SPC AC 221233

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0733 AM CDT Wed Aug 22 2018

   Valid 221300Z - 231200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE GREAT BASIN...NORTHERN ROCKIES...HIGH PLAINS...SOUTHERN NEW
   ENGLAND AND SOUTHERN MID ATLANTIC COAST...

   ...SUMMARY...
   Widely scattered strong thunderstorms with marginally severe wind
   gusts and hail will be possible in parts of southern New England,
   the High Plains and Great Basin into the southern Rockies. A few
   marginally severe wind gusts will also be possible along the
   southern Mid Atlantic coast.

   ...Great Basin/Northern Rockies...
   A positively tilted shortwave trough will move east-northeastward
   across the Intermountain West today as a Pacific cold front advances
   eastward across the region. This front should be located in western
   Montana, central Idaho and far northwestern Utah by afternoon.
   Low-level moisture should be maximized ahead of the front from
   eastern Utah into western Wyoming with surface dewpoints generally
   in the lower to mid 50s F in the lower elevations. In response,
   surface heating should increase MLCAPE values in the 1200 to 1500
   J/kg range by late afternoon as is shown on RAP forecast soundings.
   This increase in instability should result in scattered thunderstorm
   development along and ahead of the front. The instability should be
   coincident with moderate deep-layer shear supporting at least a
   marginal severe threat. 700 to 500 mb lapse rates of 7.5 to 8.0 C/km
   will be conducive for hail with discrete rotating cells. A few
   marginally severe wind gusts will be possible as well. A slight risk
   could be needed in later outlooks once the exact location of the
   strongest instability can be determined.

   ...Southern and Central High Plains...
   A shortwave ridge will move across the central and northern High
   Plains today. At the surface, a boundary with maximized low-level
   convergence should be located from southeast Colorado
   east-southeastward into the Oklahoma Panhandle. As surface
   temperatures warm during the day, convection will likely initiate in
   the higher terrain of south-central Colorado and southeastward along
   the boundary. Thunderstorms should then spread eastward into the
   lower elevations of southeast Colorado, southwest Kansas and the
   Oklahoma Panhandle. RAP forecast soundings along this corridor by
   00Z/Thursday show MLCAPE values generally between 1500 and 2500
   J/kg. The Lamar, CO forecast sounding is the anomaly peaking near
   2900 J/kg. The instability combined with 0-6 km in the 25 to 35 kt
   range should be enough for an isolated severe threat. A few
   marginally severe wind gusts will be possible with the stronger
   multicells. In spite of the relatively poor lapse rates, the
   stronger updrafts could also have hail in areas that destabilize the
   most.

   ...Southern New England...
   An upper-level trough will move eastward across southeast Canada,
   the lower Great Lakes and the Ohio Valley today. At the surface, a
   cold front will advance eastward into New England. Surface dewpoints
   ahead of this feature will be in the mid to upper 60s F aiding the
   development of a pocket of instability across southern New England
   around midday. Isolated thunderstorms are forecast to initiate along
   the front and move eastward across southern New England from late
   this morning into early afternoon. RAP forecast soundings around
   midday show weak instability in southern New England but have 0-6 km
   shear in the 30 to 40 kt range. This could be enough for marginally
   severe wind gusts with short line segments.

   ...Southern Mid Atlantic Coast...
   An upper-level trough will move eastward across the lower Great
   Lakes and Ohio Valley today as a cold front advances eastward into
   the Mid Atlantic and Carolinas. Ahead of the front, surface
   dewpoints in the mid 60s to lower 70s F should result in a corridor
   of moderate instability by afternoon along the coasts of Georgia and
   the Carolinas. Near the axis of strongest instability, the 21Z RAP
   forecast soundings at Wilmington, NC has MLCAPE around 2500 J/kg
   with 0-6 km shear of 25 kt. This combined with steep low-level lapse
   rates may be enough for marginally severe wind gusts.

   ..Broyles/Grams.. 08/22/2018

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