SPC 0830Z Day 3 Outlook

Day 3 Convective Outlook  
NWS Storm Prediction Center Norman OK
0228 AM CST Fri Nov 27 2020

Valid 291200Z - 301200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE LOWER MISSISSIPPI VALLEY/SOUTHEAST INTO THE CAROLINAS...

...SUMMARY...
Isolated strong to severe thunderstorms may occur Sunday across
parts of the lower Mississippi Valley into the Southeast and
Carolinas.

...Lower Mississippi Valley into the Southeast and Carolinas...
A closed upper low over the southern Plains Sunday morning should
move eastward across the lower MS Valley and Southeast through the
day. This upper low should begin to merge with a northern-stream
upper trough over the eastern CONUS Sunday evening into early Monday
morning. A weak surface low initially near the coast of LA is
forecast to develop northeastward across the Southeast through the
day while gradually deepening. Additional deepening of this surface
low should occur Sunday evening through the end of the period as it
continues developing northeastward along/near the spine of the
southern/central Appalachians. A warm front should lift northward
across parts of the Southeast and Carolinas through the period,
while a cold front attendant to the surface low sweeps eastward
across these regions throughout the period.

A prior frontal intrusion over the northern Gulf of Mexico may tend
to limit the prospect for substantial inland destabilization across
the Southeast through Sunday evening. Even so, low to mid 60s
surface dewpoints should advance northward into parts of southern
LA/MS/AL and the FL Panhandle in tandem with the developing surface
low and warm front through the day. Forecast soundings from various
models suggest weak instability may gradually develop with modest
daytime heating across these areas along/south of the warm front.
Strong low-level and deep-layer shear appear likely given the
enhanced wind field associated with the approaching upper low. This
high-shear, low-instability setup typically favors isolated strong
to severe convection across the developing warm sector along/ahead
of the eastward-moving cold front. Damaging winds a perhaps a couple
tornadoes would be the main threats with both low-topped supercells
and linear convection given the forecast strength of the low-level
flow and shear.

This isolated severe threat may continue across parts of GA into the
Carolinas Sunday night and early Monday morning (end of the Day 3
period), as low-level moisture return occurs from the western
Atlantic. However, even through the low-level flow across these
areas should be quite strong, surface-based instability is forecast
to remain rather weak. Given the potentially limiting factor of
modest instability, have opted to introduce a broad Marginal Risk
area from parts of the lower MS Valley/Southeast into the Carolinas.
The northern extent of any damaging wind/tornado threat remains
unclear, as there is uncertainty in how far north the low to mid 60s
surface dewpoints (which will probably be needed for surface-based
storms) will advance across the Southeast and Carolinas. If
confidence increases in a narrow corridor of greater instability
developing across the warm sector, a Slight Risk for damaging winds
and/or tornadoes may need to be included across some portion of
these regions in a later outlook update.

..Gleason.. 11/27/2020

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