday. Cyclogenesis will occur mainly tonight with the primary surface cyclone ejecting north from the DE Valley towards VT. A secondary lee cyclone over western NC should move towards the VA Tidewater, with an attendant cold front to its south that will progress east and reach the south Atlantic Coast by evening. Upper 60s to low 70s boundary-layer dew points will remain pervasive ahead of the cyclones/cold front, but boundary-layer heating will likely be more pronounced from eastern VA to north FL. Given the relatively meridional mid/upper-level flow regime and cooler temperatures aloft lagging well west, high-level lapse rates will be poor. This will likely hold MLCAPE to between 750-1500 J/kg north of FL. Deep-layer winds will increase with approach of the amplified trough, more substantially tonight, with relatively small low-level hodograph curvature expected to persist into early evening. The modest CAPE with an elongated hodograph due to the degree of vertical speed shear should favor weak/transient supercell structures within a broken line of convection along/just ahead of the surface cold front during the late afternoon and evening. The primary concern will be for locally strong gusts capable of producing sporadic damaging winds. A brief tornado is also possible, mainly centered on eastern NC tonight, with any deep convection that can linger amid meager buoyancy as low-level hodographs enlarge.
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