SPC AC 271246

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0746 AM CDT Tue Apr 27 2021

   Valid 271300Z - 281200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
   SOUTHERN PLAINS...SOUTHWEST/SOUTH-CENTRAL TEXAS AND CENTRAL HIGH
   PLAINS...

   ...SUMMARY...
   Severe-thunderstorm potential appears greatest today and tonight
   over parts of the southern Plains, southwest/south-central Texas and
   central High Plains.

   ...Synopsis...
   The dominant upper-air feature for this forecast will be a well-
   defined, synoptic-scale trough -- evident in moisture-channel
   imagery from a cyclone NV southward over southern CA and the
   subtropical Pacific west of MX.  As a vorticity maximum and
   shortwave trough now offshore from southern CA pivots through the
   southern part of the trough today, the NV cyclone will weaken
   substantially and be replaced by another cyclone over the lower
   Colorado River Valley and adjoining northwestern MX.  The associated
   500-mb low should pass the TUS area by 12z.  By then, a strongly
   positively tilted mid/upper trough should extend from that low to
   central NE and southwestward over central Baja.

   In the swath of southeasterlies located east of the synoptic trough,
   two minor but still potentially influential shortwave perturbations
   are apparent:
   1.  Over AZ and northern Sonora, forecast to eject northeastward
   across the Four Corners today to the central High Plains by 00Z.
   2.  A more-subtle feature over Chihuahua, expected to cross
   west-central/northwest TX today and reach central/eastern OK this
   evening.

   At the surface, 11Z analysis showed a low over south-central NE,
   with quasistationary front extending to a weak, diffuse low over
   southern WI.  A cold front was drawn from the NE low across
   northeastern CO.  A dryline extended from the low over west-central
   KS, northwestern OK, the TX South Plains, to near the southeastern
   corner of NM, then to the Big Bend area of TX.  The NE low will move
   northeastward along the baroclinic zone and across portions of IA
   and the IL/WI border region, reaching Lower MI by 12Z.  A hybrid
   lee/frontal-wave cyclone will develop over eastern CO today, then
   shift eastward along a frontal inflection to west-central/central KS
   overnight.  The front will move little between the lows, but behind
   the western one, will move southward through CO overnight.  The
   dryline should shift eastward over parts of west-central TX and the
   eastern Panhandle this afternoon before backing into the Permian
   Basin and above the Caprock late tonight.

   ...Southern Plains...
   Multiple episodes of strong, occasionally severe thunderstorms are
   possible across portions of OK, northwest TX and perhaps
   south-central KS, from around mid-afternoon through the sunrise hour
   tomorrow.  Localized areas of scattered to numerous thunderstorms
   are possible, and modes could vary widely from a few discrete
   supercells to dense, messy multicell clusters and bands, with
   potential for embedded bows. LEWPs and perhaps embedded heavy-precip
   supercells.  Large hail is likely, with the greatest sizes being
   from relatively discrete convection prior to clustering and/or
   nearer the dryline.  Convection should develop initially east of the
   dryline -- in a combination of low-level/warm-sector confluence/
   convergence, broad warm advection, DCVA ahead of the ejecting
   Chihuahua perturbation, and somewhat weaker EML-related CINH -- then
   later into evening, on and near the retreating dryline.

   Considerable, primarily mid/upper-level cloud cover is forecast to
   linger over the area through much of the day, restricting diurnal
   heating and, by extension, mixing and lapse rates in the boundary
   layer.  Another factor potentially holding back greater severe
   potential and dryline-proximate convective coverage is the
   shallowness of the moist layer (only about 100 mb AGL, easily mixed
   out where sufficient heating can occur).  Still, late-afternoon
   thinning/clearing of cloud cover near the dryline may promote enough
   low-level UVV to initiate convection, as should the dryline's
   westward retreat this evening into higher elevations and somewhat
   greater large-scale ascent.  Meanwhile, warm-sector convection will
   have access to increasing low-level moisture, amidst 50-60-kt
   effective-shear vectors.  Directional shear may be small for much of
   the afternoon into early evening over OK, keeping hodographs
   somewhat small and erratic in shape, but with enough speed shear to
   support storm organization.

   With stronger capping possible southward ahead of the dryline, a
   relative min may develop in severe potential across parts of
   west-central TX -- between this area and the orographically
   influenced activity moving out of Mexico that is discussed below. 
   However, the parameter space still will favor severe (including
   all-hazard supercell potential) with any storm(s) that can develop
   near the dryline.  Confidence in a substantial gap still is too low
   to carve out a fuller corridor of lower unconditional probability at
   this time.

   ...Southwest/south-central TX...
   A separate area of convection is probable perhaps as soon as late 
   afternoon, but more likely this evening and tonight, after the
   departure of the shortwave ridging and some weakening of EML-related
   CINH behind the subtropical perturbation.  This activity mostly
   should form in the Serranias del Burro and adjacent foothills, in an
   area of upslope lift and strengthening mass response to the
   developing southern mid/upper cyclone.  This activity may include
   supercells with all severe hazards (including significant to
   isolated very large hail), and should increase in coverage as it
   shifts northeastward across the adjoining Rio Grande Valley, toward
   the southeastern Edwards Plateau and southern Hill Country.

   Backed, strengthening boundary-layer flow is expected this evening,
   as the southern part of the LLJ intensifies, contributing to both
   effective SRH above 250 J/kg and effective-shear magnitudes in the
   55-70-kt range.  With rich low-level moisture (surface dew points
   upper 60s to low 70s F in a deeper layer than further north) and
   steep midlevel lapse rates, peak preconvective MLCAPE may reach 2500
   J/kg and stay that high until well into the evening (03-06Z away
   from convection).

   ...Central High Plains...
   Scattered thunderstorms should develop this afternoon along and
   north of the front, in a zone of relatively maximized low-level
   convergence north through northeast of the surface low.  The main
   concern will be large hail, with a few surface-based and elevated
   supercells expected.  However, isolated severe gusts and a marginal
   tornado threat are possible in a narrow, roughly zonal corridor
   along and just north of the boundary itself, where surface-based
   effective-inflow parcels can be realized.  Strong directional shear
   will contribute to effective-shear magnitudes mostly 35-45 kt,
   modulated by modest buoyant depth with MUCAPE around 500-1000 J/kg. 
   Curvy, somewhat hook-shaped hodographs of modest size are apparent
   in forecast soundings, but localized enhancements very near the
   boundary are probable.  The threat should diminish considerably in
   the 03-06Z time frame as instability weakens.

   ..Edwards/Jewell.. 04/27/2021

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