SPC AC 241257

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0757 AM CDT Sun May 24 2020

   Valid 241300Z - 251200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM SOUTH TEXAS
   TO EASTERN COLORADO AND THE UPPER MISSISSIPPI VALLEY...

   ...SUMMARY...
   Severe thunderstorms, with large hail and damaging wind, are
   possible today into this evening, in a broad swath from south Texas
   to eastern Colorado and the upper Mississippi Valley.

   ...Synopsis...
   In mid/upper levels, mean troughing is forecast to shift eastward
   over the Rockies through the period, as numerous shortwaves pivot
   through the associated cyclonic-flow field.  This process will
   involve height falls across the Great Plains, and eastward spread of
   southwesterlies across most of the nation between the Rockies and
   Mississippi Valley.  However, loosening of the height gradient will
   occur, conterminous with the eastward shift, resulting in little or
   no net increase in deep/speed shear over most of the central CONUS. 
   An embedded shortwave trough -- now located over UT, should shift
   eastward across CO through the period.

   Farther south, a basal/southern-stream perturbation -- well-evident
   in moisture-channel imagery over the AZ/NM border region southward
   over northwestern MX -- will pivot eastward to the Permian Basin by
   00Z.  This feature should acquire convectively generated vorticity
   in the latter half of the period and amplify as it moves toward
   central/south-central TX.  Another southern-stream shortwave trough
   was apparent from central/southern AL southward across the Gulf to
   the eastern Yucatan Peninsula/western Yucatan Channel region.  This
   trough, and associated/embedded vorticity maxima, will move slowly
   eastward to northeastward through the period.  

   Surface analysis showed a low near HON with cold front southwestward
   to a weak low near HLC, then across parts of the OK Panhandle and
   northeastern NM.  This front is expected to progress eastward to the
   upper Mississippi Valley, central KS, and southeastern NM through
   the period.  A dryline will oscillate eastward today to the TX
   Panhandle and Permian Basin region, southward into Coahuila. 

   ...Southern Plains...
   As the basal shortwave trough approaches, large-scale ascent will
   increase over the dryline this afternoon.  This will coincide with a
   rapid decrease in MLCINH from strong diurnal heating of a favorably
   moist air mass.  The result should be scattered to numerous
   thunderstorms forming near the dryline, in a north-south swath from
   the TX Panhandle to the Serranias del Burro of northern Coahuila. 
   Severe gusts and hail -- some of the hail being significant/2+
   inches in diameter -- are possible in the first few hours.  The
   threat should transition to predominantly wind as the activity
   aggregates into a more dense line with embedded bow/LEWP formations
   and possibly localized rear-inflow jet development.  Convection
   should become dominated by forward propagation and move eastward to
   the lower Plains of western OK, northwest TX, the Big Country,
   Concho Valley, Edwards Plateau, Hill Country, and Rio Grande Valley
   between DRT-LRD.  Convection growing upscale from high-terrain
   Mexican development may extend as far as parts of deep south TX and
   the middle/lower TX Coastal Plain tonight, while still at or near
   severe levels.

   Weak midlevel flow will limit deep shear, with effective shear
   generally under 25 kt over most of the area, indicating
   predominantly multicellular characteristics.  However, storm-
   scale/boundary interactions amidst the strong buoyancy may
   contribute to isolated, heavy-precip supercellular modes while
   convection still is relatively separated in western parts of the
   outlook area.  Surface dew points should recover from south to north
   behind the previous day's outflow, reaching the 60s east of the
   dryline, with 70s southward through the Rio Grande and Coastal
   Plain.  With steep midlevel lapse rates and a deep troposphere, peak
   preconvective MLCAPE values of 3500-4500 J/kg will be common.  

   ...Eastern/northern KS to upper Mississippi Valley...
   An ongoing MCS should continue to be largely subsevere for a few
   more hours as it moves along and north of a warm front analyzed
   across southern IA, with a boundary trailing from the tail end of
   the complex into southeastern NE and northern KS.  The boundary
   should decelerate through the remainder of the morning.

   Scattered thunderstorms are expected to form this afternoon
   along/ahead of the cold front and residual boundary from the morning
   convection, offering the potential for large hail and damaging
   gusts.  Activity should develop in a plume of favorable low-level
   moisture and diabatic surface heating, amidst diurnally weakening
   MLCINH.  Steep midlevel lapse rates and surface dew points generally
   in the 60s F will contribute to 2500-3000 J/kg MLCAPE, but with
   modest low/middle-level flow limiting vertical shear.  The
   predominant mode should be multicellular, with brief supercell
   characteristics possible.  MCVs moving northeastward from their
   current positions over northern and central OK may strengthen
   vertical shear and UVV on the mesoscale, in support of convective/
   severe potential across this area.  However, by contrast, related
   swaths of cloud cover and precip will inhibit diurnal heating. 
   Considerable localized variability in convective intensity and
   severe potential will exist within the corridor from KS to IA.

   ...Central High Plains...
   Widely scattered to scattered thunderstorms should form from midday
   into early afternoon across portions of the eastern mountains,
   foothills, and adjoining higher terrain of the Palmer Divide and
   Raton Mesa.  Isolated severe downdrafts are possible with the
   best-organized convection within a longer plume of precip.  

   Post-frontal northerlies will veer to some component of easterly
   across eastern CO through the afternoon, north of a weak low over
   the Raton Mesa area, as the mid/upper-level shortwave trough slowly
   approaches.  Some progs indicating a confluence/convergence axis
   near the Arkansas River or Palmer Divide.  Associated upslope flow,
   along with heating of higher terrain and accompanying erosion of
   MLCINH, will support convective development from midday into the
   afternoon.  The continued easterly components will aid storm-
   relative low-level flow, and strong directional shear will set up
   beneath mid/upper southwesterlies, yielding 40-45-kt effective-shear
   magnitudes.  Buoyancy will be restricted by a lack of more-robust
   low-level theta-e; however, modified RAOBs and forecast sounding
   suggest effectively surface-based inflow for at least a few hours,
   as activity propagates off the higher terrain and over the eastern
   CO Plains.  300-600 J/kg MLCAPE is possible atop a diurnally
   well-mixed subcloud layer, in support of localized strong-severe
   gusts.

   ...FL Keys...
   Areas of precip and embedded thunderstorms are expected to affect
   the Keys/southern Everglades region through the period, offering
   primarily a heavy rain threat (see WPC excessive-rain outlooks for
   more guidance on that potential).  A marginal tornado threat also
   may develop tonight, within a very moist, low-LCL environment with
   increasing low-level shear.  As the shortwave trough slowly crosses
   the east-central/northeastern Gulf, preceded by divergence aloft,
   and a low-level tropical wave drifts westward over the northwest
   Caribbean/Yucatan region (per NHC tropical weather discussion),
   low-level easterly gradient flow is expected to increase, especially
   just above the surface.  Time series of forecast soundings suggest
   that resultant enlarging of curved low-level hodographs may yield
   enough SRH to support rotation in some cells tonight.

   ..Edwards/Smith.. 05/24/2020

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