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    Day 2 Outlook >
Jul 18, 2018 2000 UTC Day 1 Convective Outlook
Updated: Wed Jul 18 20:00:48 UTC 2018 (Print Version | 20180718 2000Z Day 1 shapefile | 20180718 2000Z Day 1 KML)
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 Forecast Discussion
   SPC AC 182000

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0300 PM CDT Wed Jul 18 2018

   Valid 182000Z - 191200Z


   Severe thunderstorms are likely this afternoon into tonight across
   parts of the northern and central Plains including Nebraska and
   southern South Dakota.

   ...SD and NE...
   No major changes have been made to the primary risk area across
   portions of South Dakota and Nebraska. The 19Z Rapid City sounding
   shows strong midlevel northwesterly flow that will begin impinging
   on a moderately unstable environment further east. The potential
   remains for a few supercells later this afternoon capable of large
   hail and a tornado or two, followed by upscale growth this evening
   that may be accompanied by an increased severe wind risk. 

   ...Northwest ND...
   5% wind and hail probabilities have been added across portions of
   northwest ND, where thunderstorms have recently intensified. Despite
   generally weak shear across this region, moderate instability will
   support some localized hail and wind threat with the strongest

   ...Ozarks into the Lower MS River Valley...
   The Marginal Risk has been adjusted across portions of AR into the
   lower MS River Valley. Parts of western/central AR have been removed
   due to persistent cloudiness limiting destabilization across this
   area. Parts of northeast AR into western TN have been added, where
   convection has recently intensified. Portions of northeast LA into
   western MS have also been added to the Marginal area, ahead of a
   growing thunderstorm cluster that will move into an axis of
   moderate-to-strong instability, posing a threat of isolated damaging
   wind gusts.

   ..Dean/Dial.. 07/18/2018

   .PREV DISCUSSION... /ISSUED 1149 AM CDT Wed Jul 18 2018/

   ...North-central Plains/Middle MO Valley...
   Severe thunderstorms are likely this afternoon into evening and
   possibly the overnight across the region, although some uncertainty
   exists related to the most likely sub-regional corridor of severe
   storms. This is attributable to some lingering influences of prior
   outflow and multi-layer cloud cover at midday. Convection-allowing
   guidance spatial variability also exists between this morning's HRRR
   runs and other 12Z-based CAMs such as the NSSL-ARW.

   Regardless, the most probable area for surface-based convective
   initiation appears to be across west-central/south-central SD into
   far northern NEB this afternoon. With strong high-level winds
   already in place, an eastward-developing belt of increasingly strong
   mid-level westerlies in conjunction with south-southeasterly
   boundary layer winds will contribute to 35-45 kt of effective shear.
   This will be favorable for supercells capable of large hail, with
   moderate buoyancy and lengthy hodographs supportive of some larger
   hail magnitudes in spite of sub-optimal lapse rates. Some tornado
   risk may exist as well with enhanced low-level SRH in proximity to
   the surface boundary. 

   Otherwise, relatively quick upscale growth should occur by evening
   with one or more southeastward-moving clusters likely to evolve.
   This would pose an increased damaging wind risk this evening across
   parts of NEB, which could continue into the overnight potentially as
   far south as portions of KS.

   ...Lower MO Valley to Mid-South/Lower MS River Valley...
   Weak outflow/zones of differential heating will spatially influence
   sub-regional corridors of diurnally intensifying storms today within
   a very moist air mass. Some of the stronger updrafts/downdrafts will
   be capable of localized wind damage.

   ...Northern AZ/Lower CO River Valley...
   Widely scattered thunderstorms are expected later today, growing in
   coverage and becoming more clustered as they shift westward through
   afternoon into early evening. Isolated severe-caliber wind gusts are
   expected. A moist air mass, with PW of 1-1.5 inches, and steep
   low-level lapse rates in an increasingly well-mixed boundary layer
   this afternoon, will contribute to MLCAPE commonly reaching
   1000-1500 J/kg, and locally exceeding 2000 J/kg. This will
   contribute to potential for wet microbursts in cores and hybrid
   wet/dry processes in deeper/lower-ground-elevation boundary layers.
   Forecast soundings reasonably suggest (given the pattern) that a
   deep layer of easterlies in the midlevels will be the dominant
   contributor to the weak steering for early-stage convection, until
   enough upscale growth occurs with westward extent that aggregated
   cold pools force forward propagation.

   ...Coastal Carolinas vicinity...
   A warm and very moist air mass to the south of a southward-sagging
   front in conjunction with sea breeze influences could yield a couple
   of strong to locally severe thunderstorms capable of isolated
   damaging wind gusts this afternoon.



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