SPC AC 010730

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0230 AM CDT Sat May 01 2021

   Valid 031200Z - 041200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE MONDAY TROUGH
   MONDAY NIGHT ACROSS PARTS OF THE LOWER OHIO VALLEY INTO OZARK
   PLATEAU AND ADJACENT PORTIONS OF THE SOUTHERN GREAT PLAINS...

   ...SUMMARY...
   Severe thunderstorms are possible, particularly Monday evening into
   the overnight hours, in a corridor from the the lower Ohio Valley
   through the Ozark Plateau and adjacent portions of the southern
   Great Plains.  Some of this activity may pose a risk for large,
   damaging hail and strong wind gusts.

   ...Synopsis...
   Models suggest that the leading edge of large-scale mid-level
   ridging, in a southern branch of split mid-latitude westerlies, may
   begin to build inland across California and Baja during this period.
   As this occurs, initially amplified downstream troughing is forecast
   to shift east of the Four Corners states and northern Mexican
   Plateau.  Rather than as one strong, consolidated perturbation, it
   appears that a series of smaller-scale impulses will emerge, but the
   broader cyclonic regime may come in phase with troughing in the
   northern branch, east of the Rockies into an increasingly confluent
   regime across and east of the Mississippi Valley.

   While there is model consensus, in a broad sense, concerning this
   general pattern evolution, quite a bit of spread is evident among
   the output concerning the synoptic and sub-synoptic scale
   developments.  However, it does appears that a frontal zone within
   broad surface troughing will progress east-southeast of the Upper
   Midwest/Great Lakes region and central Great Plains, preceded by an
   influx of seasonably high boundary-layer moisture content, on
   seasonably strong west-southwesterly low-level flow.

   ...Ohio Valley/Ozark Plateau and adjacent southern Great Plains...
   Due to the large spread among the various deterministic model
   output, placement of severe probabilities is largely based on the
   overly smoothed out mean ensemble output of the various models. 
   However, there appears reasonable agreement that a general corridor
   from eastern Oklahoma into the lower Ohio Valley may become a focus
   for intense convective development by early Monday evening and
   continue into the overnight hours.

   Of particular concern, it appears that moderate to strong potential
   instability will develop across this region during the day, as mid
   60s to near 70F surface dew points advect northeastward ahead of the
   front, beneath east-northeastward advecting elevated mixed-layer
   air. This likely will occur in the presence of at least 30-50 kt
   west-southwesterly deep layer mean flow, with more than sufficient
   deep-layer shear to support organized convective development.  A few
   supercells are possible, but large-scale forcing for ascent
   associated within interacting perturbations within the flow probably
   will support rapid upscale growing clusters.  

   By Monday evening, it is possible that the most significant impulse
   emerging from the Southwest may be a prominent contributor to this
   forcing, across the Ozark Plateau vicinity, before spreading toward
   the lower Ohio Valley.  Given the environment, large, damaging hail
   appears possible, particularly in the early storm development, with
   the risk for severe wind gusts becoming more prominent as activity
   consolidates and organizes.

   ..Kerr.. 05/01/2021

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