Severe Storm Outlook Narrative (AC)
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ACUS03 KWNS 200720
SWODY3
SPC AC 200719

Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0219 AM CDT Sun Aug 20 2017

Valid 221200Z - 231200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS TUESDAY INTO
TUESDAY EVENING  EAST OF THE LOWER GREAT LAKES REGION SOUTHWARD
THROUGH MUCH OF THE OHIO VALLEY...

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

...SUMMARY...
Severe thunderstorms are possible Tuesday across much of Upstate New
York, southward through the Ohio Valley, accompanied mainly by the
risk for potentially damaging wind gusts.

...Synopsis...
Within the amplified main belt of mid-latitude westerlies,
northeastern Pacific upper troughing and an embedded closed low
appear likely to gradually approach the British Columbia/Pacific
Northwest coast during this period.  As this occurs, downstream
ridging will likely continue shifting eastward across the Canadian
prairies, while a mid/upper cyclonic circulation within much weaker
lower latitude flow elongates and perhaps accelerates slowly
northeastward near the California coast.

Some further amplification of downstream troughing across the Great
Lakes into the Northeast is possible.  Within this regime, one
vigorous shorter wavelength impulse is forecast to pivot through the
Great Lakes region, and may contribute to forcing for significant
surface cyclogenesis east northeast of the Great Lakes through
Quebec by late Tuesday night.  South/southeast of this impulse, the
increasingly sheared remnants of a perturbation emerging from the
Southwestern monsoonal regime may continue developing east
northeastward, across and north/east of the Ohio Valley, portions of
the Ozark Plateau and south central Plains.

In association with the deepening surface cyclone, a notable cold
front is forecast to advance eastward and southward through much of
the Northeast, Ohio and Tennessee Valleys and south central Plains
by the end of the period.  Forcing for ascent and destabilization
along and ahead of this feature may support considerable
thunderstorm activity Tuesday through Tuesday night, at least some
of which probably will be accompanied by a risk for severe weather.

...Northeast into the southern Plains...
Spread evident within and among the output of the various models
precludes greater certainty concerning the details of the potential
convective evolution and more precise severe weather probabilities
for this period.  In general, though, weak deep layer flow and shear
may limit pre-frontal severe thunderstorm probabilities to the
southwest of the Ohio Valley.  The degree of instability and timing
of the arrival of stronger forcing for ascent are more of a concern
northeast of the Great Lakes region, into northern New England.

Strongest mid/upper flow may tend to lag to the west of the
southeastward advancing cold front.  Still, weak to moderate
pre-frontal boundary layer destabilization, coupled with
strengthening of cyclonic lower/mid tropospheric wind fields to
30-50 kts, probably will be conducive to severe storm development
Tuesday into Tuesday evening.  Aided by mid/upper forcing for
ascent, storms are expected to initiate to the lee of Lakes Ontario
and Erie, probably evolving into one or more organized lines
accompanied by potentially damaging wind gusts while spreading
toward the Champlain/Hudson Valleys and northern Mid Atlantic Coast
region.  Similar additional activity seems likely to initiate
southwestward across the Ohio Valley, before spreading southeastward
into the central Appalachians and Tennessee Valley Tuesday evening.

..Kerr.. 08/20/2017

$$


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