SPC AC 051250

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0750 AM CDT Sun May 05 2024

   Valid 051300Z - 061200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
   SOUTH TEXAS...

   ...SUMMARY...
   Scattered strong to severe thunderstorms are possible across parts
   of South Texas today.  Isolated locally severe thunderstorms are
   also possible across the Mid-South and upper Ohio Valley.

   ...Synopsis...
   A substantial synoptic-scale trough has moved inland over the
   Pacific Coast States and NV, anchored by a cyclone now centered near
   the NV/OR border.  The 500-mb low should move erratically eastward
   through the period, reaching southern WY by 12Z tomorrow, with a
   strong shortwave trough southward over western CO and NM. 
   Associated height falls will shift eastward through the
   Intermountain West, Rockies, and eventually High Plains.  As that
   occurs, a convectively reinforced, southern-stream perturbation --
   apparent in moisture-channel imagery from western OK to central/
   southeast TX -- will move eastward to the lower Mississippi Valley. 
   An embedded MCV, initially evident over southern OK, should move
   northeastward toward the Ozarks through 00Z.  Another MCV may be
   located over northeast TX, and should move over AR today.

   At the surface, 11Z analysis showed a weak low over northwestern OH,
   near a frontal triple point, with warm front arching across eastern
   OH, western WV and eastern KY.  This front represents the western
   rim of a cold-damming airmass over the central Appalachians, and
   will be slow to erode from the west as the cold front catches up
   today.  The cold front extended from the low across portions of
   northern/western KY to northern AR, to a frontal-wave low over
   northeastern OK, related to the northern MCV.  The front then
   extended across eastern OK, and north-central/southwest TX.  The
   trailing part of the cold front is expected to lose definition
   amidst:
   1.  A vast field of convective outflow air already in place across
   much of central/north/east TX, OK and AR, as well as
   2.  Intensifying low-level warm advection on both sides late in the
   period, contributing to frontolysis.

   ...South TX to Mid-South/lower Mississippi Valley...
   A dissipating convective complex is moving eastward across portions
   of LA, followed by a still-robust but mostly non-severe line of
   thunderstorms over southeast TX.  The trailing line may produce
   isolated gusts near severe limits for a few more hours, but should
   move into an airmass modified by the leading activity.  Meanwhile,
   the boundary layer farther north -- ahead of a belt of precip and
   embedded thunderstorms now over eastern OK and the Arklatex --
   should destabilize diabatically through the afternoon.  That, along
   with favorable low-level moisture (surface dewpoints generally in
   the mid/upper 60s F), should offset modest mid/upper-level lapse
   rates enough to support peak MLCAPE in the 1000-1500 J/kg range over
   much of the outlook area.  Scattered thunderstorms are expected --
   perhaps in the form of intensification along the northeastward-
   shifted leading edge of the current convection.  With modest
   low-level and deep shear, severe potential should remain marginal
   and isolated, in the form of gusts and hail.  Mesobeta-scale
   mass-response effects ahead of the MCVs might help organization over
   parts of the Mid-South region later today.

   Farther southwest, the aggregate outflow boundary from the morning
   complexes as analyzed from near HOU-COT and into northern Coahuila. 
   This boundary should stall, then drift northward over south-central
   TX and the adjoining Rio Grande Valley today.  The boundary should
   focus additional thunderstorm development from midday through the
   afternoon, some of which may become severe multicells or supercells
   with large hail, damaging gusts and at least marginal tornado
   potential.  Though flow beneath the 700-mb level should be modest
   (generally 5-10 kt), two factors will favor severe potential:
   1.  Backed flow near the boundary, with maximized low-level
   vorticity, augmentation of weak ambient shear, and favorable
   storm-relative low-level winds on the mesoscale.
   2.  Favorable buoyancy, with MLCAPE commonly 2000-3000 J/kg along
   and south of the boundary, will be fostered by strong diurnal
   heating, rich low-level moisture (70s F surface dewpoints) and weak
   CINH.

   In the absence of notable mid/upper support behind the shortwave
   trough, convection may diminish rapidly through the evening, as the
   boundary layer stabilizes from both nocturnal cooling and spreading
   outflow air.

   ...Upper Ohio Valley vicinity...
   Episodes of widely scattered to scattered thunderstorms are expected
   to move roughly eastward across this outlook area today, with
   isolated damaging (but mainly subsevere) gusts possible.
   Activity will occur as weak MLCINH is breached by diabatic heating
   and frontal lift.  In the pocket of favorable boundary-layer air
   between the warm and cold fronts, 60s F surface dewpoints should
   contribute to MLCAPE in the 1000-1500 J/kg range.  Glancing
   influences of a northern-stream trough -- passing north of the area
   from the Upper Great Lakes across northeastern ON and adjoining QC
   -- will include somewhat tightened height gradients and enhanced
   mid/upper winds.  However, low-level flow will remain weak and
   veered to southwesterly, with weak vertical shear in the lowest 500
   mb.  Mode therefore should be multicellular, with convection
   weakening considerably as low-level instability decreases near and
   after sunset.

   ..Edwards/Broyles.. 05/05/2024

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