SPC AC 161301

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0701 AM CST Sun Jan 16 2022

   Valid 161300Z - 171200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER SOUTHERN
   AND EASTERN FLORIDA AND COASTAL CAROLINAS...

   ...SUMMARY...
   Isolated strong to severe storms may impact parts of the Florida
   Peninsula/Keys and coastal Carolinas today into this evening, with a
   marginal threat for severe gusts or a tornado.

   ...Synopsis...
   Large-scale mid/upper-level troughing will be maintained over the
   eastern CONUS through the period, with two primary perturbations
   involved:
   1.  A shortwave trough -- apparent in moisture-channel imagery from
   northwestern ON across ND to eastern MT -- and forecast to move
   rapidly southeastward to parts of the Ohio Valley and southern
   Appalachians by 12Z tomorrow.
   2.  A closed cyclone now over MS, with accompanying trough from TN
   to the northwestern Gulf, should move east-northeastward to the
   western Carolinas by 00Z, then eject to the Mid-Atlantic and weaken
   overnight as the upstream perturbation approaches the larger-scale
   trough position.

   At the surface, the main low related to the MS cyclone was analyzed
   at 11Z over extreme east-central AL, with occluded front to the
   central part of the FL-GA line.  The cold front was drawn from there
   southward past CTY and across the eastern Gulf. The synoptic warm
   front was drawn across the JAX area then northeastward over Atlantic
   waters off SC and well offshore southern NC.  A separate marine/
   warm-frontal zone was evident around the southwestern to
   southeastern peninsular FL coastline, and should diffuse northward
   gradually through the morning and afternoon.  The initial surface
   low should shift erratically north-northeastward along the
   southern/central Appalachians through the period as the pressure
   center of an occluded cyclone.  An occasional secondary low is
   possible along the triple point farther east and southeast, moving
   northeastward over the eastern Carolinas and southeastern VA. 
   Though the Atlantic warm front may move slightly inland prior to
   cold frontal passage, overland destabilization to its south is in
   question, as discussed below.

   ...FL...
   A wavy, pre-cold-frontal squall line -- containing several embedded
   LEWP/bowing structures, is moving inland from the west-central FL
   coastline, and should continue to cross the southwest FL coastal
   areas and east-central/south-central FL through the morning.  A few
   supercells in the modifying Gulf warm sector also may move over/near
   the Keys or inland on the peninsula a short distance before being
   overtaken by the main convective band.  Isolated damaging to
   marginally severe gusts are possible, and a marginal tornado threat
   also exists.

   Though the inland boundary layer is destabilizing gradually due to
   theta-e advection from the Straits and extreme eastern Gulf, this
   process will remain incomplete prior to the line, as the
   aforementioned marine boundary slowly diffuses.  This will limit the
   overall magnitude and coverage of the severe threat, which is kept
   at marginal unconditional probabilistic levels for this outlook
   cycle.  Inflow-layer parcels inland will be more elevated with
   northward and inland extent, atop stronger MLCINH, while deep-layer
   shear will decrease from north to south with greater distance from
   the mid/upper low.  Effective-shear magnitudes of 30-40 kt should be
   common over southern FL and the Keys.  Still, somewhat favorable
   low-level hodographs (effective SRH 150-250 J/kg) suggest continuing
   potential for occasional storm-scale rotation, both with the main
   line and any preceding, surface-based discrete cells.  The main
   convective band should clear the FL mainland and Keys by
   midafternoon.

   ...Coastal Carolinas...
   From midday into early evening, the squall line also affecting FL is
   expected to extend northeastward across the Atlantic off GA, with
   its northwestern fringe brushing the coastline from SC to the
   eastern Outer Banks of NC.  A conditional threat exists for isolated
   damaging gusts and/or a tornado may occur along immediate coastal
   areas.

   Before that convective band extends northward to near the coast,
   expect extensive, persistent, antecedent clouds/precip into the
   boundary layer, both north of the warm front and over the nominal
   warm sector inland to its south.  This will substantially inhibit
   inland destabilization near and east of the triple point across the
   eastern edge of the Piedmont and coastal plain.  A narrow sliver of
   surface-based buoyancy -- which may be confined mostly below optimal
   lightning-generation layers by poor mid/upper-level lapse rates --
   may develop along the coast, with MLCAPE generally less than 300
   J/kg.  This will be related mainly to boundary-layer theta-e
   advection ahead of the squall line, in a partially modified,
   return-flow air mass from the Atlantic.  Vertical shear will be
   favorable, however, with a 60-70-kt southeasterly LLJ contributing
   to effective SRH of 250-350 J/kg, amidst roughly 35-40 kt
   effective-shear magnitudes.  Convection will be more intense
   offshore where ocean-air heat fluxes and less convection ahead of
   the squall line foster greater buoyancy.  The main convective band
   should clear the Outer Banks by around 03Z.

   ..Edwards/Broyles.. 01/16/2022

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