SPC AC 130600

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0100 AM CDT Sun May 13 2018

   Valid 131200Z - 141200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER A PORTION OF
   THE SOUTHERN PLAINS AND THE MIDWEST...

   ...SUMMARY...
   A few strong to severe thunderstorms are possible today from parts
   of the Midwest to the Mid Atlantic, the southern/central Plains, and
   a portion of the Colorado Front Range into the Laramie Mountains
   vicinity of Wyoming. Damaging wind and hail will be the primary
   threats.

   ...Synopsis...

   A synoptic blocking pattern will persist today with cutoff upper low
   over the Great Basin and Intermountain West and a downstream upper
   ridge over the Southeast States. At the surface a quasi-stationary
   front will persist from eastern VA through the OH Valley and west
   into KS where it will intersect a dryline extending south through
   west TX.

   ...Midwest...

   Richer low-level moisture with mid-upper 60s F surface dewpoints
   will advect through the warm sector to the south of a
   quasi-stationary front currently situated from the OH Valley through
   northern MO. A warm elevated mixed layer will expand east above the
   moistening warm sector and frontal zone contributing to very strong
   instability (2000-3000 J/kg MUCAPE) and a strong cap. Shortwave
   ridging and strong convective inhibition should limit thunderstorm
   development over much of this region during the day. However
   vorticity maximum currently approaching the southern Rockies will
   crest upper ridge this evening and tonight which, along with a
   strengthening low-level jet, should contribute to sufficient
   isentropic ascent for storms to initiate along/north of the front.
   Most CAM solutions suggest this activity will eventually grow
   upscale, possibly becoming a forward propagating MCS. Initial threat
   will be large hail across IA, but the threat for damaging wind may
   increase as storms grow upscale from eastern IA into northern IL and
   IN.

   ...Southern and central Plains...

   Influx of richer low-level moisture beneath steep lapse rates
   associated with a warm elevated mixed layer should result in strong
   instability (2500-3000 J/kg MLCAPE) in the southern and central
   Plains warm sector. Strong diabatic warming and dryline mixing
   should eventually erode the cap, and storms are expected to initiate
   from northwest TX into the TX Panhandle and eventually move east
   into western OK during the evening. Despite weak vertical shear
   (25-30 kt, 0-6km) The thermodynamic environment appears very
   favorable for wet downbursts and large hail before activity weakens
   later in the evening. 

   ...Central Rockies region...

   Storms should once again initiate in upslope flow regime where winds
   aloft and vertical shear along southeast periphery of upper low will
   be sufficient for a few rotating updrafts.  MUCAPE from 500-800 J/kg
   and 50+ kt effective shear will support a threat for supercells
   capable of large hail. In addition, steep lapse rates and large
   temperature-dewpoint spreads will promote a threat for downburst
   winds. At this time, marginal instability could be a limiting factor
   for a more robust severe threat, but an upgrade to slight risk might
   be warranted in later updates. 

   ...Central Appalachians to Middle Atlantic region...

   Backdoor front has shunted warm sector south of the stronger winds
   aloft. Most storms (modulated by vorticity maxima embedded within
   the west-northwest flow regime) will remain confined to north of the
   quasi-stationary front. However any storms developing near this
   boundary may become capable of producing a few strong to damaging
   wind gusts as the boundary layer destabilizes during the afternoon
   and early evening.

   ..Dial/Karstens.. 05/13/2018

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