SPC AC 251251

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0751 AM CDT Mon May 25 2020

   Valid 251300Z - 261200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF WEST
   TEXAS TO PARTS OF SOUTH TEXAS...

   ...SUMMARY...
   The main severe-thunderstorm threat will be this afternoon into
   tonight, across parts of west, central and south Texas.

   ...Synopsis...
   In mid/upper levels, a high-amplitude, synoptic-scale trough will
   move eastward from the Rockies across the Plains States through the
   period, in between regimes of strong mean ridging over the East and
   zonal flow across the Pacific Northwest.  Several shortwave troughs
   of varying amplitudes will traverse the cyclonic flow near the
   trough, and southerly to southwesterly flow farther east.  Among
   them is a well-defined shortwave trough -- now apparent in
   moisture-channel imagery over NM, the ELP area and northern
   Chihuahua.  This feature will pivot east-southeastward to the
   eastern border of TX/NM and the Big Bend area by 00Z.  The trough
   will proceed eastward to northwest/central TX and Coahuila
   overnight, perhaps acquiring additional, convectively generated
   vorticity.

   Downstream, a series of lower-amplitude perturbations, including
   MCVs, will move north-northeastward across the lower elevations of
   the Plains States, lower/middle Missouri Valley, and Upper Midwest.
   A separate shortwave trough -- now located over the extreme east-
   central/northeastern Gulf near FL -- will eject northeastward over
   FL through the period, likely deamplifying in the process.

   The surface analysis showed a low near LSE with slow-moving cold
   front southwestward across eastern KS, western OK and the TX South
   Plains. The Midwest segment of the front should become
   quasistationary, while a low develops over OK near an MCV, joins the
   front, and moves northward to eastern KS.  A dryline over far west
   TX will move slightly eastward to the lower Pecos Valley region this
   afternoon. 

   ...TX...
   Scattered thunderstorms are expected today across parts of
   west/southwest TX east of the front and dryline.  A mix of
   multicells and messy supercells is expected, offering a threat for
   all severe types.  Upscale aggregation into another wind-dominant,
   expansive MCS is anticipated across southwest TX into the Edwards
   Plateau, Hill Country and Rio Grande Valley regions around DRT.  The
   complex should move/expand across parts of south-central, deep south
   and middle coastal areas of TX through the overnight hours. 
   Considerable uncertainty exists on how far the MCS will progress
   before 12Z tomorrow, and how severe it may remain approaching the
   nocturnal minimum in surface-based instability, but this outlook
   accounts for the faster scenarios and stronger storm-scale ascent
   that often verify for forward-propagational complexes. 

   The prior day's major MCS has left a considerable decrease in
   boundary-layer moisture quality and depth across the outlook area
   compared to previous days.  However, with a weaker EML: 
   1.  Less MLCINH and theta-e will be needed to support deep
   convective development, and
   2.  Forecast soundings suggest existing/remnant moisture, moist
   advection from lower elevations of northeastern MX into southern
   parts of the outlook area, along with strong and sustained diurnal
   heating under a high sun angle and many hours of clear to partly
   cloudy conditions, still can yield favorable buoyancy.  Peak
   preconvective MLCAPE should range from around 1000 J/kg over the
   South Plains to around 3000 J/kg on the Rio Grande near DRT.  This
   will support afternoon thunderstorm development over parts of the
   lower Pecos Valley/Permian Basin regions, perhaps extending to the
   southern South Plains and/or northern Coahuila.  

   Vertical shear, though not intense anywhere, may be relatively
   maximized nearer to the Rio Grande, beneath a 500-250-mb speed max
   basal to the trough.  Return-flow moisture advection into south TX
   tonight, both at and above the surface in low levels, will help to
   sustain the complex, by which time cold-pool-forced lift will be the
   main mechanism for maintaining the convection.

   ...Ozarks to upper MI...
   Widely scattered to scattered thunderstorms in clusters should form
   today into this evening, offering locally strong-severe gusts. 
   Within this broader corridor, activity may be concentrated on the
   mesoscale by a combination of pockets of sustained surface-based
   heating-destabilization, with MCV-related mesoscale enhancements to
   1) large-scale UVV, 2) low-level convergence and 3) low/midlevel
   winds above the surface.  Forecast soundings suggest that, away from
   persistent cloud cover, 1500-2500 J/kg MLCAPE may develop.  In such
   a setting, isolated/short-lived supercells cannot be ruled out. 
   However, general lack of both stronger lapse rates and deep shear
   will render the severe threat marginal in intensity, isolated in
   coverage and sporadic temporally. 

   ...Southern FL...
   Ahead of the shortwave trough, an MCV is evident over the Straits,
   offshore from the Upper Keys, moving northward.  To its north,
   low-level convergence and shear should be relatively maximized in
   the associated mass response, overlapping broader/weaker ascent
   related to the Gulf shortwave.  This will continue to encourage
   scattered to numerous showers and widely scattered/embedded
   thunderstorms, amidst weak lapse rates and modest CAPE, but also
   weak MLCINH, very rich moisture, and low LCL.  A few cells may
   exhibit transient storm-scale rotation amidst 100-200 J/kg effective
   SRH, and a tornado cannot be ruled out.  Isolated strong-severe
   downdrafts also are possible.  This activity should spread northward
   up the coast of southeastern FL through afternoon.  Additional
   development is possible this afternoon and evening over the western
   Straits and Keys, once the mesoscale area of downward vertical
   motion behind the MCV has passed, though this convection should
   occur amidst substantially weaker low-level flow/shear than the
   initial/MCV-related episode.

   ..Edwards/Smith.. 05/25/2020

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