SPC AC 010538

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1238 AM CDT Sat May 01 2021

   Valid 021200Z - 031200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SUNDAY INTO SUNDAY
   EVENING ACROSS PARTS OF SOUTHEAST TEXAS...SOUTHERN AND EASTERN
   LOUISIANA...AND MUCH OF MISSISSIPPI INTO ADJACENT PORTIONS OF
   SOUTHWESTERN TENNESSEE AND WESTERN ALABAMA...

   ...SUMMARY...
   A few severe thunderstorms are possible Sunday into Sunday evening
   across much of the lower Mississippi Valley.  Strong to severe
   thunderstorm development is also possible east of the Colorado Front
   Range into adjacent portions of the central Great Plains.

   ...Synopsis...
   Mid/upper ridging extending north-northeastward from the subtropical
   eastern Pacific into the western U.S. is becoming increasingly
   suppressed, as short wave perturbations within the westerlies
   emanating from the mid-latitude Pacific progress inland around its
   crest.  This includes one significant impulse on the southern fringe
   of the westerlies, which is forecast to migrate inland across the
   northern California coast early today, followed by another by late
   tonight.  It appears that the trailing impulse may dig more sharply
   southeastward into the Sierra Nevada vicinity by 12Z Sunday, in
   response to upstream large-scale ridge amplification.  During the
   day Sunday, through Sunday night, models indicate substantive trough
   amplification downstream of the ridging, across the southern Great
   Basin into the Four Corners states.

   In response to these developments, the remnants of a mid-level low
   emerging from the northern Mexican Plateau are forecast to
   accelerate from north central Texas into the lower Ohio Valley, with
   deamplifying troughing pivoting to its south/southeast, across the
   lower Mississippi Valley into the Cumberland Plateau/southern
   Appalachians by late Sunday night.  It appears that this feature
   will be accompanied by the inland (and northeastward) progression of
   surface troughing along the western periphery of a large surface
   ridge centered over the southern mid-latitude/subtropical western
   Atlantic.

   Primary surface cyclogenesis is forecast take place from
   southeastern Colorado through the Texas Panhandle, into northwest
   Texas/southwestern Oklahoma, beneath a broadening plume of warm and
   capping elevated mixed-layer air advecting eastward across much of
   the southern Great Plains by late Sunday night.  
    
   Farther north, a surface cold front is forecast to overtake surface
   troughing across the central Great Plains into Upper Midwest, as
   cold surface ridging noses southward to the lee of the northern
   Rockies, in association with mid-level troughing digging across the
   central Canadian/U.S. border area.

   ...Upper Texas coast/lower Mississippi Valley...
   If anything, latest model output appears a little less clear
   concerning severe weather potential for Sunday.  It still appears
   that potentially cool/dry boundary layer air across and
   east/northeast of the lower Mississippi Valley, associated with the
   Southeastern surface ridging, will substantively moisten beneath a
   north-northeastward developing low-level jet (around 30-40 kt at 850
   mb).  How quickly this occurs is becoming less certain, as moisture
   return likely will be initially elevated, and convection developing
   ahead of the approaching mid-level impulse may slow boundary-layer
   warming.  Still, with mid 60s+ F surface dew points forecast to
   spread inland across the lower Mississippi Valley vicinity, moderate
   destabilization beneath relatively cold mid-level air associated
   with the short wave trough appears possible.  If this occurs, a 50+
   kt mid-level jet streak will contribute to an environment conducive
   to organized convective development, including supercells. 
   Depending on how convection evolves across Louisiana into
   Mississippi, it is possible that the mid-level cold core could
   become a focus for strong to severe storm development across parts
   of central and southern Arkansas late Sunday afternoon and evening.

   ...Front Range into adjacent central Great Plains...
   Northeast of the larger-scale troughing digging into the Four
   Corners states, strongly difluent and divergent high-level flow may
   aid convective development within moistening post-frontal upslope
   flow to the east of the Colorado Front Range.  By mid to late Sunday
   afternoon, it appears that destabilization beneath steepening
   lower/mid tropospheric lapse rates may become sufficient to support
   severe storms capable of producing large hail along and north of the
   Palmer Divide.  As this convection tends to develop with favorable
   mid/upper support toward a dryline/cold front intersection across
   east central Colorado into western Kansas by Sunday evening,
   low-level thermodynamic profiles and shear may become increasingly
   conducive to an organizing convective system posing a risk for
   strong surface gusts.  Once lingering uncertainties concerning
   destabilization become better resolved, it is possible that higher
   severe probabilities will be needed in later outlooks for this
   period.

   ..Kerr.. 05/01/2021

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