SPC AC 210541

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1241 AM CDT Fri Sep 21 2018

   Valid 211200Z - 221200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON AND EVENING...SOUTH AND EAST OF LAKES ERIE AND ONTARIO...

   ...SUMMARY...
   Severe thunderstorms are possible across parts of the lower Great
   Lakes region late this afternoon and evening, accompanied by the
   risk for potentially damaging wind gusts.

   ...Synopsis...
   It appears that the influence of mid/upper subtropical ridging will
   become increasingly confined to the southern tier of the U.S. during
   this period, with generally zonal belts of mid-latitude westerlies
   prevailing across the northern tier and Canada.  Within the latter
   regime, one significant short wave trough is forecast to accelerate
   northeast of the upper Great Lakes region through Quebec and
   adjacent portions of the Northeast.  It appears that fairly strong
   cyclogenesis will proceed in association with this feature, with the
   center of the broad surface low forecast to migrate northeast of
   Lake Superior through Newfoundland and Labrador by 12Z Saturday.  A
   cold front trailing from the cyclone may advance southeastward
   through all (to the east of the Rockies) but the Gulf and south
   Atlantic Coast states.

   ...Lower Great Lakes region into Northeast and Ohio Valley...
   A relatively cool low-level air mass is expected to linger across
   the northern Mid Atlantic Coast region through much of New England,
   beneath relatively warm and warming mid/upper levels.  Guidance
   suggests that this stable stratification will linger through at
   least late this evening, with pre-frontal destabilization limited to
   a corridor of stronger boundary layer heating and low-level moisture
   return from the lower Ohio Valley through the lower Great Lakes
   region, southern Ontario and western Quebec.  Colder air aloft
   associated with the short wave appears likely to lag to the west of
   the cold front even as it approaches this region, but it appears
   that peak afternoon mixed layer CAPE will still exceed 1000 J/kg,
   and could approach 2000 J/kg.  

   In the presence of 40-50 kt flow in the 850-500 mb layer, at least
   as far south as the Lake Erie vicinity, the environment appears
   likely to become conducive to organized severe storm development. 
   This may include low-level hodographs initially large and clockwise
   curved, beneath strengthening southwesterly high level flow, across
   parts of northwestern Pennsylvania into western New York state. 
   These profiles would be favorable for supercells with a risk for
   tornadoes, given storm initiation.  However, forcing to support
   thunderstorm development remains unclear until late afternoon, when
   it appears that a perturbation near/just southeast of the short wave
   within the westerlies will approach the lower Great Lakes region. 
   As this occurs, thunderstorm development along or just ahead of the
   cold front may generally coincide with a transition to more
   unidirectional lower/mid tropospheric flow.  But an evolving line of
   storms probably will be accompanied by the risk for severe wind
   gusts.  Thunderstorms may develop southwestward into progressively
   weaker deep layer mean flow (and corresponding diminishing severe
   wind potential), toward the lower Ohio Valley through early evening.

   Farther east, thunderstorm activity is expected to weaken after dark
   as it advances into the Hudson/Champlain Valleys and northern New
   England.  Model forecast soundings do suggest some pre-frontal
   boundary-layer based destabilization is possible across this region
   overnight.  However, beneath lingering warm mid/upper levels, this
   does not appear likely to be deep enough to support thunderstorms,
   though showers accompanied by locally strong surface gusts may not
   be out of the question.

   ..Kerr/Squitieri.. 09/21/2018

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