SPC AC 181656

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1156 AM CDT Sun Mar 18 2018

   Valid 181630Z - 191200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE
   SOUTHERN PLAINS TO THE SOUTHEAST STATES...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
   CENTRAL/SOUTHERN PLAINS TO THE SOUTHEAST STATES...

   ...SUMMARY...
   Thunderstorms with large hail and a tornado risk are possible
   especially across North and East Texas late this afternoon into
   evening, with additional severe thunderstorms possible across other
   parts of the south-central Plains to Southeast States.

   ...North/East Texas and far Southern Oklahoma to ArkLaTex...
   The potential exists for locally intense storm development later
   this afternoon/early evening, but a number of questions remain
   regarding the likelihood and locations/coverage of deep convective
   development. Along these lines, various convection-allowing models
   have shown semi-dramatic sub-regional spatial shifts between 00Z and
   12Z of preferred severe corridors later today, while various HRRR
   runs have also exhibited considerable run-to-run variability
   (supercells vs. essentially no development) this morning.

   Scattered showers and some thunderstorms along with multi-layer
   cloud cover remain prevalent across the region at midday, related to
   east/northeastward-transitioning southern-stream forcing for ascent
   embedded with a strong (100+ kt at 250 mb) subtropical jet stream.
   In the wake of this scattered precipitation/thicker clouds, cloud
   breaks and some additional low-level moist influx should allow for
   relatively rapid destabilization especially across
   north-central/east-central Texas to the near/east of a dryline and
   along/south of an east/southeastward-extending front.

   But given some hindered heating through early afternoon and the
   early timing of the aforementioned southern-stream impulse, the
   likelihood and extent of deep convective development later today
   remains questionable, particularly given that areas such as
   north-central Texas may be influenced by subsidence aloft during
   peak heating. Accordingly, the overall scenario seemingly remains
   largely conditional, with the notion that any sustained
   surface-rooted development could evolve into supercells capable of
   very large hail and some tornado risk in the presence of steep
   mid-level lapse rates, moderate buoyancy, and favorable hodographs
   for large diameter (potentially significant 2+ inch) hail. 

   Further outlook refinements/adjustments can be expected with the 20Z
   update.

   ...Lower Mississippi Valley to Southeast States...
   A weak impulse or two (and/or MCV) should semi-focus thunderstorm
   clusters across the region today, such as is occurring across
   southern Mississippi at midday. While buoyancy will not be overly
   strong, 40+ kt effective shear could support some bowing segments
   and possibly even a transient supercell or two. Damaging winds and
   possibly some hail should be the primary concerns.

   ...Portions of Texas Panhandle/western Oklahoma into Kansas...
   At least some severe-weather potential should exist later this
   afternoon through early evening with anticipated low-topped storm
   development across the region. Very limited moisture will exist
   within/north of a narrowing warm sector, but steep lapse rates and
   moderately strong wind profiles could support some strong to severe
   storms across the Texas Panhandle/northwest Oklahoma northeastward
   into southern Kansas.

   ..Guyer/Marsh.. 03/18/2018

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