SPC AC 281246

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0746 AM CDT Wed Apr 28 2021

   Valid 281300Z - 291200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   SOUTHWEST TO NORTH-CENTRAL TEXAS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   ENHANCED RISK...AND ALSO OVER PARTS OF THE LOWER GREAT LAKES
   REGION...

   ...SUMMARY...
   Severe thunderstorms are possible across parts of the southern
   Plains to southwest Texas this afternoon and evening. Other severe
   storms may develop in parts of the lower Great Lakes region.  In
   both areas, the most intense thunderstorms may produce large hail
   and wind damage.  A tornado threat also may develop in the southern
   Plains and southwest Texas.

   ...Synopsis...
   A synoptic-scale split-flow pattern will dominate mid/upper levels
   over most of the CONUS, with a through-cutting northern stream from
   the Canadian Rockies across the northern Plains, Great Lakes and
   Northeast.  A strong, positively tilted trough is evident from the
   central High Plains through a cyclone over AZ and across central
   Baja.  The cyclone should shift eastward to eastern NM and far west
   TX through the period, with the trough extending from KS across
   northwestern MX.  Downstream, a shortwave trough -- apparent in
   moisture-channel imagery over the upper Mississippi Valley region --
   will cross the upper Great Lakes region through midday, penetrating
   the mean ridge, then crossing the lower Great Lakes and
   northern/inland Mid-Atlantic midday through afternoon, with some
   deamplification.

   At the surface, 11Z analysis showed weak lows near BGM, MKG and RFD,
   along a wavy/quasistationary frontal zone. A cold front extended
   from the northern IL low across northern MO, south-central KS, the
   TX Panhandle, and central NM.  A dryline extended from the front
   over the Panhandle southward across the Permian Basin to eastern Big
   Bend region.  That segment of the front from MO across the southern
   Plains is forecast to make a net southeastward shift through the
   period, while moving more slowly/erratically east of the Mississippi
   River.  By 00Z, the front should be located across western NY, the
   Lake Erie area, northern portions of IN/IL, western MO, southwestern
   OK, and the Permian Basin region.  From north-south, the front will
   slowly overtake both the dryline and a prefrontal convective
   boundary discussed below.

   ...Southern Plains to Southwest TX...
   A broken and episodic belt of thunderstorms is expected through much
   of the period from the Ozarks region across OK to southwest TX.  See
   tornado watch 116 and latest mesoscale discussion (446 at outlook
   time) for near-term guidance.

   The greatest overall severe threat and most-favorable parameter
   space should develop this afternoon and evening from north TX to the
   Rio Grande Valley near DRT.  In that corridor, a few tornadoes are
   possible, along with large hail and damaging winds.  Some of the
   hail be significant (2 inches or larger diameter), especially in the
   Rio Grande to southern Hill Country area, where very deep, well-
   organized supercells are possible with hailstones to near 4 inches
   in diameter.

   An outflow/convergence boundary, much of which still had reinforcing
   convection to its west, was analyzed from northwestern AR across
   east-central/south-central OK and north-central TX (west of the
   Metroplex), north of SJT and near OZA and 6R6.  Some westward/
   northwestward retreat of the activity/outflow boundary is probable
   today, especially during mesoscale spatiotemporal lulls in
   convection to its west.  Increasing large-scale ascent and mass
   response to the approaching mid/upper cyclone indicate an enhanced
   severe threat today across this region -- both near the boundary and
   with more unconditional potential for late-afternoon/evening
   supercell development moving out of the Serranias del Burro region
   and across the Rio Grande.

   Low-level lift and vorticity should be maximized near the boundary,
   locally enlarging hodographs within an already favorable broader-
   scale CAPE/shear regime for supercells. Diurnal heating along and
   east of the boundary, rich low-level moisture (surface dew points
   commonly upper 60s to low 70s F and 1.5- to 2-inch PW), and steep
   midlevel lapse rates, will support MLCAPE from 1500-2500 J/kg over
   north TX to 22000-3000 J/kg over the Hill Country and Rio Grande --
   all amidst 50-60-kt effective-shear magnitudes.  Though with weaker
   moisture and deep shear, steep midlevel lapse rates and at least
   marginal CAPE will support severe hail and perhaps gusts or a
   tornado possible this afternoon and evening with convection
   developing on either side of the cold front (west of the warm-sector
   boundary activity) over OK and northwest TX.

   ...Lower Great Lakes region...
   Widely scattered to scattered thunderstorms should develop this
   afternoon and this evening along and south of the front, offering
   sporadic large hail and damaging gusts.  The deep-layer environment
   should destabilize and become buoyant fairly rapidly during midday
   and into the afternoon as these processes occur essentially
   simultaneously:
   1.  Moisture/theta-e advection from the southwest and
   west-southwest, contributing to surface dew points increasing into
   the 50s to locally near 60 F;
   2.  Increasing large-scale ascent and lapse rates aloft ahead of the
   approaching Great Lakes mid/upper perturbation;
   3.  Diurnal heating and related steepening of boundary-layer lapse
   rates.  Some mixing associated with this process may offset moist
   advection enough to slow the dew-point increase; still, MLCINH still
   should be reduced to near zero in much of the area such that only
   weak lift from the front, lake boundaries or orographic forcing will
   be needed to initiate convection.

   Accordingly modified model soundings indicate a deeply buoyant layer
   with 500-1200 J/kg peak/preconvective MLCAPE.  A dominant westerly
   surface component will limit low-level shear and keep hodographs
   small, though mid/upper-level winds will be strong enough to support
   effective-shear magnitudes commonly in the 35-45-kt range.  This
   will support mixed modes, with multicells, a few supercells and
   small bows possible.  The threat should wane during mid/late evening
   as nocturnal cooling/low-level stabilization offset advective
   processes in reducing buoyancy.

   ...Mid Mississippi Valley/lower Ohio Valley...
   Scattered thunderstorms are expected to develop predominantly south
   of the front through the evening hours, offering at least a marginal
   damaging-gust and large-hail threat.  A tornado cannot be ruled out
   as well, with storm- to mesobeta-scale processes being the main
   determinant.  This activity will be supported by a broad plume of
   low-level warm and moist advection, and diurnally weakened MLCINH in
   preconvective areas, away from the northeastern rim of the southern
   Plains EML.  Surface dewpoints in the 60s F already are common
   across this corridor and should remain so, supporting areas of
   1000-1500 J/kg MLCAPE -- locally/briefly higher this afternoon. 
   40-50-kt effective-shear magnitudes should become common as the
   buoyant layer deepens into stronger flow aloft.

   Parts of this region my realize a greater severe threat than the
   "marginal" probabilities currently depicted, conditional on
   sufficient organization and upscale/cold-pool growth of any
   thunderstorm clusters that can develop.  If so, unconditional wind
   probabilities in particular may need a boost in a succeeding update,
   as mesoscale convective/environmental trends warrant.

   ..Edwards/Jewell.. 04/28/2021

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z