SPC AC 210554

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1254 AM CDT Sat Oct 21 2017

   Valid 211200Z - 221200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS
   OKLAHOMA...NORTH TEXAS...SOUTHEAST KANSAS...SOUTHWEST
   MISSOURI...EXTREME NORTHWEST ARKANSAS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE SOUTHERN
   PLAINS TO THE UPPER MIDWEST...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
   SOUTHERN PLAINS TO THE UPPER MIDWEST SURROUNDING THE SLIGHT RISK...

   ...SUMMARY...
   Scattered severe thunderstorms are expected especially across
   portions of the south-central Plains late this afternoon and
   evening, with very large hail, damaging winds, and a few tornadoes
   possible.

   ...Synopsis...
   An upper-level trough centered over the Great Basin and northern
   Intermountain Region Friday evening will continue to dig/amplify
   from the northern/central High Plains this morning toward the
   central/southern Plains and Upper Midwest by early Sunday. An
   east/southeastward-moving cold front will be a primary focus for
   severe thunderstorm development later today, with a frontal wave and
   dryline also factors across the southern Plains.

   ...Southern/Central Plains...
   An active second-season severe-weather day is expected across the
   region by late afternoon and evening. Ample moisture and moderate
   buoyancy, especially by mid/late October standards, will support
   intense thunderstorm development during the mid/late afternoon hours
   into evening as upper trough-related forcing for ascent overspreads
   the frontal zone. Low to middle 60s F surface dewpoints, as
   currently found across much of OK/TX, will become increasingly
   established by afternoon northward across eastern KS into far
   eastern NE, IA, and southern MN. 

   It seems likely that storms will initially develop and increase
   during the mid/late afternoon (roughly 21Z-23Z) near the
   east/southeastward-moving cold front spanning far eastern NE/western
   IA and northeast/south-central KS into northern/western OK. While a
   couple of initial supercells capable of hail could occur, a
   relatively quick transition to a linear mode seems likely within
   this corridor in vicinity of the advancing cold front with damaging
   winds becoming the main concern.

   Farther south, a somewhat greater spatiotemporal window for
   semi-discrete supercells will likely exist across southern OK into
   western north TX where deep-layer shear vectors will be somewhat
   more line-normal oriented. Current thinking is the most probable
   semi-discrete-supercell-related very large hail and potential for a
   couple of tornadoes should be focused across
   southwest/west-central/south-central OK in vicinity of the surface
   low/dryline-related triple point between 22Z-02Z.

   Quasi-linear bands of storms will be increasingly prevalent through
   the mid/late evening hours with damaging winds being the primary
   hazard, although a couple of tornadoes cannot ruled out across
   central into eastern OK with any line-embedded circulations amid
   adequate low-level shear/SRH. These linearly organized storms should
   otherwise tend to gradually weaken during the overnight hours as
   they move into south-central MO/west-central AR/southeast OK and the
   Ark-La-Tex region.

   ...Upper Midwest...
   Although low-level moisture and buoyancy will be considerably more
   limited into the Upper Midwest including parts of IA into MN/western
   WI, strong south-southwesterly deep-layer winds (850mb winds around
   40 kt) will promote relatively shallow linear/bowing segments that
   rapidly spread north-northeast. The strongest convection during the
   evening hours may be capable of damaging gusts and perhaps a
   tornado.

   ...West-central/southwest TX...
   While the coverage/likelihood of storm development along the dryline
   is not certain, a few supercells capable of large hail cannot be
   ruled out during the late afternoon and early evening from the Low
   Rolling Plains south-southwestward toward the Edwards Plateau and
   Big Bend vicinity. A more likely scenario will be for increased
   storm coverage later this evening into the overnight as the cold
   front overtakes the dryline with some hail and wind being possible.

   ..Guyer/Cook.. 10/21/2017

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