SPC AC 280555

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1255 AM CDT Fri Apr 28 2023

   Valid 281200Z - 291200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS CENTRAL
   TEXAS...

   ...SUMMARY...
   Scattered storms are forecast to produce damaging hail and severe
   winds from late afternoon through evening from north-central Texas
   to the middle and lower Rio Grande Valley. Isolated strong to severe
   storms may also occur over parts of the Southeast into the central
   Appalachians.

   ...Synopsis...
   Latest water-vapor imagery and analyses depict a mid-level shortwave
   trough undergoing slight deepening over the central Rockies. This is
   within a strong cold advection regime, behind a southward-surging
   cold front over the Rockies and central High Plains. Both of these
   features are expected to continue southward over the next 24 hours,
   with the upper trough likely reaching northern Mexico and the
   attendant cold front sweeping into central TX by around 21 UTC this
   afternoon. Additionally, a second, less intense, upper perturbation
   is noted over the mid-MS Valley migrating to the east/northeast. A
   weak surface low associated with this feature is forecast to lift
   into parts of the upper OH Valley, with modest lee cyclogenesis east
   of the Appalachians. Thunderstorm development under the upper low
   and in the vicinity of the lee low appears probable around peak
   heating. 

   ...Central to southern Texas...
   00 UTC soundings from across the High Plains/Rockies sampled steep
   mid-level lapse rates in excess of 8 C/km. This corroborates recent
   RAP analysis that show the lapse rate plume extending over NM into
   west TX. Northwesterly 700-500 mb mean flow will advect these lapse
   rates into central TX over the next 12-18 hours. At the surface,
   modest moisture advection is occurring over central TX to the
   coastal plain, amid weak surface pressure falls over west TX. Mid
   50s to low 60s dewpoints should migrate into central TX by early
   afternoon. This moisture return will become more pronounced through
   the morning hours as the surface low deepens, and shifts eastward in
   tandem with the approaching upper wave. This combination of
   increasing low-level moisture and steepening mid-level lapse rates
   should establish a corridor of 3000-3500 J/kg MLCAPE from southern
   to central TX by early to mid afternoon. The positive tilt of the
   upper trough over southern NM will spread 50-60 knot zonal mid-level
   winds over the warm sector, with effective bulk shear values
   approaching 50 knots. 

   Diurnal warming, combined with broad ascent in the vicinity of the
   surface low over central TX, may allow for convective initiation as
   early as 18-20 UTC, near a diffuse triple point and/or along a
   sharpening dryline roughly parallel with the I-35 corridor. Initial
   storm modes will likely be discrete supercells, given favorable flow
   orientation away from the zone of initiation and elongated
   hodographs. These early cells will pose a risk for large to very
   large (2+ inch) hail as well as a tornado threat - especially in the
   vicinity of the surface warm front where low-level helicity will be
   maximized. However, the window/longevity for discrete supercells is
   uncertain. Recent surface observations across the High Plains show a
   faster frontal progression than depicted by most guidance. As such,
   the arrival of the cold front into central TX may foster rapid
   upscale growth into a squall line faster than expected by 00z CAMs.
   The primary hazard is expected to quickly transition to severe wind
   once the MCS becomes established and propagates southeastward
   towards the TX Coastal Plain. A strong line-orthogonal deep-layer
   shear component suggest this MCS may be fairly progressive and
   capable of a few 75+ mph wind gusts. However, the eastward and
   westward extent of the wind threat is uncertain, given a tight
   buoyancy gradient across east TX and the potential for frontal
   undercutting across south-central TX.

   ...Upper OH River Valley to the Mid-Atlantic...
   Daytime heating under the upper-low should bolster MLCAPE values to
   500-1000 J/kg across the upper OH Valley region and into the
   Mid-Atlantic by peak heating this afternoon. While mid-level lapse
   rates will be meager, deep-layer shear values on the order of 25-30
   knots should support some storm organization with an attendant
   hail/damaging wind threat. Damaging gusts may be more common across
   parts of the Mid-Atlantic where daytime highs in the mid 70s will
   support steep lapse rates in the 0-1 km layer that may allow for
   more efficient downdraft acceleration. 

   ...Florida...
   A convective cluster ongoing over the central Gulf of Mexico is
   forecast to migrate eastward, reaching the FL Peninsula by around 12
   UTC. Diurnal warming ahead of this cluster of storms should support
   increasing buoyancy with MLCAPE values reaching near 1500 J/kg by
   mid-afternoon. 30-40 knot mid-level flow on the southern periphery
   of the eastern wave will allow for some elongation of the deep-layer
   hodograph conducive for some organization of semi-discrete
   cells/clusters. The threat for severe hail/damaging winds will
   likely be greatest across the eastern FL coast, where diurnal
   warming will be maximized ahead of any convection.

   ..Moore/Darrow.. 04/28/2023

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