SPC AC 210549

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1249 AM CDT Tue Aug 21 2018

   Valid 211200Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON AND
   EVENING ACROSS MUCH OF THE UPPER OHIO VALLEY AND CENTRAL
   APPALACHIANS VICINITY...

   ...SUMMARY...
   A few severe thunderstorms are possible across the upper Ohio Valley
   and central Appalachians vicinity this afternoon and evening,
   accompanied by a risk for damaging wind gusts, and perhaps the risk
   for an isolated tornado or two across parts of western Pennsylvania
   into adjacent southern portions of western New York.

   ...Synopsis...
   Models suggest that large-scale troughing, evolving within the main
   branch of mid-latitude westerlies across the central Canadian
   provinces, will dig southeastward across much of the Upper Midwest
   and Great Lakes region by late tonight.  Ahead of (and to the south
   of) this feature, a closed low within a weaker southern branch
   already appears to be devolving into an open wave, which is forecast
   to accelerate east/northeast of the middle Mississippi Valley
   through the lower Great Lakes region by early this evening, before
   gradually becoming absorbed within the broader-scale northern branch
   troughing.  It appears that a  modest surface cyclone accompanying
   this feature may once again begin to slowly deepen during the day,
   before more rapid deepening commences this evening or overnight,
   northeast of the lower Great Lakes region into southwestern Quebec.

   At the same time, guidance indicates that an elongated upper low
   will linger across the northern intermountain region, within
   positively tilted troughing in the southern branch of the
   mid-latitude westerlies.  To the southeast, a prominent mid-level
   subtropical high center is forecast to redevelop east of the
   southern Rockies into the southern High Plains.  One or two
   convectively generated or enhanced perturbations may progress
   through weak to modest flow on its western through northern
   periphery.

   ...Lower Great Lakes/Upper Ohio Valley/Appalachians/Mid Atlantic...
   Severe weather potential associated with the accelerating short wave
   trough and accompanying cyclone will continue to be hindered by the
   lack of colder mid-level air and generally modest to weak
   low/mid-level lapse rates.  Additionally, cloud cover may limit
   insolation through much of the warm sector of the evolving cyclone,
   and highest moisture content air (characterized by precipitable
   water in excess of 2 inches) may become confined to the east of the
   Appalachians (within generally weak flow) by the beginning of the
   period.

   Still, boundary layer dew points may linger near 70F through the day
   ahead of the cold front associated with the surface low, across much
   of the upper Ohio Valley/Allegheny Plateau region.  This is where a
   corridor of modest heating may contribute to mixed-layer CAPE in
   excess of 1000 J/kg by this afternoon, as a belt of 30-40+ kt
   southwesterly mid-level flow overspreads the region.  This
   environment could become conducive to organized convective
   development posing primarily a risk for damaging wind gusts through
   early evening.

   In closer proximity to the surface cyclone migrating across southern
   Ontario this afternoon, low-level hodographs may become large enough
   on the southern periphery of a 30 kt 850 mb jet (where boundary
   layer instability appears likely to be more favorable) to support
   some risk for a tornado or two across parts of western Pennsylvania
   into adjacent southern portions of western New York state.

   It is possible that thunderstorms could spread eastward or develop
   within surface troughing to the lee of the central Appalachians by
   early this evening, accompanied by a risk for potentially damaging
   wind gusts.  Otherwise, severe weather potential east of the
   Appalachians remains more unclear, largely due to the likely
   continuing presence of weak to negligible instability to the north
   of the Mid Atlantic, and generally modest to weak flow and shear
   where stronger instability could develop across parts of
   southeastern Virginia and eastern North Carolina.

   ...Central High Plains...
   A consistent signal persists within model output concerning
   potential for thunderstorm development and possibly an evolving
   cluster north/east of the Sangre de Cristo Mountains and Raton Mesa
   vicinity late this afternoon into tonight.  This appears to be
   associated with a perturbation migrating around the subtropical high
   center.  It still seems possible that this could be accompanied by
   at least some risk for severe wind and hail.  However, most guidance
   suggests that mixed-layer CAPE across this region may only reach
   1000-1500 J/kg, for a boundary layer that may not become
   particularly hot/deeply mixed (for the time of year).  Given
   uncertainties concerning the strength of the low/mid-level flow and
   shear, the extent of the severe weather threat seems limited.

   ..Kerr/Gleason.. 08/21/2018

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