Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 221708
SWODY2
SPC AC 221707

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1207 PM CDT Sat Jul 22 2017

Valid 231200Z - 241200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS MUCH OF THE
MIDWEST...OHIO VALLEY...AND MID-ATLANTIC...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK ACROSS BROAD PORTIONS OF THE GREAT
LAKES...MIDWEST...OHIO VALLEY...MID-ATLANTIC...SOUTHERN PLAINS...AND
MID-SOUTH...

...SUMMARY...
A broad zone of risk for storms and isolated severe risk will reside
over portions of Great Lakes/mid Atlantic/central Appalachians area
on Sunday.  Isolated severe storms may also occur from Oklahoma
eastward to the Mid-South during the afternoon.

...Synopsis...
A broad, synoptic-scale mid-level trough will migrate southeastward
across the Great Lakes on D2/Sunday, resulting in broadly cyclonic
30-45 knot mid-level flow across much of the Ohio Valley and
Mid-Atlantic.  Farther west, a persistent mid/upper ridge will
remain centered near the Four Corners region, displacing the main
belt of westerlies across southwestern Canada and the adjacent
northern Rockies.

At the surface, weak low pressure centered over western Pennsylvania
will migrate slowly eastward through the day, and a trailing wind
shift will migrate southward from the central Appalachians westward
to Oklahoma/northern Arkansas.  Stout surface ridging will persist
across New England and another area of high pressure will migrate
southward across the northern Plains.

...Areas of the Midwest and Ohio Valley eastward to the
Mid-Atlantic...
Weak convection will likely be ongoing in portions of the region at
the start of the forecast period, which may result in localized
areas of relatively stable conditions into the afternoon.
Nevertheless, model consensus suggest that a broad area of moderate
to strong instability will develop during the day as insolation
results in heating of a very moist low-level airmass.  The
combination of this unstable, weakly capped airmass and subtle
forcing for ascent beneath the broad mid-level trough and along
remnant outflow/surface boundaries will foster the development of
scattered to potentially widespread convection during the afternoon
and early evening hours.  These storms will pose a primarily
diurnally driven threat for damaging wind gusts - especially with
clusters and linear segments that can evolve throughout the
afternoon and particularly in the Slight risk area where deep shear
profiles are the strongest.  Models seem to be less aggressive with
convective coverage in Illinois/Indiana and vicinity, although the
background synoptic environment supports clusters/bands of storms in
these areas with an appreciable wind/hail threat into the evening.

Farther west into Oklahoma/northern Arkansas, deep shear is meager
at best as wind fields are generally less than around 15 knots
throughout the troposphere.  Nevertheless, hot surface temperatures
and steep low-level lapse rates will foster pulse-type storms with
sporadic strong-to-severe downbursts being the primary threat.

..Cook.. 07/22/2017

$$


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