SPC 2000Z Day 1 Outlook

Day 1 Outlook Image

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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