SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0656 AM CST Mon Feb 15 2021

Valid 151300Z - 161200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
THE FLORIDA PANHANDLE...NORTHERN FLORIDA...EXTREME SOUTHEASTERN
ALABAMA...SOUTHERN GEORGIA...AND IMMEDIATE COASTAL EASTERN SOUTH
CAROLINA AND SOUTHERN NORTH CAROLINA...

...SUMMARY...
Severe thunderstorms are possible from the Florida Panhandle into
southern Georgia through the evening, and over the coastal Carolinas
overnight. Damaging gusts and a couple tornadoes will be the main
concerns.

...Synopsis...
A mid/upper-level longwave trough will persist over the central
CONUS through the period, regulating a large belt of cyclonic flow
now covering the Rockies and Plains States.  The most important/
embedded perturbation influencing sensible weather will continue to
be a synoptic trough -- now evident in moisture-channel imagery from
south-central KS across western OK, central TX and Coahuila.  This
feature will eject northeastward across the mid Mississippi and
lower Ohio Valleys through the period, while deamplifying. By 12Z
tomorrow, the southern rim of the associated 500-mb vorticity field
should be crossing central portions of MS/AL.

At the surface, the associated frontal-wave low was analyzed over
the Gulf approximately east of CRP and south of LCH, with cold front
southwestward over east-central MX.  A warm front was drawn from the
low eastward across the northern Gulf near buoy 42365 then eastward
over central FL, where it was quasistationary.  The low is forecast
to move across the Mississippi River mouth around 15Z then inland
near PNS about 18Z, when a trough will extend northward to another
weak low in the cool air over northern AL.  The lows should combine
and reach extreme eastern TN near TYS by 00Z, with occluded/cold
front southward to near TLH then southwestward over the east-central
Gulf.  The warm front should move northward over the central/eastern
FL Panhandle, extreme southeastern AL and southern Ga before being
overtaken by the cold front.  The northeastern limb of the warm
front also should get very close to the SC/NC coast overnight.

...Coastal northeast Gulf...
A band of scattered thunderstorms is forecast to evolve through the
remainder of the morning and midday over the Gulf, near the surface
low and trailing cold front, and extending poleward of the low on an
elevated version of the frontal zone toward the MS/AL coastlines. 
This activity should move northeastward across the Gulf Coast from
there eastward through the day.  As it does so, encountering
progressively more surface-based parcels near and south of the warm
front, the associated severe threat will spread obliquely inland
across the FL Panhandle and southeastern AL this afternoon, before
reaching southern GA late afternoon and evening.  Potential for
isolated, marginally severe hail will extend somewhat more inland
than the damaging-gust and tornado threats, with suitably moist
elevated inflow parcels located under colder air aloft and favorable
deep shear.

Along and south of the warm front, the buoyancy/shear parameter
space will be favorable for supercells and bowing segments within
the main convective band, and isolated supercells cannot be ruled
out to its east.  Mid-60s to low-70s F surface dew points are
expected in that regime, supporting 1000-1500 J/kg MLCAPE near the
coast, decreasing northward to less than 500 J/kg just north of the
warm front.  Expect strengthening mid/upper winds supporting
50-65-kt southwesterly effective-shear vectors, oriented just
slightly rightward of the main corridor of convective forcing.  The
southern fringe of a 45-60-kt LLJ will contribute to mildly curved
but long hodographs with 200-300 J/kg effective SRH -- greatest
along the warm front.

...Coastal Carolinas...
Elevated thunderstorms should spread northeastward over interior
areas tonight, as a northward extension of the Gulf regime. A
separate, prefrontal low-level convergence zone is forecast to
support increasing thunderstorms tonight, initially from roughly 
the GGE-ILM corridor southward across Atlantic waters, the northern
portion of which should affect coastal areas from there past HSE
through the end of the period.  A damaging-gust threat may develop
very near the coastline, where boundary-layer instability may be at
least marginally favorable, with tornado potential from any mature
supercell that can cross the coast.  Farther northwestward, isolated
strong-severe gusts may penetrate the near-surface stable layer;
that potential will lessen with inland extent.

The northwestern margin of the most substantial severe potential,
relative to the coastline, may be just a few miles either way. 
Planar guidance across this region, from a preponderance of
operational models and ensemble (HREF, SREF, GEFS) members, now
indicates MLCAPE developing somewhat inland, with 250-500 J/kg
values possible, and 1000 J/kg just offshore.  Buoyancy will
increase with proximity to the Gulf Stream, where vertical
heat/moisture fluxes will provide the most sustained airmass
modification.  Scrutiny of forecast soundings from some of them
still show shallow near-surface stable layers at the bottom of the
lifted layer yielding surface- based effective-inflow parcels. 
Vertical shear will be favorable, with 45-55-kt effective-shear
magnitudes and effective SRH around 200-300 J/kg.

..Edwards/Smith.. 02/15/2021

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