SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0656 AM CST Sat Feb 13 2021

Valid 131300Z - 141200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
CENTRAL/SOUTHWESTERN FLORIDA...

...SUMMARY...
Strong to marginally severe thunderstorms are possible today through
tonight across parts of central and southwestern Florida.

...Synopsis...
In mid/upper levels, a high-latitude blocking pattern will start to 
break down as 1) a high now over northern Yukon redevelops
southeastward across the Northwest Territories and 2) a long-lived
cyclone over southern AB moves slowly eastward to northwestern ON. 
As this occurs, a speed max and associated shortwave trough --
initially near the CA/OR coastline -- will dig southeastward in a
strongly difluent upper pattern across the Great Basin today.  This
perturbation will expand into a synoptic-scale trough by the end of
the period, from the Four Corners region southward across
northwestern MX.  

In turn, the mid/upper trough now over southern NM and northwestern
MX -- with embedded 500-mb low apparent over Chihuahua -- should
pivot across northeastern MX and south TX through the period.  By
12Z, this trough should reach the middle TX Coast and western Gulf,
while weakening rapidly amidst a broadening cyclonic-flow field
related to the expansive upstream troughing.  Downstream, a more-
subtle shortwave trough -- now evident in moisture-channel imagery
over the western Gulf -- will eject east-northeastward over the
central/northeastern Gulf, weakening substantially by the time ot
reaches FL around the end of the period. 

At the surface, a quasistationary to northward-drifting frontal zone
was drawn across north-central FL just south of a DAB-LEE-CGC line,
and is being overtaken from west-east by a convective complex moving
off the Gulf.  A frontal-wave low was drawn behind the convection
near buoy 42039, with slow-moving but strong cold front
southwestward across the north-central/west-central Gulf.  This low
should weaken over time, with a new one forming tonight over the
central Gulf along the front, and ahead of the ejecting
southern-stream shortwave trough.

...FL...
A loosely organized band of thunderstorms over coastal areas of
west-central FL -- and adjacent Gulf waters -- will proceed
east-northeastward across the peninsula through the rest of the
morning, offering the threat for isolated strong-severe gusts. A
brief tornado cannot be ruled out near and just south of the frontal
zone, where low-level SRH/vorticity will be relatively maximized
based on the modified TBW wind/thermal RAOB. See SPC mesoscale
discussion 73 for near-term details.  

This activity has been very poorly progged by most 00Z convection-
allowing guidance including HREF members, as well as HRRR runs from
then through 10Z, when finally forced by reflectivity input to
depict (underdone) convection initially.  The atmosphere clearly is
not obedient to the models.  Observationally, based on satellite and
radar trends, associated cloud cover and precip should impede
diurnal destabilization over northern and parts of central FL,
effectively shunting the baroclinic zone southward on the mesoscale
and casting increasing doubt on severe potential with northern
extent.  

An additional round or two of strong/isolated severe convection,
with potential for embedded supercells/bow structures, is possible
this afternoon and/or tonight over parts of central/southern FL.
Some of this activity will interact with the boundary reinforced/
relocated southward by morning convection, while also accessing
less-disturbed warm-sector inflow parcels.  In the warm sector over
west-central/southwestern FL late tonight, forecast soundings
indicate that rich boundary-layer moisture will offset slightly
warming midlevel temperatures (around 500 mb) enough to yield
500-1000 J/kg MLCAPE, though low-level shear/hodographs will
diminish with southward extent.  In association with such
convection, a relatively concentrated corridor of potential may
develop within the broader marginal area; however, mesoscale
uncertainties preclude assigning greater unconditional probabilities
at this time.

..Edwards/Smith.. 02/13/2021

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