SPC AC 230551

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1251 AM CDT Fri Apr 23 2021

   Valid 231200Z - 241200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON INTO TONIGHT ACROSS PARTS OF EASTERN TEXAS...MUCH OF
   CENTRAL AND NORTHERN LOUISIANA...ADJACENT SOUTHERN ARKANSAS AND
   SOUTHWESTERN MISSISSIPPI...

   ...SUMMARY...
   Severe thunderstorms are possible today across the southern Great
   Plains into the lower Mississippi Valley.  This may include the
   evolution of large, organized cluster of storms across the Piney
   Woods vicinity into the lower Mississippi Valley by this evening,
   which could produce a swath or two of damaging wind gusts and
   perhaps a couple of tornadoes.

   ...Synopsis...
   Deep mid-level troughing, with a developing embedded low, has
   emerged from the Arctic latitudes and is forecast to become
   entrenched across much of interior Canada during this period,
   downstream of a blocking mid-level high which has become centered
   across Alaska.  To the southeast of the low, a similar preceding
   perturbation, and associated deep surface cyclone, appear likely to
   gradually turn northward across and north of the Canadian Maritimes.


   As this lead system gradually loses influence on the Northeast,
   models indicate that a belt of westerlies emanating from the
   mid-latitude Pacific will continue to become a more prominent
   influence across much of the remainder of the United States.  This
   regime includes a significant short wave trough now turning eastward
   across parts of the Southwest and northwestern Mexico, as it comes
   in phase with a belt of westerlies emanating from the subtropical
   eastern Pacific. It appears that this feature will accelerate
   east-northeastward, across southern portions of the Great Plains
   into the lower Mississippi Valley by late tonight.

   In lower levels, while another intrusion of cold air surges south of
   the Canadian/U.S. border, to the east of the northern Rockies,
   through much of the northern Great Plains, potentially cold air
   associated with a preceding intrusion will, at least initially, be
   slow to lose influence across much of the Southeast into lower
   Mississippi Valley.  However, a substantive return flow of moisture
   is ongoing off the western Gulf of Mexico into the southern Great
   Plains, and this will tend to spread east-northeastward in advance
   of the impulse emerging from the Southwest/Mexican Plateau, and
   broad surface troughing developing eastward across the southern
   Great Plains.

   ...Southern Great Plains through the central Gulf states...
   Beneath steep mid-level lapse rates associated with elevated
   mixed-layer air, daytime heating is expected to contribute to
   moderately large mixed-layer CAPE, in at least a narrow corridor
   along a developing dryline from the eastern Texas Panhandle into the
   Hill Country vicinity by this afternoon.  Low clouds may slow
   destabilization within a developing wedge of returning
   boundary-layer moisture across central/southeast Texas into the
   north central Gulf coast vicinity, while trajectories emanating from
   surface ridging slow boundary-layer moistening across and northeast
   of the lower Mississippi Valley.

   Further complicating warm sector boundary-layer destabilization,
   various model output suggests that thunderstorm activity may
   initiate by midday east of the dryline, across parts of central
   Texas into the Texas coastal plain.  It appears that this may occur
   in response to forcing associated with a subtropical perturbation
   preceding the primary short wave trough, aided by lower/mid
   tropospheric warm advection, and guidance indicates a steady
   increase in thunderstorm development while spreading
   east-northeastward through the afternoon.

   Increasing inflow of at least weakly destabilizing boundary-layer
   air probably will support a gradual intensification and upscale
   growth in the presence of strong deep-layer shear.  This activity
   may tend to outpace the most substantive boundary-layer
   destabilization as it spreads into and through the lower Mississippi
   Valley by this evening.  However, of primary concern, most model
   output suggest that this convection will be accompanied by the
   development of a mesoscale area of low pressure, and substantive
   strengthening of wind fields in the 850-700 mb layer.  It appears
   that this will include speeds to 50-70+ kt.  Although forecast
   soundings suggest that neutral to stable near-surface lapse rates
   may persist across and particularly east-northeast of the Piney
   Woods/lower Mississippi Valley vicinity tonight, there still appears
   considerable potential for one or more swaths of potentially
   damaging wind gusts.  If low-level lapse rates are able to
   destabilize at least a bit more than currently forecast, a greater
   risk for tornadoes could also develop.

   It remains unclear what, if any, influence early convective
   development will have on potential for late afternoon thunderstorm
   development along the dryline, as well as along a zone of
   differential surface heating across western/northern Oklahoma into
   southwest Missouri.  However, at least scattered storm development
   is possible, in an environment that may become conducive to large
   hail and locally strong surface gusts.  The surface boundary
   intersection could become a focus for another organizing cluster of
   storms across southwestern/central Oklahoma this evening.

   ..Kerr.. 04/23/2021

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