SPC AC 271952

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0252 PM CDT Sun Apr 27 2025

   Valid 272000Z - 281200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR MUCH OF THE
   GREAT PLAINS...

   ...SUMMARY...
   Isolated to scattered severe thunderstorms are possible mainly this
   evening into early morning Monday across parts of the Great Plains.
   Large to very large hail, severe gusts, and a few tornadoes are all
   possible.

   ...20z Update...
   The previous forecast remains on track with no appreciable changes
   required. Signs of broad-scale ascent beginning to overspread the
   High Plains west of the surface trough/dryline are evident in latest
   GOES imagery. Shallow cumulus is beginning to develop from western
   NE southward into west TX, which suggests that convective initiation
   within this corridor should become more probable within the next few
   hours as we approach peak diurnal heating and dryline circulations
   are maximized. West Texas Mesonet observations are sampling higher
   dewpoints (by about 2-3 F) and stronger surface convergence than
   anticipated by most model solutions. This suggests that the
   probability of realizing the potent convective environment may be
   increasing across the TX Panhandle (though confidence in storm
   coverage remains too limited to warrant any upgrades). Further
   north, recent RAP analyses show the tightest low-level lapse rate
   gradient across western KS, indicative of a tighter/stronger dryline
   circulation. This trend supports some recent high-res guidance that
   shows convective initiation between 21-00z across this region and
   warrants maintaining the current risk probabilities. Similarly,
   steady surface pressure falls across western NE/SD suggest that
   ascent is strongest across the central High Plains region, which
   should favor at least isolated thunderstorm development this
   afternoon. See the previous discussion below for additional details.

   ..Moore.. 04/27/2025

   .PREV DISCUSSION... /ISSUED 1130 AM CDT Sun Apr 27 2025/

   ...Synopsis...
   Water-vapor imagery this morning shows a mid- to upper-level low
   over NV.  A belt of strong southwesterly mid to high-level flow will
   extend from the base of the western U.S. trough through the
   southern/central High Plains later today into tonight.  A pair of
   smaller-scale disturbances are forecast to eject east-northeastward
   into the central High Plains and Black Hills vicinity during the
   mid-late evening.  Concurrently, an upper ridge over the eastern
   half of the Great Plains states will shift eastward over the MS
   Valley towards daybreak Monday.  

   Late morning surface analysis shows a developing low centered over
   southeast WY with a lee trough/dryline extending southward through
   the southern High Plains.  The cyclone will deepen near the WY/NE/CO
   border through early evening before developing northeast into
   central SD late tonight.  

   ...Southeast MT...Western Dakotas...
   A sharpening lee trough is forecast by late this afternoon amidst
   appreciable heating and moistening, especially across western SD. 
   Model guidance shows a narrow plume of 500-2000 J/kg MLCAPE (highest
   over SD) developing by mid-late afternoon as large-scale ascent
   begins to overspread the region.  Isolated to widely scattered
   thunderstorms are forecast to develop from the northern NE Panhandle
   to the Black Hills, and north of the developing low over southeast
   MT by late afternoon.  The stronger storms will gradually move into
   richer moisture with an accompanying risk for mainly large hail and
   severe gusts.  Gradual cooling during the evening will favor a
   transition to elevated storms evolving with time as a LLJ
   strengthens tonight.  A severe risk will probably develop farther
   east in association with WAA into parts of the eastern Dakotas and
   MN late.

   ...Central/Southern High Plains...
   Visible-satellite imagery indicates a very moist boundary layer with
   a mix of a cumulus field across central and north TX and eastern
   NM/CO and stratus from the TX Caprock northward through OK to NE. 
   The depth and quality of moisture sampled by the 12z Midland,
   Amarillo, and Dodge City raobs is considered seasonably moisture
   rich, and is located beneath an elevated mixed layer.  Additional
   moistening through advection and evaporation will contribute to 60s
   dewpoints east of a dryline/lee trough by late afternoon.  Strong
   heating is forecast near the dryline from western NE southward
   through West TX, which will result in a weakly capped but moderate
   to very unstable airmass by 21z.  Large-scale forcing for ascent
   across the region will be nebulous, but low-level convergence along
   the dryline amid little to no convective inhibition may result in
   convective initiation. 

   With dryline convergence as the mechanism for convective initiation,
   predictability is low.  Dryline circulations will likely be
   strongest across the TX Panhandle/West TX where temperatures west of
   the dryline will be warmest, but low-level convergence will likely
   maximize in the CO/NE/KS border intersection vicinity near a
   developing surface low.  As a result, these two areas may have a
   greater risk for thunderstorms (isolated, widely spaced storm
   coverage) late this afternoon into the evening. 

   The overall environment will be very supportive of supercells with
   any storms that do form, with large to very large hail as the
   primary risk initially. If any storms manage to develop and become
   sustained, a strengthening low-level jet will support a tornado
   threat during the evening with any storms that persist (or develop
   along the retreating dryline).  Nocturnal cooling and increased CINH
   by mid to late evening will act to eventually weaken storm activity
   in this region.

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