SPC AC 041249

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0749 AM CDT Tue Apr 04 2017

   Valid 041300Z - 051200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS
   AFTERNOON/EVENING FROM NORTHEAST OKLAHOMA INTO SOUTHWEST MISSOURI...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
   INTO TONIGHT FROM CENTRAL/EASTERN OKLAHOMA TO NORTHWESTERN ARKANSAS
   AND CENTRAL MISSOURI...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE TONIGHT
   ACROSS PARTS OF THE NORTH CENTRAL/NORTHEAST GULF COAST...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   PLAINS/OZARKS SLIGHT RISK...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS NORTH
   FLORIDA TODAY...AND FROM SOUTHEAST LOUISIANA AND NORTHERN
   ALABAMA/NORTHWESTERN GEORGIA LATE TONIGHT...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
   ACROSS EASTERN PENNSYLVANIA AND VICINITY...

   ...SUMMARY...
   Scattered strong to severe thunderstorms will be possible across
   parts of the south-central Plains into the Ozarks this afternoon and
   early evening, and across parts of the central Gulf Coast states
   late tonight into early Wednesday morning.  Isolated strong storms
   will also be possible today across part of north Florida and in the
   vicinity of eastern Pennsylvania.

   ...I-44 corridor from northeast OK into MO this evening...
   A compact midlevel shortwave trough over northern NM this morning
   will progress eastward to central OK this evening and the Ozarks
   overnight.  An associated surface cyclone initially in northwest TX
   will develop northeastward along a stalled front near I-44 in OK, in
   advance of the midlevel trough.  Strengthening wind profiles and
   ascent will immediate precede the midlevel trough and surface
   cyclone and support strong-severe thunderstorm development this
   afternoon in OK, and the storms will spread into MO/AR overnight.

   The primary limiting factor to a more robust severe-weather risk
   will be marginal moisture return in the warm sector of the cyclone. 
   The maritime tropical air mass was shunted southward into the north
   central and northwest Gulf of Mexico by the system the prior two
   days.  Residual boundary layer dewpoints in the low-mid 50s, aided
   some today by evapotranspiration but offset by vertical mixing, are
   expected up to the stalled front from central OK into southwest MO
   by mid-late afternoon.  This moisture, along with afternoon surface
   temperatures in the 70s and midlevel lapse rates of 8 C/km or
   greater, will drive MLCAPE of 1000-1500 J/kg.  This buoyancy,
   combined with effective bulk shear in excess of 50 kt, and
   relatively cool midlevel temperatures, will support a risk for
   supercells with large hail near and just ahead of the surface
   cyclone/triple point in beginning 21-23z in OK and continuing into
   MO through late evening.  Storms may also develop southward along
   the cold front into eastern OK and northwestern AR, though the
   strongest/most persistent storms are expected along the path of the
   cyclone along the I-44 corridor from northeast OK into southwest MO
   through early tonight.

   The tornado risk is limited mostly by the marginal low-level
   moisture.  There might be a narrow zone along the stalled front
   (immediately ahead of the surface cyclone) where moisture will be
   sheltered some from vertical mixing, and where low-level hodographs
   will by the largest with the most streamwise vorticity.  A discrete
   storm moving along this narrow zone could produce a tornado before
   near-surface cooling/stabilization ends the threat just after
   sunset. 

   ...North central Gulf coast into Dixie early Wednesday...
   In response to cyclogenesis with the shortwave trough approaching
   the Ozarks, the maritime tropical air mass will return northward
   late tonight and early Wednesday in a strengthening warm advection
   regime.  Elevated thunderstorm development is expected from 06-12z
   from southeast LA into at least central AL.  Large hail will be the
   main risk early Wednesday across the northern parts of the Slight
   Risk area.  Farther south near the surface warm front, some of the
   convection could become rooted near or at the surface.  Deep-layer
   and low-level shear will favor supercells, and profiles will become
   supportive of some tornado and/or damaging wind risk from 09-12z. 
   The main uncertainties will be the specific corridor of storm
   development and the narrow time window for surface-based storms. 
   Thus, 5% tornado probabilities appear most appropriate within this
   outlook period.

   ...North FL today...
   A lingering band of storms along a composite outflow boundary will
   sag southward through the day across north FL.  Isolated strong
   gusts will be possible the first half of the day within this band,
   before the convection weakens.

   ...Eastern PA and vicinity this afternoon...
   A midlevel trough and an associated/diffuse surface trough will
   cross PA during the day.  Lingering low-level moisture and some
   surface heating will contribute to weak surface-based instability
   across this area by midday/early afternoon, and will be sufficient
   for at least isolated thunderstorm development.  Given somewhat cool
   midlevel temperatures and effective bulk shear of 30-40 kt, a
   low-end hail/wind risk will be possible this afternoon with the
   stronger/more organized cells.

   ..Thompson/Goss.. 04/04/2017

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