SPC AC 171956

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0256 PM CDT Sat Jun 17 2023

   Valid 172000Z - 181200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF THE SOUTHERN HIGH PLAINS...EXTENDING INTO THE SOUTHERN PLAINS AND
   THE LOWER MISSISSIPPI VALLEY...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PARTS OF
   WESTERN INTO CENTRAL IOWA...

   ...SUMMARY...
   Severe thunderstorms, capable of producing damaging gusts, large
   hail, or tornadoes, are possible over a vast area from Colorado to 
   Florida, and central Texas to the lower Missouri Valley. One or more
   damaging wind swaths may occur across the southern Plains into the
   lower Mississippi Valley this evening into tonight.

   ...20Z update...
   The enhanced risk was expanded southward and eastward to cover much
   of northern Texas into the lower MS Valley. An elongated
   west-to-east corridor of strong to potentially extreme instability
   is becoming established, from parts of western TX into MS, with the
   latest mesoanalysis showing up to 5000 J/kg MLCAPE in spots, with
   nearly the entire southern CONUS overspread by strong mid-level flow
   and associated 50+ kts of effective bulk shear. At least one or more
   MCSs are expected to develop from upscale-growing supercells and/or
   multicellular clusters tonight, and rapidly propagate eastward in
   the aforementioned favorable environment. Some high resolution
   guidance shows one MCS traversing central/northern OK, with another
   MCS moving across northern/central TX into the lower MS Valley. Any
   of the MCSs that organize will have the potential to produce an
   extensive swath of damaging gusts, and at least a few instances of
   65+ kt gusts are possible. Still, details in MCS evolution, from
   northern OK, to central TX, remains nebulous, and it is unclear
   which MCS would dominate and have the potential to become
   long-lived. As such, within the broader Category 3/Enhanced risk
   area, there may be a gap in severe potential. Similarly, confidence
   is also too low to introduce Category 4/Moderate Risk probabilities
   this outlook. Nonetheless, the existence of a widespread area of
   favorably overlapping strong shear/CAPE poised to experience
   scattered severe storms with some upscale growth suggests that
   broader Enhanced-Risk probabilities are warranted.

   Significant-severe hail probabilities in the Southeast were extended
   farther west into southern MS to account for the possibility of 2+
   inch diameter hail occurring with ongoing supercells in
   central/southern MS. Here, MRMS mosaic radar data shows MESH cores
   already approaching 2 inches in spots. The rest of the outlook
   remains largely on track, with only slight adjustments made to
   probabilities to reflect the latest observations or guidance
   consensus.

   ..Squitieri.. 06/17/2023

   .PREV DISCUSSION... /ISSUED 1140 AM CDT Sat Jun 17 2023/

   ...Southeast CO to Arkansas...
   Morning water vapor imagery shows a well-defined shortwave trough
   over southwest CO.  This feature will cross the Rockies and begin
   affecting the High Plains this afternoon, promoting scattered
   thunderstorms off the high terrain and foothills.  Initial supercell
   storms over southeast CO will be in a region of steep mid-level
   lapse rates and moderate CAPE, favorable for large hail.  As
   activity moves/builds eastward into southwest KS, the northern TX
   Panhandle, and northwest OK, it will encounter high CAPE values and
   increasingly strong low-level winds.  Supercells with very large
   hail, significant wind damage potential, and a few tornadoes may
   affect this area during the evening.  Upscale growth of this cluster
   of storms into a fast-moving bowing complex remains likely, with
   activity moving across northern OK into northwest AR tonight.

   ...North TX...
   A 50-60 knot westerly mid-level jet max is accompanying the
   aforementioned shortwave trough over CO.  This wind max will move
   across the TX Panhandle this evening, with enhanced forcing for
   ascent affecting the dryline over west TX.  This will lead to rapid
   supercell development in a very moist and unstable environment. 
   Very large hail and damaging winds are expected with the stronger
   cells.  Several CAM solutions favor splitting supercells, with
   left-movers tracking northeastward toward the Red River through the
   evening with a continued risk of significant severe.  Therefore have
   extended the ENH risk into these areas.

   ...MS/AL/FL...
   Morning model guidance shows a subtle shortwave trough currently
   over west TN tracking southeastward into AL, with an associated
   40-50 knot mid-level wind max moving across MS.  Strong daytime
   heating of a very moist boundary layer ahead of these features
   should result in scattered severe thunderstorm development by mid
   afternoon over parts of southeast MS, southern AL, and the FL
   Panhandle.  Given the strength of the deep-layer shear, supercell
   structures capable of large hail and damaging winds are expected.  A
   tornado or two is also possible as the storms approach the coast
   late this afternoon.  Activity should move offshore after dark.

   ...Western IA...
   A well-defined remnant MCV is noted this morning over
   central/eastern NE.  Southerly low-level winds ahead of this
   circulation will maintain near 70F dewpoints over western IA, where
   strong heating is occurring.  This should lead to scattered
   afternoon thunderstorms.  Forecast soundings show sufficient
   vertical shear for organized multicell or occasional supercell
   storms capable of hail and damaging winds.  Therefore have added a
   SLGT risk to this region for this afternoon and early evening.

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