MARGINAL RISK FOR SEVERE STORMS TODAY SOUTH FLORIDA!

 

Forecast Discussion

   SPC AC 181230

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0630 AM CST Sun Feb 18 2024

   Valid 181300Z - 191200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER THE FLORIDA
   KEYS AND A SMALL PART OF SOUTH FLORIDA...

   ...SUMMARY...
   Isolated to scattered thunderstorms, associated with a marginal
   wind/tornado threat, will be possible across part of south Florida
   and the Keys today.

   ...Synopsis...
   A generally progressive shortwave pattern is expected across most of
   the CONUS.  A northeastern Pacific cyclone -- initially centered
   near 38N137W -- is forecast to approach (but stay offshore from) the
   West Coast this period.  Associated frontal forcing and large-scale
   DCVA/cooling aloft should contribute to increasing general-
   thunderstorm potential from mid/late afternoon into tonight across
   portions of northern/central CA.

   Meanwhile, a downstream/midlatitiude shortwave trough was evident in
   moisture-channel imagery over the Mid-South and Arklatex regions,
   while a southern-stream perturbation was fairly prominent in the
   same imagery over the western Gulf.  By 00Z, the midlatitude trough
   is expected to amplify and pivot from strongly positively tilted
   initially to only slightly positive over MS and the Tennessee Valley
   region.  Overnight, this perturbation should develop a closed 500-mb
   low , reaching northern GA by 12Z, with trough south-southwestward
   across the FL Panhandle to the northeastern Gulf.  As this occurs,
   the southern-stream/Gulf perturbation will eject east-northeastward
   and deamplify, reaching the extreme eastern Gulf just west of the FL
   Peninsula by 00Z.  This perturbation should cross the peninsula and
   move over Atlantic waters by 06Z. 

   In response to those developments, a slow-moving/wavy surface
   frontal zone -- analyzed at 11Z from near TMB to about 35 nm N of
   EYW -- may oscillate northward on mesobeta to local scales while
   remaining over extreme southern mainland FL for much of the day. 
   With the approach of the Gulf shortwave trough aloft, a weak/
   frontal-wave low now drawn about 200 nm west of EYW should ripple
   along the front and across south FL by 00Z, to a position just
   offshore from the FLL-PBI corridor.  As that low proceeds east-
   northeastward just north of the Bahamas, the cold front should move
   southeastward past mainland FL and the Keys by 09Z. 

   ...South FL/Keys...
   A band of convection, with isolated to scattered embedded
   thunderstorms, is apparent in satellite, distant radar and lightning
   data over the extreme eastern Gulf west of the Dry Tortugas.  This
   activity is expected to be maintained and perhaps intensify somewhat
   while move east-northeastward, affecting parts of the southern
   mainland either side of the front, as well as the Keys, from midday
   into part of the afternoon.  This activity may be preceded by more
   isolated thunderstorms developing in the warm sector and crossing
   the front.  Marginal potential exists for supercells or embedded
   LEWP/bow formations, offering a damaging-gust or brief tornado risk.
    
   A richly moist boundary layer will persist south of the front,
   characterized by surface dewpoints from the upper 60s to the low 70s
   F.  Even then, buoyancy will be weak due to poor mid/upper-level
   lapse rates, including a couple prominent stable layers noted in the
   12Z KEY sounding, which will keep peak/preconvective MLCAPE values
   generally in the 400-800 J/kg range.  The lack of more-robust
   instability/buoyancy will be a limiting factor for severe potential
   amid favorable deep shear, and potentially favorable localized
   low-level shear.  Strong mid/upper-level winds will persist, with
   flow commonly 60-75 kt in the 500-250-mb layer.  This will
   contribute to 45-55-kt effective-shear magnitudes.  Low-level
   hodographs should be long but not particularly enlarged, with the
   possible exception of localized boundary influences, such as the
   front just east of the low.

Excessive rainfall outlook

Marginal risk for severe storms