SPC AC 201730

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1230 PM CDT Tue Aug 20 2019

   Valid 211200Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS WEDNESDAY ACROSS
   PARTS OF THE NORTHERN MID ATLANTIC STATES AND NEW ENGLAND...

   ...SUMMARY...
   Severe thunderstorms are possible across parts of the northern Mid
   Atlantic Coast states into New England Wednesday.  Additional strong
   to severe storms are possible in a corridor from the central Plains
   into the lower Ohio Valley, and near the Rockies.

   ...Synopsis...
   The stronger westerlies appear to be undergoing amplification, but
   may remain largely confined to Canada and the northern tier of the
   U.S. through this period.  Within this regime, larger-scale
   mid/upper troughing is forecast to progress inland of the British
   Columbia and Pacific Northwest coast, through the Canadian Rockies
   and northern U.S. intermountain region by late Wednesday night.  As
   this occurs, sharp downstream ridging should shift eastward across
   interior Canada, while large-scale troughing to the east develops
   across Ontario/Quebec and the Great Lakes region, toward the
   northern Atlantic coast.

   Models indicate the eastern troughing will include an embedded
   mid-level low, with at least a couple of significant perturbations
   migrating through the broader cyclonic flow.  One of these is
   forecast to be in the process of digging into the upper Great Lakes
   region early Wednesday, before accelerating eastward and
   northeastward into and through Quebec by 12Z Thursday.  It appears
   that associated forcing for ascent will support significant surface
   cyclogenesis across Quebec, with the most rapid deepening to the
   northwest of the St. Lawrence Valley Wednesday night.  

   A front trailing from the developing cyclone is expected to advance
   through much of the Upper Midwest, mid Missouri Valley and northern
   Plains by early Wednesday, before continuing southeastward
   into/through New England, the Ohio and middle Mississippi Valleys
   and the central Plains by 12Z Thursday.  The front will be preceded
   by the remnants of convective outflow from the large ongoing
   convective system (now advancing into the lower Ohio Valley), which
   may still be evident across the central Appalachians and lower Ohio
   Valley early Wednesday.

   Seasonably moist air along and ahead of pre-frontal surface
   troughing across parts of the Northeast, and the convective
   outflow/cold front extending westward across the central
   Appalachians into the Rockies, appears likely to contribute to
   moderate to strong potential instability with daytime heating
   Wednesday.  This is expected to provide support  for areas of strong
   thunderstorm development, some of which will probably pose at least
   some risk for severe wind and hail.

   ....Northeast...
   The extent to which to which the stronger surface cyclogenesis
   impacts convective potential across the region during this period
   remains unclear, given the apparent overnight timing of the most
   rapid surface deepening.  Additionally, it is possible that the
   associated surface cold front may become a focus for thunderstorm
   development, but this may be mostly over portions of southeastern
   Ontario and southwest Quebec prior to Wednesday evening.  However,
   the primary convective potential seems likely in association with
   forcing for ascent accompanying a remnant convectively generated or
   enhanced perturbation, which is forecast to develop northeastward
   ahead of the primary troughing within the westerlies, from the lee
   of the lower Great Lakes region through New England by Wednesday
   evening.

   A belt of enhanced southwesterly lower/mid tropospheric flow (on the
   order of 30-40+ kt), coupled with at least modest destabilization
   (CAPE of 1000-2000+ J/kg) near/ahead of a developing pre-frontal
   surface trough, may provide an environment conducive to organized
   severe storm development across parts of eastern New York and
   Pennsylvania and much of New Jersey through much of New England. 
   Potentially damaging wind gusts appear the primary hazard, but there
   may be at least some risk for a tornado or two, particularly across
   the Hudson/Champlain Valley vicinity into western New England
   Wednesday afternoon.

   ...Ohio and middle Mississippi Valleys into high Plains...
   It appears that the focusing boundaries for moderate to strong
   boundary layer destabilization and thunderstorm development will be
   south of the stronger shear associated with the westerlies. 
   However, thermodynamic profiles characterized by relatively steep
   lapse rates may still be favorable for convection capable of
   producing hail and strong wind gusts associated with downbursts and
   convectively generated cold pools.  Too much uncertainty still
   exists concerning sub-synoptic features to attempt to delineate
   areas of higher ("slight risk") severe probabilities at this time.

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

   ..Kerr.. 08/20/2019

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