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
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.
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
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