SPC AC 200548

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1248 AM CDT Thu Sep 20 2018

   Valid 211200Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM CENTRAL AND
   EASTERN OHIO EAST/NORTHEAST INTO NEW ENGLAND...

   ...SUMMARY...
   Potential for severe storms -- posing primarily a damaging-wind risk
   -- is expected across the Northeast and Ohio Valley Friday.

   ...Synopsis...
   Mid- and upper-level troughing, initially moving across the Upper
   Great Lakes region, is forecast to advance quickly
   east-northeastward, crossing southern Ontario and the lower Great
   Lakes region, before eventually reaching the Canadian Maritimes and
   New England.  In the wake of this trough, westerly/zonal flow will
   prevail across the northern U.S., in advance of the
   next/low-amplitude eastern Pacific trough -- expected to reach the
   Pacific Northwest late.  Farther south, weak troughing will drift
   across parts of the central and southern Plains, while a ridge
   persists over the Southeast.

   At the surface, the main feature of interest will be a
   strong/rapidly advancing cold front, trailing southwestward from a
   deepening low progged to lie near eastern Lake Superior at the start
   of the period.  As this low moves quickly northeastward across
   Quebec through the period, the trailing front will cross the Great
   Lakes region and later New England, while settling more slowly
   southeastward across the Midwest/Ohio Valley with time.  By the end
   of the period, the boundary should extend from near the New England
   coast southwestward to the Mid-Atlantic region, and then
   west-southwest across the Tennessee River Valley and Mississippi
   Delta region to the Red River Valley.

   ...Parts of the Ohio Valley area eastward across the Northeast...
   A band of showers and thunderstorms is forecast to redevelop along
   the advancing cold front by midday/early afternoon, as diurnal
   heating allows some destabilization to occur -- particularly from
   the Midwest to PA/NY.  Farther northeast, much weaker instability is
   expected into New England.  Still, as the line of storms advances,
   strong flow aloft through a deep layer should allow
   organization/upscale growth into a semi-solid squall line, which
   should be crossing the Ohio Valley and lower Great Lakes region
   through sunset.  While marginal hail will be possible with a few of
   the stronger updrafts, mainly from western New York southwestward,
   damaging winds will likely be the primary severe risk.  The wind
   potential should continue into the evening as the storms spread east
   of the Lower Great Lakes, but with diurnal cooling underway, the
   already meager instability expected along and east of the
   Appalachians should eventually result in a gradual decrease in wind
   risk.  Still, given strength of flow aloft, gusty/locally damaging
   winds may persist through much of the period as the front approaches
   the New England coast -- even after lightning has diminished within
   the frontal convective band.

   ...MAXIMUM RISK BY HAZARD...
   Tornado:   5%     - Slight
   Wind:     15%     - Slight
   Hail:     15%     - Slight

   ..Goss.. 09/20/2018

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