SPC AC 031250

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0750 AM CDT Mon May 03 2021

   Valid 031300Z - 041200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM EASTERN
   OKLAHOMA TO PARTS OF THE OHIO VALLEY...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM
   NORTHWEST TEXAS TO ILLINOIS AND EASTERN KENTUCKY...AND SEPARATELY
   OVER PARTS OF ALABAMA...GEORGIA AND THE CAROLINAS...

   ...SUMMARY...
   The greatest severe-thunderstorm threat appears to be from this
   afternoon through tonight, in a corridor from eastern Oklahoma to
   parts of the Ohio Valley.  Damaging wind, hail and tornadoes all are
   possible.

   ...Synopsis...
   A progressive mid/upper-level pattern will characterize this period,
   led by a synoptic-scale trough now evident in moisture-channel
   imagery from western CO southward across western Chihuahua.  A
   series of low-amplitude shortwave perturbations and vorticity maxima
   will pivot through the associated cyclonic flow as the trough moves
   eastward.  By 00Z, the main trough should extend from the central/
   southern Rockies southward past southern NM and the ELP area.  By
   12Z, the trough should be located from central KS to western OK and
   the TX Big Bend area.  Downstream, satellite and composited radar
   imagery depict a well-develop MCV over the northeastern KS/
   northwestern MO region, produced by last night's MCS over western
   KS.  The vorticity max, and associated shortwave trough, should move
   eastward across central IL by 00Z, then weaken over OH/PA by 12Z.

   At the surface, 11Z analysis indicated an elongated low-pressure
   area from central WI to northeastern KS, with centers over
   southwestern WI and near TOP.  The latter is associated with the
   MCV.  A cold front connects the lows, and extends southwestward over
   the TX Panhandle and east-central NM.  Another, initially separate
   area of low pressure was drawn over the Red River region near SPS,
   with wavy, diffuse warm front eastward across AR, TN and southern
   NC.  The warm front will become more poorly defined through the day
   while shifting northward around areas of convection and outflows. 
   The northern low will move eastward over Lower MI through the
   period, as the trailing cold front crosses the remaining mid/upper
   Mississippi Valley, parts of the Ozarks, most of OK, and
   central/southwest TX. 

   ...Eastern OK to Ohio Valley...
   The greatest probable concentration of convection (including severe
   potential) and also the greatest mesoscale uncertainties exist in
   this corridor, where a complex convective scenario may unfold. 
   These are two of the most probable regimes -- which may become
   linked tonight over parts of the lower Ohio Valley:

   1.  Widely scattered to scattered thunderstorms possible this
   afternoon and evening across parts of eastern IA, eastern MO and IL,
   with at least isolated damaging gusts, marginal hail and conditional
   tornado potential.  Convective potential from the mid Mississippi
   Valley to the Ohio Valley should be influenced by the MCV, and
   related enhancements to both deep-layer flow/shear and mesoscale
   DCVA/lift aloft.  Some moisture recovery and low-level warm
   advection, along with some diurnal heating, may boost peak
   preconvective MLCAPE into the 1000-1500 J/kg range, with modest deep
   shear but strong pre-MCV forcing aloft.  A conditional threat for a
   well-organized severe event exists with this, and probabilities have
   been raised a level over much of the area.  These may need further
   refinement as mesoscale trends and 12Z and later guidance come into
   better focus. 

   2.  Scattered to numerous thunderstorms are possible from late
   afternoon into tonight, in a corridor from eastern OK across the
   Ozarks, ultimately building into parts of TN/KY overnight.  Large,
   perhaps very large/significant hail will be possible, especially
   relatively early in the convective period with more-discrete cells.
   A few tornadoes and scattered damaging wind (some potentially
   exceeding hurricane force) may occur as activity grows upscale and
   shifts eastward to northeastward overnight. 

   Convection should develop rather fast over the eastern OK/western AR
   region late this afternoon and evening, in a regime of steep
   EML-driven lapse rates aloft, rich boundary-layer moisture, and a
   prefrontal low-level convergence zone gradually overcoming capping
   at the base of the EML.  Modified forecast soundings suggest surface
   dew points in the upper 60s F and 8-8.5 deg C/km midlevel lapse
   rates contributing to MLCAPE commonly 2500-3500 J/kg, with 30-40 kt
   effective-shear magnitudes supporting a blend of multicells and
   supercells.  Buoyancy will diminish gradually with northeastward
   extent into the Ohio Valley, but still more than sufficient for
   sustaining a complex of severe thunderstorms with severe wind,
   sporadic large hail and a few tornadoes. 

   These two most-probable regimes also may be preceded tonight across
   parts of the Ohio Valley, by hail-producing thunderstorms occurring 
   in a corridor of strong low-level warm advection and moisture
   transport. 

   ...Southern High Plains to Red River region... 
   Isolated to widely scattered thunderstorms are possible from late
   this afternoon through this evening across this corridor, moving
   rapidly eastward while offering large to very large hail and
   sporadic severe gusts.  

   This activity should develop along and behind the surface cold
   front, but nonetheless, with a ribbon of CAPE/shear parameter space
   for supercells, and enough inflow-layer moisture to support
   efficient (and potentially destructive) hail production from the
   most persistent convection.  The northerly flow component just
   behind the front will enable two favorable features atypical for
   post-frontal regimes in these parts:
   1.  Curved low-level hodographs with favorable vertical shear and
   high SRH for fast eastward-moving cells, despite the potential
   displacement of the entire hodograph east of the meridian. 
   Effective SRH and deep shear may vary greatly in short distances,
   given the flow geometry and narrowness of deepest buoyancy, but peak
   values around 200-300 J/kg and 50-60 kt are possible, respectively.
   2.  Temporary moisture advection and vertical mixing leading to an
   axis of surface-based to nearly surface-based instability,
   ultimately shunted southward via boundary-layer cold advection, but
   with an axis of 500-1500 J/kg MLCAPE from extreme east-central NM
   across the South Plains to northwest TX.  Any convection lasting
   into mid/late evening also may reach much richer moisture tonight
   over north-central TX and adjoining parts of extreme southern OK. 
   Some progs indicate an ultimate merger between this activity and the
   trailing part of the eastern/southeastern OK regime discussed above.

   ...AL to Carolinas corridor...
   Scattered thunderstorms are expected to develop throughout the day
   in a corridor of relatively maximized low-level lift, from
   central/eastern AL across northern GA into portions of the
   Carolinas.  Sporadic large hail, damaging gusts and a couple
   tornadoes are possible.  

   A messy mixture of multicellular and supercellular modes is
   possible, given the presence of favorably strong mid/upper winds but
   general/gradual veering of surface winds, with only around 30-40 kt
   effective-shear magnitudes.  Moisture is expected to be rich, with
   surface dew points already upper 60s to near 70 F over southern
   parts of the area, and forecast to increase northeastward through
   moist advection.  Cloud cover will slow diabatic heating, but a
   combination of weak heating and warm advection -- in an already
   weak-CINH regime -- should lead to MLCAPE values in the 1000-1500
   J/kg range around the south and east edges of most of the main
   convective plume -- somewhat higher over AL where lapse rates aloft
   will be stronger but deep-layer forcing slightly weaker as a
   trade-off.  The overall threat should diminish this evening as
   activity moves southeastward toward southern GA and the SC Coast,
   though sporadic damaging gusts and isolated hail still will be
   possible.

   ..Edwards/Jewell.. 05/03/2021

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