SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0652 AM CST Wed Jan 27 2021

Valid 271300Z - 281200Z


Thunderstorms will pose a marginal severe-weather threat today
across southern Georgia and extreme northern Florida.

In mid/upper levels, a high-amplitude trough will remain just
offshore from the Pacific Coast through the period, as one
accompanying cyclone now offshore from OR ejects northeast and is
replaced by another developing into essentially the same area by 12Z
tomorrow.  Associated cold air aloft will contribute to marginal
instability for thunder near portions of the CA Coast.  Downstream,
a strong shortwave trough was apparent in moisture-channel imagery
over portions of KS/OK.  This perturbation will deamplify somewhat
-- but still remain well-defined -- as it accelerates eastward
across the Mid-South to the Tennessee Valley today.  By the end of
the period, the trough should reach the eastern Carolinas. 

As that occurs, a weak surface low -- analyzed at 11Z near MAI --
should ripple east-northeastward along what is now a quasistationary
frontal zone extending from southeastern LA across southern GA.  The
low should move offshore around 00Z and merge with another cyclone
developing in the cool sector over SC.  The combined cyclone should
deepen substantially late tonight as it crosses Atlantic waters
progressively farther offshore from GA and the Carolinas.  Behind
the low this evening, the boundary should accelerate southeastward
across the outlook area as a strong cold front, reaching south FL by

...Southern GA/northern FL, eastern FL Panhandle...
Widely scattered to scattered thunderstorms are expected to move
eastward across the area, mainly ahead of the surface front.  A few
embedded/transient supercells are possible.  A marginal damaging-
wind/tornado threat exists through late afternoon.  

This regime will be bracketed on the north by relatively stable air
north of the frontal zone.  The shortwave trough and related
DCVA/cooling aloft will pass well north of the area, minimizing
direct upper support and keeping deep-layer lapse rates weak.  This
will keep MLCAPE around 500 J/kg or less in most areas.  Still,
associated tightening of height gradients will strengthen deep/speed
shear over the area, contributing to effective-shear magnitudes of
45-55 kt, despite veered near-surface winds.  This flow geometry
will limit low-level shear and lift, though:
1.  Weak MLCINH should enable convective initiation with available
frontal and near-cyclone lift anyway, and 
2.  Forecast soundings do indicate enough hodograph curvature to
support pockets of marginal (around 150 J/kg) effective SRH,
especially midday over parts of the eastern FL Panhandle and
southern GA.  The southwesterly surface flow also will advect
better-modified Gulf air with greater moisture across the area,
supporting greater eastward longevity of convection. 

Weaker boundary-layer buoyancy and deep-layer lift are forecast with
southward extent and time this evening over northern FL, reducing
the severe potential.

..Edwards/Gleason.. 01/27/2021

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