SPC 1730Z Day 2 Outlook
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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