SPC AC 131940

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0140 PM CST Sat Mar 13 2021

   Valid 132000Z - 141200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON AND EVENING ACROSS PARTS OF THE EASTERN TEXAS PANHANDLE...

   ...SUMMARY...
   Numerous severe storms are expected late this afternoon into tonight
   across west Texas into western Oklahoma and southwestern Kansas. The
   most significant severe weather threat, including the potential for
   long-lived supercells capable of producing strong tornadoes is
   expected across the eastern Texas Panhandle into portions of the
   Texas Big Country through early evening.

   ...20Z Outlook Update...
   Slight adjustments have been made to the categorical and
   probabilistic lines, mainly to account for the progression of 
   synoptic features and associated destabilization.

   A narrow corridor of moderate boundary-layer destabilization along
   and just ahead of an eastward advancing dryline still appears likely
   to become the focus for considerable intense thunderstorm
   development late this afternoon and evening across western Texas. 
   Southerly low-level jet strengthening (40-60 kt around 850 mb)
   forecast across parts of the Texas Big Country into the eastern
   Texas Panhandle will contribute to substantive further enlargement
   of clockwise-curved low-level hodographs by around 00-01Z, along
   with a corresponding increase in convective organization and peak in
   severe weather potential early this evening.

   ..Kerr.. 03/13/2021

   .PREV DISCUSSION... /ISSUED 1029 AM CST Sat Mar 13 2021/

   ...Southern High Plains...
   No change to the long-track/strong tornado threat driving the MDT in
   the eastern TX Panhandle. Primary changes this update are to add a
   30 SIG wind area across a portion of western north TX, expand the 5
   tor area south along the dryline into the Edwards Plateau region,
   and broaden the western gradient of the threat probabilities for a
   potentially slower dryline evolution with initial afternoon
   development.

   A deep closed low near the AZ/UT border will progress east across
   the southern Rockies through the end of the period. In association
   with an embedded jet streak rotating northeastward around the
   eastern periphery of the low, a surface cyclone will deepen as it
   ejects north-northeast across the TX Panhandle this afternoon into
   southwest KS tonight. The cyclogenesis will draw the surface
   warm sector northward across western OK and the TX Panhandle to the
   east of a developing dryline, which will focus severe storm
   development this afternoon/evening.

   Regenerative elevated convection has persisted this morning from
   northeast NM northeastward into southwest KS, within the exit region
   of the mid-upper jet. Mid-level lapse rates/buoyancy will be
   sufficient for a threat of isolated severe hail. Farther south, the
   warm sector will spread north today, beneath very steep mid-level
   lapse rates of 8-9 C/km. Greater boundary-layer heating is expected
   initially from the TX South Plains south, before spreading north
   towards late afternoon. A plume of moderate buoyancy with MLCAPE of
   1500-2000 J/kg is expected south of the warm front and east of the
   dryline at peak heating.

   Initial surface-based storm development is expected by early-mid
   afternoon along the dryline in the vicinity of I-27 in the TX
   Panhandle. These initial storms will move off the dryline and
   maintain more discrete character as they encounter a moist,
   destabilizing warm sector with strong vertical shear. Given
   boundary-layer dewpoints in the low 60s, afternoon temperatures in
   the upper 60s to lower 70s, effective bulk shear near 70 kt, and
   effective SRH in excess of 250 m2/s2, there will be a few hour
   window for a couple of supercells to produce long-track, strong
   tornadoes and very large hail centered on the east central/southeast
   TX Panhandle.  

   To the west and south of the discrete supercells, squall line
   development is expected by early evening as a Pacific cold front
   overtakes the dryline and the stronger height falls/ascent
   overspread northwest to west-central TX. Embedded supercell
   structures within the line will be capable of producing all hazards.
   Effective bulk shear in excess of 60 kt will also support a few hour
   window for bowing structures. With 50-60 kt 700-mb flow expected
   both ahead of and behind the squall, along with pronounced surging
   of the Pacific front, a threat for significant severe wind gusts may
   be realized as well. The overall severe threat should largely weaken
   with eastward extent during late evening to early overnight as a
   result of diminishing buoyancy in western OK towards central
   portions of TX.

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