SPC AC 241240

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0640 AM CST Wed Nov 24 2021

   Valid 241300Z - 251200Z

   ...NO SEVERE THUNDERSTORM AREAS FORECAST...

   ...SUMMARY...
   Thunderstorms are possible this evening and tonight from parts of
   the southern Plains into the mid Mississippi Valley.

   ...Synopsis...
   The mid/upper-level pattern over the CONUS will be dominated by a
   positively tilted mean trough, extending from a progressive cyclone
   over norther Hudson Bay, across the northern Plains, central High
   Plains and Four Corners, to northern Baja.  The northern part, from
   the Rockies northeastward, will contain numerous vorticity maxima
   and small, progressive shortwaves contributing to the net eastward
   shift of the synoptic trough through the period, across the
   northern/central Plains and toward the mid/upper Mississippi Valley.
    
   Farther southwest, one southern-stream shortwave trough -- now over
   eastern AZ and Sonora -- will extend from south-central KS to far
   west TX by 00Z, weakening thereafter as it ejects northeastward. 
   The next substantial shortwave trough upstream will develop this
   evening over portions of CA/NV, digging south-southeastward
   overnight.  By 12Z tomorrow, a closed 500-mb low should develop over
   northern Baja, with an intervening split-flow/col pattern developing
   over parts of AZ/NM.  In between the shortwave troughs, showers and
   perhaps brief/small cumulonimbi may occur this afternoon in weak
   low-level moisture, as higher terrain is heated over parts of
   southeastern AZ around the Chiricahua Mountains to the NM bootheel
   and southward into MX.  While a rogue lightning flash cannot be
   ruled out, thunder potential appears too conditional, isolated and
   unorganized for a general/10% area. 

   At the surface, 11Z analysis showed a cold front from far northern
   ON through a low near STC, southwestward to another low near GBD,
   then across southern CO. This front will move southeastward to near
   a DBQ-MKC-PNC-CDS-CVS line by 00Z.  By 12Z tomorrow, the front
   should reach northeastern Lower MI, southern IL, north-central AR,
   and northeast to south-central TX and northern Coahuila. 

   ...Southern Plains to Mid Mississippi Valley...
   Isolated to scattered thunderstorms are expected to develop this
   evening and overnight in two primary regimes:
   1.  Along and behind the cold front, where the boundary (surface
   through low levels) impinges on a progressively modifying/
   moistening warm sector.
   2.  A belt of broad low-level confluence/convergence ahead of the
   front in the warm sector, with weak CINH, related to the
   return-flow/warm-advection regime.

   Considerable veering with height of low-level flow is expected,
   along with strong upper/anvil-level winds.  However, midlevel flow
   weaknesses generally will keep effective-shear magnitudes below 35
   kt, based on forecast soundings sampling warm-sector/surface-based
   environments and elevated buoyancy behind the front.  A northward-
   narrowing wedge of 60s F surface dew points should reach eastern OK
   and perhaps the western Ozarks prior to frontal passage.  Beneath
   modest mid/upper-level lapse rates, this will support MLCAPE in the
   500-1000 J/kg range over central/north TX, decreasing to less than
   300 J/kg over northwestern AR.  Similar values of MLCAPE are
   expected just behind the front at the same latitudes.  The most
   intense cells may produce small hail or strong gusts, but severe
   potential appears too minimal for a categorical area at this time.

   ..Edwards/Jewell.. 11/24/2021

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