SPC AC 281254

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0754 AM CDT Thu May 28 2020

   Valid 281300Z - 291200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   MISSISSIPPI/ALABAMA AND SOUTHERN TENNESSEE...AS WELL AS SOUTHWEST
   TEXAS NEAR THE RIO GRANDE...

   ...SUMMARY...
   Within a very extensive swath of marginal severe-storm potential,
   two areas appear to have the most-concentrated hail/wind threats
   today:  parts of Mississippi/Alabama and southern Tennessee, as well
   as southwest Texas near the Rio Grande.

   ...Synopsis...
   The upper-air pattern from the CONUS westward across the
   northeastern Pacific will remain one characterized by split flow and
   cutoff lows, albeit with a deamplifying trend beginning in this
   period.  The Ozarks-area cyclone and attached trough are forecast to
   devolve into a weakening, open-wave perturbation.  By 12Z tomorrow,
   the resulting shortwave trough should extend from the lower Ohio
   Valley, northern MS, northern LA and southeast TX, somewhat in phase
   with a convectively induced/enhanced vorticity maximum over
   northeastern MX.  This will occur as a northern-stream trough --
   evident in moisture-channel imagery from the MB-ON border to the
   Dakotas -- pivots to the James Bay region, Lake Superior and the
   upper Mississippi Valley.  Strong synoptic-scale ridging will shift
   eastward across the AZ, the Great Basin and northern Rockies.

   At the surface, a low was analyzed over east-central IA, with
   diffuse, quasistationary front across eastern MO, central AR, and
   southeast/south-central TX.  A secondary cold front was organizing
   from the low southwestward across southwestern OK to northwest TX
   and southeastern NM.  The secondary front will overtake the
   northward-retreating leading boundary over south TX this afternoon
   and evening.

   An extensive area of at least marginal, localized severe potential
   is evident from the interior Mid-Atlantic to the Gulf States and
   southeastern NM, with a northward break, then near parts of the
   central Rockies in low-level upslope flow.  This discussion will
   focus on two relative concentrations of severe threat within the
   broader swath.

   ...TX near Rio Grande...
   Widely scattered thunderstorms are expected to develop today over
   the western and northern parts of the outlook area.  Thunderstorms
   regionally will be forced by a combination of lift from surface
   heating, the southward-moving baroclinic zone, and orographic
   forcing over the Big Bend region and northern Coahuila.  Additional
   convection is possible this evening farther east as the frontal zone
   impinges on richer inflow-layer moisture.  Large hail and severe
   gusts will be the main concerns.

   Height rises are expected as the Ozarks cyclone/trough weaken and
   move away from the area.  Nonetheless, associated northwesterly flow
   aloft and strong directional shear will persist, despite weak
   low-level winds.  This will contribute to 30-40-kt effective-shear
   magnitudes, though the lack of more robust low-level flow will limit
   hodograph size.  Steep midlevel lapse rates, along with some
   moisture return through an air mass affected by prior days'
   convective complexes, will combine to yield MLCAPE in the 2000-3000
   J/kg range across much of this region.  More-robust moisture return
   is probable this evening over south-central TX and the lower Rio
   Grande Valley, which will be available to any convection that can
   develop anew along the combined frontal zone, or evolve upscale from
   diurnally initiated convection from Coahuila.  That contingency
   appears increasingly probable based on more-recent guidance and the
   observational trends in the lower Rio Grande Valley, so the outlook
   has been expanded on the east side.

   ...MS/AL...
   Widely scattered to scattered thunderstorms should develop this
   afternoon and move northeastward across this region, offering
   sporadic strong-severe gusts and large hail.  Activity should form
   atop a weakly capped, strongly heated and increasingly moist
   boundary layer.  Peak preconvective MLCAPE may reach 1000-1500 J/kg,
   despite modest midlevel lapse rates, with a well-mixed subcloud
   layer supporting maintenance of gusts/hail to the surface.  The
   outlook area generally represents a relative max in expected
   convective coverage within the broader moist plume occupying the low
   levels to the southeast of the ejecting cyclone/trough, along with
   associated modest increase in deep shear.  A cyclonically curved
   belt of relatively strong mid/upper winds will contribute to
   30-40-kt effective-shear magnitudes and favorable upper-level
   ventilating flow, in support of convective organization.  Weak
   low-level winds and lack of stronger buoyancy will be limiting
   factors.

   ..Edwards/Smith.. 05/28/2020

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