SPC AC 241245

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0745 AM CDT Wed Mar 24 2021

   Valid 241300Z - 251200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
   CENTRAL TEXAS TO SOUTHERN OKLAHOMA AND EASTWARD ACROSS THE ARKLATEX
   REGION...

   ...SUMMARY...
   Large hail and damaging to severe thunderstorm winds will be
   possible over parts of the southern Plains into the Arklatex, mainly
   this evening into tonight.

   ...Synopsis...
   A mid/upper-level longwave trough will persist over the western
   CONUS through this period and into day 2, as a series of
   perturbations traverse the associated cyclonic-flow field.  One that
   already has -- a compact cyclone evident in moisture-channel imagery
   over IA and southern MN -- will devolve to an open wave today as it
   ejects northeastward.  By 00Z the trough should be over Lake
   Superior, the MI Upper Peninsula and Lake Michigan, weakening
   further as it moves northeastward toward southwestern QC overnight.

   The main upper-air feature for this forecast will be a broad cyclone
   -- currently on the margins of being an open-wave trough -- with a
   circulation center apparent over the northern part of the AZ/NM
   border.  By 00Z this should manifest as a synoptic-scale open trough
   from southern CO across central/southern NM and the ELP area, then
   southwestward over northwestern MX.  By 12Z tomorrow, as another
   perturbation and speed max dig south-southeastward across the
   Pacific Northwest to reinforce the longwave trough -- the southern
   Rockies trough should weaken somewhat and shift to southern NE,
   western parts of KS/OK/TX, to the Big Bend region.

   At the surface, a low related to the leading/ejecting shortwave
   trough was drawn at 11Z near RST, with warm front eastward over
   portions of WI/northern lower MI and southern ON.  A cold front
   extended from the low across northern MO, southern OK, and
   west-central TX, to another low near INK.  The cold front will move
   slowly eastward/southeastward today, extending by 00Z from the low
   over Upper MI to northern IL, northwestern AR, south-central/
   southeastern OK, north-central TX, and the Edwards Plateau.  A new
   low should form along the front by about 06Z, over northeast TX or
   southeastern OK.  By 12Z, the southern low should move northeastward
   over western AR, with cold front across east TX and deep south TX.

   A dryline will develop today over central/southwest TX as the
   warm/moist sector spreads inland.  The dryline will be overtaken by
   the cold front from north-south with time.  A marine/warm front over
   the northern Gulf and south-central TX will shift erratically inland
   through the period, perhaps impeded or delayed most by a strong
   outflow-reinforced cold pool now analyzed over parts of southern LA
   and the adjoining Gulf.  Still, even that part of the baroclinic
   zone should retreat somewhat inland over LA/MS by late tonight.

   ...Southern Plains/Arklatex...
   Scattered thunderstorms are forecast to form late this afternoon
   through this evening over parts of central and north-central TX,
   becoming severe near the frontal zone and east of the dryline, as
   large-scale lift increases with the approach of the mid/upper
   trough.  Additional development is possible this evening into
   tonight across more of central/south-central TX from the eastern
   Hill Country eastward/northeastward past the Balcones Escarpment. 
   Large hail and damaging gusts are the main concerns.  The northern
   part of the resulting swath of thunderstorms may spread across the
   Arklatex and into parts of southern AR overnight, maintaining a
   threat for hail and wind damage.  Though the tornado threat is more
   conditional, and low-level hodographs not yet ideal over most of the
   area, a tornado cannot be ruled out given the strength of the deep
   shear, increasing quality of moisture return with time, and
   potential for a few surface-based supercells.

   As the marine front moves diffusely northward today, associated
   theta-e advection and moistening will increase surface dewpoints
   into the 60s across much of central and parts of north TX. 
   Meanwhile, height falls and a tightening mid/upper height gradient
   will spread over the region, with large-scale ascent increasing
   under the left-exit region of a cyclonically curved, 120-140 kt jet
   core at 250-300-mb.  Diminishing MLCINH should support episodic
   convective development, with MLCAPE in the 1500-2000 J/kg range over
   south-central TX (but with stronger capping) and 1000-1500 J/kg in
   north TX, diminishing considerably into southern OK.  Effective-
   shear magnitudes should increase into the 50-60-kt range this
   evening.

   Another area of briefly strong convection may form during late
   afternoon and evening across parts of northwest Texas and
   southwestern OK, rooted in a layer of warm advection and frontal
   forcing around 800 mb.  With MUCAPE generally under 500 J/kg, but
   strong speed shear, brief hail/gusts near severe limits may occur. 
   The severe threat will be limited by lack of greater buoyancy, and
   by the low-level stable layer.

   ...Central Gulf Coast, lower Delta region...
   Scattered to widely scattered thunderstorms are possible
   episodically throughout the period, as low-level warm advection and
   moisture transport persist atop an extensive outflow pool and north
   of the marine front.  Though rooted in elevated unstable/inflow
   layers, this activity may become organized enough on a sporadic
   basis for isolated, marginally severe hail.  Bowing structures also
   have been observed over the cold pool already this morning, and
   strong/damaging gusts penetrating the stable frontal/boundary layer
   cannot be ruled out, especially near the coast where that layer is
   shallowest.  Though devoid of any substantial upper support, minor
   perturbations moving along the broad southwest-flow field, and near
   a 500-300-mb layer speed max - may support episodic convection over
   the region, with isentropic lift to LFC continuing.  MUCAPE
   1000-2000 J/kg and 50-70-kt effective-shear magnitudes will support
   the potential for organized strong to locally severe convection,
   albeit rooted aloft.

   Late this evening and tonight, this regime will shift northward
   across northeastern LA, southeastern AR and central/northern MS, and
   western/northern AL.  The western part of this warm-frontal
   convection may link with the eastward-shifted extent of initially
   separate southern Plains/Arklatex regime described above.  Most of
   the activity over the Delta and Mid-South regions prior to 12Z
   should remain elevated, but continuing a threat for isolated,
   marginally severe hail and gusts.  A tornado cannot be ruled out in
   any convection close to the warm front that encounters surface-base
   inflow, but this potential is very conditional.

   ...Southeastern Lower MI, Lake Erie, OH...
   Isolated to widely scattered thunderstorms may develop southeast of
   the surface low and cold front, in a zone of warm-sector confluence/
   convergence that will be diurnally heated to minimize MLCINH.  While
   the bulk of strongest deep-layer forcing, mature convection, and
   associated strong-severe gust potential may be over adjoining parts
   of ON, isolated thunderstorms (especially over northern OH and that
   portion of Lower MI from the Thumb southward) may approach severe
   levels before moving quickly out of the area.  This arc of thunder
   potential will include 40-50 kt effective-shear magnitudes with peak
   MLCAPE potentially reaching around 500 J/kg where the most surface
   heating can take place.  At this time, the potential appears too
   conditional and isolated to warrant a categorical outlook.

   ..Edwards/Leitman.. 03/24/2021

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