Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 230601
SWODY2
SPC AC 230600

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
0100 AM CDT Sun Jul 23 2017

Valid 241200Z - 251200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE NORTHERN PLAINS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER THE CENTRAL
APPALACHIANS REGION AND EAST TO THE MID ATLANTIC/SOUTHERN NEW
ENGLAND COASTAL AREAS...

...SUMMARY...
Isolated strong/severe storms may occur over portions of the
northern Plains vicinity, as well as across parts of the central
Appalachians and mid Atlantic region.

...Synopsis...
As mid-level troughing moves eastward across the Northeast, and a
second trough lingers near the West Coast, large-scale ridging will
continue to affect much of the country.  A strong low crossing the
Canadian Prairies may also affect convective potential over the
north-central states late in the period.

At the surface, a cold front accompanying the Canadian upper low
will shift into the northern Plains later in the period, while a
weaker system crosses the Northeast in conjunction with the upper
trough traversing this region.

...Northern Plains...
As afternoon heating of a modestly moist boundary layer ensues
across the northern Plains, isolated afternoon storms may develop,
aided by ascent near the Black Hills, and along a weak trough
progged to reside atop the area.  Moderate mid-level west-northwest
flow could allow a couple of cells to organize, and become capable
of producing evaporatively aided damaging gusts and/or hail.

Later, the eastward advance of an compact upper low/trough into the
Canadian Prairies will support progression of a surface cold front
across the northern Plains through the second half of the period.
While the strongest large-scale ascent should remain north of the
international border, a band of frontal storms -- most likely over
North Dakota -- may evolve during the late afternoon/early evening
hours and shift east with time.  Locally damaging winds and hail
would be possible, though coverage of risk and associated/parent
convection remains in question at this time.  As such, will maintain
only MRGL/5% risk at this time, but upgrade to 15% probability could
be required in later forecasts -- particularly over northern
portions of North Dakota.

...Central Appalachians to the mid Atlantic/southern New England
coasts...
Strong storms may be ongoing at the start of the period across
portions of the southern New England vicinity, shifting
eastward/offshore through the morning.  Meanwhile farther west into
the central Appalachians, cooling aloft associated with advance of
the upper trough atop some low-level heating will support pockets of
modest CAPE across portions of the PA/NY vicinity.  This should
support isolated to scattered afternoon storm development, with a
few stronger cells likely to evolve given amply strong flow aloft.
While flow is expected to weaken with southwestward extent, stronger
destabilization across WV and southwestward into the mid South, may
allow limited severe risk to extend as far southwest as TN.  Storms
across the region should peak in intensity by late afternoon,
diminishing through the evening hours with the loss of diurnal
heating.

..Goss.. 07/23/2017

$$



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