Severe Storm Outlook Narrative (AC)
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ACUS02 KWNS 210628
SWODY2
SPC AC 210627

Day 2 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
0127 AM CDT Mon Aug 21 2017

Valid 221200Z - 231200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS TUESDAY AFTERNOON
AND EVENING ACROSS MUCH OF THE OHIO VALLEY AND LOWER GREAT LAKES
REGION INTO PORTIONS OF WESTERN NEW ENGLAND...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS
SURROUNDING AREAS OF THE NORTHEAST AND THE TENNESSEE/MISSISSIPPI
VALLEYS INTO THE SOUTH CENTRAL PLAINS...

CORRECTED DAY OF WEEK IN SLIGHT RISK HEADLINE

...SUMMARY...
Severe thunderstorms are possible Tuesday afternoon and evening
across the lower Great Lakes region and Ohio Valley into parts of
the Northeast, accompanied mainly by the risk for potentially
damaging wind gusts.  Scattered strong thunderstorms may also impact
parts of the Ozark Plateau into the south central Plains, with at
least some risk for severe weather.

...Synopsis...
Short waves may remain slowly progressive within the amplified main
belt of mid-latitude westerlies, roughly centered near the
Canadian/U.S. border area during this period.  As one larger-scale
trough and embedded closed low approach the British Columbia coast,
a mid/upper cyclonic circulation within much weaker lower latitude
flow may become increasingly deformed while shifting inland across
California.

East of large-scale ridging shifting across the Canadian Prairies,
and east of the northern U.S. Rockies, there may be some further
amplification of large-scale troughing across the eastern Canadian
provinces, Great Lakes and Northeast.  One significant shorter
wavelength impulse is forecast to pivot through the Great Lakes
region and contribute to forcing for fairly strong low/mid-level
cyclogenesis east northeast of the Great Lakes region, through
Quebec.  Merging into the southern/eastern periphery of the
increasingly cyclonic regime, this feature may be preceded by the
increasingly sheared remnants of a perturbation emerging from the
Southwestern monsoonal regime.

In association with the deepening surface cyclone, a notable cold
front appears likely to advance eastward and southward through much
of the Northeast, Ohio and Tennessee Valleys and south central
Plains by late Tuesday night.  Forcing for ascent, and
destabilization supported by seasonably high moisture content along
and ahead of it, are expected to support the development of
considerable thunderstorm activity Tuesday into Tuesday night.  At
least some of this will probably be accompanied by a risk for severe
weather.

...Northeast into the southern Plains...
Generally weak deep layer mean flow/shear and relatively warm
mid-levels still appear limiting factors to pre-frontal severe storm
potential near/southwest of the lower Ohio Valley.  East of the
lower Great Lakes into New England, lingering uncertainties
concerning the magnitude of boundary layer destabilization are also
precluding higher severe probabilities than currently indicated.
This seems particularly the case across parts of eastern Ohio and
western Pennsylvania into Upstate New York, where strengthening deep
layer shear and forcing for ascent appear most favorable for
organized severe storm development Tuesday afternoon and evening.

Still, with at least weak to moderate boundary layer destabilization
and strengthening of cyclonic flow in the 850-500 mb layer to 30-50+
kt, the environment probably will become conducive to the evolution
of one or more broken lines of thunderstorms capable of producing
potentially damaging surface gusts.  This appears likely to initiate
by early Tuesday afternoon (17-20Z) to the lee of Lakes Ontario and
Erie, before spreading toward the Champlain/Hudson Valleys and
northern Mid Atlantic Coast region, while developing southwestward
across the Ohio Valley, into Tuesday evening.

..Kerr.. 08/21/2017

$$


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