SPC AC 171955

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0255 PM CDT Mon May 17 2021

   Valid 172000Z - 181200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS THE TEXAS
   SOUTH PLAINS/WESTERN NORTH TEXAS AREA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE CENTRAL
   HIGH PLAINS INTO THE SOUTHERN PLAINS REGION...

   ...SUMMARY...
   Severe thunderstorms remain likely across the southern Plains this
   afternoon and tonight. The potential for very large hail, a few
   tornadoes, and damaging winds will be focused across a broad part of
   west/northwest Texas.

   ...Discussion...
   Forecast areas and reasoning laid out in prior outlooks continue to
   adequately reflect current expectations overall, with respect to the
   evolving severe weather scenario.  As such, little change to outlook
   lines is required at this time.  The main exception is over southern
   Louisiana, where small/rotating cells persist near and east of a
   small mesolow in the vicinity of Calcasieu Lake, with at least one
   brief tornado reported.  Given ample low-level shear, the very moist
   boundary layer, and presence of the mesolow, sufficient risk for
   additional/brief tornadoes warrants SLGT risk upgrade in this area. 
   For additional short-term information, please refer to SWOMCD #652.

   Otherwise, aside from a slight westward expansion of 5% wind/2%
   tornado probability in north central New Mexico, no other
   substantive changes appear necessary to the current outlook.

   ..Goss.. 05/17/2021

   .PREV DISCUSSION... /ISSUED 1138 AM CDT Mon May 17 2021/

   ...West/southwest/north Texas and southwest Oklahoma...
   An active severe-weather day is expected particularly across
   west/southwest Texas later this afternoon/evening. In the base of an
   eastward-moving upper trough, an east/northeastward-ejecting
   high-level jet analyzed over northern Mexico this morning will reach
   the southern High Plains by around peak heating/early evening,
   leading to a further strengthening of mid/high-level westerlies that
   are already seasonally strong (12Z Midland TX observed sounding).

   Related to convection overnight/early this morning across the Low
   Rolling Plains and north-central Texas, an outflow boundary
   continues to settle/decelerate south into the Permian Basin/Concho
   Valley including Midland/San Angelo areas, east-southeastward toward
   the Texas Hill Country. Considerable convective overturning and
   stabilization has occurred in association with this
   convection/outflow across the Low Rolling Plains and western North
   Texas, although steady modification/northward mixing of the boundary
   should occur especially across the Permian Basin and West Texas
   South Plains later today. It is in relative proximity to this
   boundary and the north/south-oriented dry line that will have the
   highest probability of intense surface-based storm development later
   today. Intense heating/ample convergence will likely lead to such
   development by late afternoon. 

   These storms will move into an environment of very high instability
   (MLCAPE over 3000 J/kg), steep mid level lapse rates exceeding 8.0
   C/km, and effective shear values of 40-50 kt. Intense supercells
   capable of very large hail (some hailstones in excess of 3" in
   diameter are viable) are expected, particularly within the Moderate
   Risk area across west/southwest Texas. Although low-level winds are
   not forecast to be overly strong, the residual influence of the
   outflow boundary and robust instability will support the possibility
   of a few tornadoes. Over time, storms should eventually
   cluster/organize this evening toward the Low Rolling Plains/Big
   Country with damaging wind potential also increasing as storms
   progress east-southeastward.

   ...Southeast Colorado/south-central High Plains...
   A moist post-frontal environment in conjunction with low-level
   upslope flow will be conducive for at least isolated severe
   thunderstorms by mid/late afternoon initially across southeast
   Colorado/northeast New Mexico including the Raton Mesa vicinity.
   Although deep-layer shear will not strong, organized multicells and
   some transient supercells can be expected with large hail and
   possibly some tornado risk. These storms should move into additional
   parts of the region including the Texas/Oklahoma Panhandles and
   southwest Kansas by early evening with mostly a hail/wind risk
   continuing.

   ...Southeast Texas/western Louisiana...
   A loosely organized cluster of storms persist across southeast Texas
   into western/southern Louisiana late this morning. While deep-layer
   shear and buoyancy are not overly strong, the moist environment and
   relatively strong low-level shear/SRH have been supportive of
   transient supercell circulations. This risk may continue this
   afternoon with at least some potential for a brief tornado and/or
   locally damaging wind gust, particularly in areas closer to the
   immediate coast.

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z