SPC AC 071251

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0751 AM CDT Thu May 07 2020

   Valid 071300Z - 081200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS MUCH OF
   OKLAHOMA TO SOUTHERN KANSAS...NORTHWEST/NORTH TX TO PARTS OF THE
   ARKLATEX...AND THE EASTERN TX PANHANDLE...

   ...SUMMARY...
   Severe hail and damaging thunderstorm gusts are expected late this
   afternoon into tonight across parts of the southern to central
   Plains.

   ...Synopsis...
   In mid/upper levels, the synoptic pattern will be dominated by a
   large, well-defined cyclone moving southeastward across northern ON,
   and northwest flow across the central CONUS.  A smaller mid/upper
   cyclone -- now evident in moisture-channel imagery over southern MT
   -- is expected to devolve gradually into a compact shortwave trough
   as it moves southeastward across the central Plains, reaching MO by
   12Z tomorrow.

   At the surface, 11Z analysis depicted a surface low over
   southwestern SD, with cold front southwestward across southeastern
   WY.  This front is forecast to strengthen, move southeastward across
   the central Plains today, and merge this evening with an initially
   separate area of cyclogenesis across the northeastern NM/TX
   Panhandle region.  By the end of the period, the front should extend
   from a low over southeastern MO, across central/western AR, and
   north-central to far west TX.  A warm front was drawn from a low
   over southeastern NM southeastward across southwest/south-central TX
   to the northwestern Gulf.  This warm front will move northward
   through the day and become more diffuse on its western limb, across
   the southern High Plains.  Meanwhile, as moist advection occurs in
   the airmass to its east, a dryline will take shape from northern
   Coahuila northward to near the Capriock and eastern TX Panhandle. 
   The cold front will overtake the dryline from north-south tonight.

   ...Portions of OK/KS/AR and northwest/west TX...
   A narrow corridor of favorably moist return flow is expected to
   occur across west TX, western OK and into at least southern/central
   KS ahead of the dryline and cold front.  This plume will spread at
   least marginally favorable theta-e northward from the Rio Grande,
   through the afternoon and into early evening ahead of the cold front
   and dryline, and should underlie a combination of steep midlevel
   lapse rates associated with an EML and favorable deep shear for
   organized convection.  Two regimes with some spatial overlap will
   post the bulk of severe-thunderstorm potential this period, in rough
   chronological order:

   1.  DRYLINE:  Isolated to widely scattered thunderstorms are
   possible late this afternoon into early evening, initiating in a
   relatively intensely heated area along or just east of the dryline
   late this afternoon.  Some progs indicate a dryline bulge forming in
   the southeast Panhandle/northwest TX region near CDS, which may
   concentrate low-level convergence/lift to supplement that from
   ambient thermodynamic forcing.  While the potential is rather
   conditional due to concerns over stronger capping and related
   hindrance to storm coverage, the parameter space across the
   southeast Panhandle/southwestern OK/northwest TX area suggests the
   threat for any storms that form to become discrete, somewhat
   high-based supercells, offering very large, destructive hail over 2
   inches in diameter.  55-60-kt effective-shear magnitudes and 300-500
   J/kg effective SRH are possible by the 22-03Z time frame. A
   well-mixed boundary layer, beneath 3000-3500 J/kg MLCAPE, also
   supports the threat for severe downdrafts with such storms.

   Potential may be shorter-lived and more isolated southward along the
   dryline, with greater distance from the most favorable large-scale
   support and deep shear.  Strong MLCINH is likely, given the stout
   EML sampled by the 12Z DRT/MAF RAOBs.  However, at least a brief
   bout of severe hail/gusts is possible with any sustained storm that
   develops in that corridor. 

   2.  COLD FRONT:  This may become the most concentrated regime for
   severe.  The front should impinge upon the northern fringe of the
   moisture plume across portions of the eastern TX Panhandle,
   northwestern OK and at least southern KS this evening.  This area
   will be in increasing proximity to stronger large-scale ascent aloft
   than farther south, as the shortwave trough approaches.  In concert
   with associated destabilization aloft and weakening CINH, the front
   should provide sufficient low-level lift to overcome remaining
   capping and initiate discrete to semi-discrete cells.  That activity
   then may become denser and aggregate cold pools with time, forming a
   QLCS within a few hours.  Until that evolution occurs, large hail
   and isolated severe gusts will be possible, especially with any
   associated supercells that can mature before linear mode takes over.
    Then the main threat will transition to wind, with isolated severe
   hail.  The wind threat will be relatively/locally maximized in any
   bowing segments and LEWP features.  

   Although the return-flow regime will be incomplete, with surface dew
   points generally in the 50s to low 60s F, modified soundings suggest
   1000-2000 J/kg MLCAPE in the immediate preconvective environment
   over northwestern/north-central OK, decreasing to around 500 J/kg in
   central KS.  The remains of a well-mixed subcloud layer will support
   maintenance of hail and acceleration of downdraft gusts to the
   surface.  Local gusts near hurricane force may occur.  Strongly
   veering winds with height will support 200-500 J/kg effective SRH
   and 55-65-kt effective-shear magnitudes.  Therefore, despite the
   somewhat meager moisture, at least a marginal threat for tornadoes
   exists this evening and tonight with line-embedded mesovortices.  An
   increasingly moist LLJ, strengthening to 55-65 kt, may help to
   maintain any resulting squall line through most of tonight,
   potentially as far southeast as parts of AR and northeast/north-
   central TX, though surface-based instability will decrease markedly
   with eastward extent.

   A relatively dense corridor of severe gusts may develop with this
   scenario, especially across parts of OK, and this may need further
   upgrading of unconditional wind probabilities in succeeding
   outlooks, as later guidance and mesoscale trends warrant.

   ..Edwards/Dial.. 05/07/2020

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