SPC AC 161730

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1230 PM CDT Thu Aug 16 2018

   Valid 171200Z - 181200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE NORTHEAST...

   ...SUMMARY...
   Isolated severe thunderstorms with mainly a strong to damaging wind
   threat will be possible Friday afternoon and evening across parts of
   the Northeast and Mid-Atlantic.

   ...Synopsis...
   Within large-scale troughing over Ontario/Quebec and the Upper
   Midwest/Great Lakes region, several shortwave troughs will develop
   east-northeastward across the Northeast, Mid-Atlantic, and OH Valley
   through the period. A 30-40 kt south-southwesterly low-level jet is
   forecast to overspread parts of the Northeast by Friday afternoon.
   Farther west, a shortwave trough will move eastward across the
   Pacific Northwest and northern Rockies. At the surface, a weak
   surface low should develop northeastward into northern New England
   by Friday evening, with a trailing cold front extending
   southwestward from this low across the OH/TN Valleys into the
   Mid-South and southern Plains.

   ...Northeast and Mid-Atlantic...
   A warm front should lift northward over much of the Northeast
   through the day, and a very moist low-level airmass, with dewpoints
   in the upper 60s to lower 70s, will likely be in place to the south
   of this front. Even through mid-level lapse rates will remain poor,
   as diurnal heating occurs instability should increase into the
   1000-2000 J/kg range across southern VT/NH into eastern NY, western
   MA/CT and vicinity. Although relatively stronger mid-level flow may
   remain mostly displaced to the north of this region, forecast
   soundings from the NAM/RAP show sufficient veering/strengthening
   with height to support 25-35 kt of effective bulk shear. A mix of
   multicells and marginal supercell structures should occur, and most
   high-resolution guidance suggests convection will probably develop
   along a pre-frontal trough by Friday afternoon and subsequently
   spread eastward through the evening. Have introduced a Slight risk
   for part of the Northeast for the potential of isolated strong to
   damaging winds with one or more bowing clusters. A tornado or two
   may also occur along and south of the warm front given the forecast
   strength of the low-level jet.

   ...OH/TN Valley into the Southeast and Southern Plains...
   Thunderstorms will likely be ongoing across parts of the OH/TN
   Valleys Friday morning, and this will have a great influence on
   better convective potential in the afternoon. Have
   maintained/expanded Marginal risk across the OH Valley as some
   guidance is now suggesting additional thunderstorms could form along
   the cold front in the afternoon, and mid-level flow may be enough to
   support 25-30 kt of effective bulk shear and a few multicell
   clusters. Across the TN Valley into parts of the Southeast and
   southern Plains, mid-level flow and resultant shear should remain
   generally weak. Even so, ongoing convection Friday morning may
   re-strengthen along and ahead of the cold front and spread
   southeastward with time. Across this broad region, strong to locally
   damaging winds may occur on an isolated basis.

   ...Southern/Central High Plains...
   Isolated to scattered thunderstorms may develop across the higher
   terrain of the southern/central Rockies and then spread eastward
   into the southern/central High Plains through Friday afternoon and
   evening. This may be in response to a weak mid-level perturbation
   rounding the upper ridge over the western states. Forecast soundings
   from the NAM/RAP show sufficient shear in conjunction with weak to
   moderate instability to support a Marginal risk for hail and
   strong/gusty winds with any thunderstorms that occur.

   ...Northern Rockies/Great Basin...
   Steep mid-level lapse rates and large-scale ascent attendant to a
   shortwave trough will promote some destabilization by mid afternoon.
   However, instability should remain marginal (MLCAPE at or below 500
   J/kg) owing to limited low-level moisture. Storms are expected to
   develop over the higher terrain during the day and spread eastward
   through the evening. Inverted-V type forecast soundings showing a
   deeply mixed boundary layer will an isolated risk for strong/gusty
   downdraft winds from late afternoon through early evening.

   ...MAXIMUM RISK BY HAZARD...
   Tornado:   2%     - Marginal
   Wind:     15%     - Slight
   Hail:      5%     - Marginal

   ..Gleason.. 08/16/2018

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