SPC AC 101244

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0744 AM CDT Sat Apr 10 2021

   Valid 101300Z - 111200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   WESTERN/NORTHERN FLORIDA...SOUTHERN GEORGIA...THE FLORIDA
   PANHANDLE...AND EXTREME SOUTHEASTERN ALABAMA,...

   ...SUMMARY...
   A broad area of at least marginal severe-thunderstorm threat is
   apparent today from the Southeast to parts of the Ohio Valley and
   Great Lakes States.

   ...Synopsis...
   A mid/upper-level split-flow pattern will continue trough the period
   over the central CONUS, related to a cyclone now evident in
   moisture-channel imagery over eastern KS/western MO.  The 500-mb low
   should move to western/central IL by 00Z, then pivot east-
   northeastward to northwestern IN by 12Z tomorrow.  Embedded in the
   cyclonic flow to its southeast is a shortwave trough with
   convectively enhanced vorticity field -- evident in moisture-channel
   imagery over the Mid-South/Tennessee Valley region.  This
   perturbation will move eastward to northeastward today and elongate,
   reaching extending from southern OH to the western Carolinas by 00Z,
   then becoming diffuse overnight.

   The primary surface low -- analyzed at 11Z near COU -- will shift
   northeastward under the mid/upper low today, with the cyclone
   becoming stacked through tonight.  The associated cold front --
   drawn at 11Z from the low to near MEM, GLH, HOU and COT -- should
   reach central IN, east-central portions of KY/TN, and central/
   southwestern AL by 00Z.  Over the Deep South, the front is preceded
   by an extensive area of convection and associated cold pool,
   spreading across southeastern AL, the FL Panhandle, and southeastern
   LA at this time, and just offshore from the MS Coast.  A marine warm
   front was analyzed from south FL northwestward across southwestern
   peninsular coastal waters, to the AAF/PFN area and into the leading
   parts of an MCS over extreme southeastern AL.  

   ...Gulf Coast to Great Lakes/Ohio Valley...
   An extensive MCS and related cold pool are moving eastward and
   southeastward over portions of southeastern AL, the FL Panhandle,
   central Gulf Coast, and adjacent waters.  Damaging gusts, a brief
   tornado, and isolated, marginally severe hail remain possible.  See
   SPC mesoscale discussion 359 for near-term details.  

   This activity has progressed considerably faster than progged last
   evening, and the regional outlook areas are being adjusted
   accordingly in both magnitude and spatial orientation.  Its
   well-defined pressure perturbation should progress across the
   northeastern Gulf, with the most intense convection likely being
   over water by late morning into afternoon, where inflow-layer
   theta-e will be the greatest.  Over land, the MCS and its outflow
   boundary will overtake the marine front faster than the front can
   progress northeastward/inland.  Still, a partially modified boundary
   layer and areas of inland diabatic heating will destabilize the
   boundary layer enough to support some potential for damaging gusts
   to reach the surface across portions of northern and western FL with
   the weakened but still well-defined remnants of the complex. 

   Farther north, from midday through the afternoon, an arc of widely
   scattered to scattered convection, including some thunderstorms, is
   expected to develop along the cold front near the western rim of the
   "Marginal" area from IL southeastward, perhaps as far as middle/
   eastern TN.  Because of a combination of trajectories from
   1.  An incompletely modified, former continental/polar boundary
   layer over the Appalachians region and 
   2.  Convectively processed boundary-layer air near the Gulf Coast,
   ...instability across much of the region will be very limited today,
   with modest midlevel lapse rates over the northern 1/2-2/3 of the
   outlook area.  

   Where deep ascent is stronger near the cyclone, instability will be
   weak.  Still, enough moisture should be present to support patchy
   MLCAPE around 500-800 J/kg from the Tennessee Valley northward. 
   Strong mid/upper winds are forecast over much of the region, but
   with a nearly unidirectional deep-layer wind field limiting bulk
   shear.  These offsetting factors suggest severe potential is
   non-zero, but marginal and conditional, across a broad corridor. 

   ...Carolinas...
   Scattered thunderstorms may develop this afternoon and evening
   across western parts of the Carolinas and move northeastward,
   offering at least isolated potential for severe gusts.  A
   conditional threat exists for a small MCS to form and concentrate
   the severe risk more, but too much mesoscale uncertainty remains to
   outlook such an area at this time. 

   Deep-layer flow and large-scale ascent each should increase with
   time this afternoon ahead of the shortwave trough, while a
   strengthening LLJ boosts low-level shear and hodograph size. 
   Extensive cloud cover should spread over the area for much of the
   day from the Gulf Coast convection, limiting heating.  Still, enough
   theta-e advection and diffused diabatic heating may occur to support
   500-1000 J/kg MLCAPE by mid/late afternoon.  The extensive
   longitudinal coverage and faster-than-progged progress of the Gulf
   Coast MCS, and its resultant influences on the instability and mass
   fields across this region, lend considerable uncertainty to both
   coverage and intensity of development ahead of the shortwave trough.
    However, as mesoscale diagnostic and short-fused model trends
   warrant, an upgraded unconditional area may be needed in one of the
   succeeding updates.

   ..Edwards/Broyles.. 04/10/2021

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