SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0757 AM CDT Thu Apr 08 2021

Valid 081300Z - 091200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
THE FLORIDA PANHANDLE ACROSS THE SOUTHERN APPALACHIANS TO PARTS OF
THE GREAT LAKES REGION...

...SUMMARY...
Isolated severe thunderstorms are possible today from parts of the
Florida Panhandle across the southern Appalachians to parts of the
Great lakes region.

...Synopsis...
Split flow in mid/upper levels will persist over the central CONUS,
related to a pronounced cyclone now centered over northwestern MO. 
As a strong/upstream shortwave trough moves southeastward from the
Pacific Northwest across the northern Rockies, the cyclone will
eject northeastward.  By 00Z, the 500-mb low should be located over
eastern IA, moving slowly northward to northeastward toward western
WI by the end of the period. 

At the surface, 11Z analysis showed a low over north-central IA,
with occluded/cold front arching eastward over the DBQ area then
southeastward across central IL, western portions of KY/TN, western
MS, and southwestern LA, becoming quasistationary across the
lower/middle TC Coastal Plain to near LRD.  The low should meander
around northern IA and southern MN through most of the period as the
mid/upper cyclone approaches.  Along an inverted trough extending
northward from the surface low, a second closed low may form between
09-12Z over northern MN.  The front will move eastward across IN and
parts of OH and Lower MI through the period, while also crossing
into eastern TN/KY and northwestern GA, and decelerating/stalling
over MS/AL.  The trailing part of the boundary will become a warm
front moving northward over parts of central/east TX today and
tonight, and then over LA/MS overnight. 

...Great Lakes to southern Appalachians...
Widely scattered to scattered thunderstorms are expected to form
from late morning through afternoon along/ahead of the front, in at
least a broken arc, initially from the southern Lake Michigan area
across southern IN to middle/eastern TN.  Isolated damaging to
severe gusts will be the main concern, though a tornado cannot be
ruled out. 

Overall, most of the richest low-level moisture and highest theta-e
will be displaced in the Gulf Coast States, south of the area of
strongest deep-layer forcing (both large-scale warm advection,
deeper low-level mass convergence nearer the cyclone, and frontal
lift).  The greatest potential for overlap will be across the
corridor roughly between CVG-CHA, but with mean-wind/deep-shear
vectors having a substantial component parallel to the convective
axis.  As such, and with effective shear on the margins (vector
magnitudes 30-40 kt over much of the area), the main modes will be
quasi-linear and multicellular, but with brief supercell modes and
embedded mesocirculations possible.  The warm sector will become
more narrow and incompletely modified with northward extent, only
somewhat offset by colder air aloft in generating buoyancy.  MLCAPE
should be in the 500-1000 J/kg range in the western portions of the
outlook area, where greater diurnal and midlevel destabilization
each are possible, and across some parts of eastern TN/KY where
moisture is greater.  Convection should diminish in intensity and
coverage with eastward extent late this afternoon into evening, upon
encountering even more-meager instability. 

...AL/GA/FL Panhandle...
Farther south from the southernmost Appalachians (AL/GA) and Gulf
Coast, times series of forecast soundings reasonably depict
weakening low-level hodographs with time through the day, as
boundary-layer instability strengthens from diabatic surface
heating.  With the deep-layer cyclone already well-removed from this
region and moving away, little appreciable upper support is
expected, and the main influence on convective development will be
the front (with modest lift) and mesoscale boundaries.  A somewhat
favorably moist prefrontal air mass will remain in place, with
surface dew points in the 60s F and pockets of sustained heating
enabling 1000-2000 J/kg MLCAPE.  This will occur beneath the
eastern/northeastern edge of the mid/upper speed max (500-250-mb
layer) supporting 45-55-kt effective-shear magnitudes.  As such, any
sustained convection that can develop may become at least briefly
supercellular and/or assume bowing/LEWP configurations, locally
yielding a severe threat.  However, overall convective coverage is
in question, keeping the unconditional threat marginal.

..Edwards/Dial.. 04/08/2021

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