Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 211716
SWODY2
SPC AC 211715

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1215 PM CDT Fri Jul 21 2017

Valid 221200Z - 231200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM ILLINOIS
EASTWARD TO THE MID-ATLANTIC...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS NORTHERN
MINNESOTA/WISCONSIN AND VICINITY...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS IN A BROAD AREA
FROM THE CENTRAL PLAINS AND UPPER MISSISSIPPI RIVER VALLEY EASTWARD
TO THE MID-ATLANTIC...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS CENTRAL
AND SOUTHERN ARIZONA...

...SUMMARY...
A broad area of severe weather risk is apparent from portions of the
northern and central Plains east across the Midwest to the mid
Atlantic region.  A few severe storms may also occur across portions
of southern Arizona and along the Nebraska/Kansas border.

...Synopsis...
A mid-level belt of quasi-zonal flow will extend from the northern
Rockies eastward across the Great Lakes and Mid-Mississippi Valley
and on to the Mid-Atlantic throughout the day Saturday.  A shortwave
trough will migrate from southern Manitoba to the western Great
Lakes throughout the forecast period as well.  At the surface, one
or two MCSs will be ongoing at the beginning of the period across
northern Illinois/southern Lower Michigan and vicinity.  This
activity will migrate eastward and augment the position of a surface
boundary, which should generally extend from west-to-east from
northern Kansas into Indiana/Ohio and on to New Jersey.  Farther
north, weak surface cyclogenesis will occur across Minnesota during
the day, eventually developing into Wisconsin after dark in
conjunction with the aforementioned mid-level shortwave.

...Illinois eastward to the Mid-Atlantic...
Considerable uncertainty exists regarding the evolution of a morning
MCS that should be located in the general vicinity of northern
Illinois/southern Michigan at the beginning of the forecast period.
This activity should propagate eastward through noon and pose a
marginal risk of damaging wind gusts during the morning hours.
Parts of Lower Michigan may not destabilize much in the wake of
morning convection and attendant severe probabilities have been
reduced in this outlook as a result.

Later in the day, models suggest some re-intensification of the MCS
and/or remnant boundaries from Pennsylvania westward to Indiana/Ohio
as moderate to strong surface-based destabilization occurs within
the undisturbed pre-convective airmass.  Deep shear profiles favor
storm organization, with an evolution into forward-propagating
linear segments expected into the afternoon.  Damaging wind gusts
and large hail are likely in this region - and portions of
Ohio/Pennsylvania may need an upgrade in later outlooks once
finer-scale details of convective evolution become clearer.
Furthermore, an isolated tornado threat may evolve in Pennsylvania,
where low-level shear parameters (although modest) become maximized
in the early evening due to veering of flow with height.

Isolated to scattered convection should develop as far west as the
Mississippi River in Illinois along and ahead of a surface boundary
across the region and within a strongly unstable airmass (MUCAPE
values exceeding 4000 J/kg).  15%/slight risk probabilities have
been maintained in the area to address this threat.

...Minnesota/Wisconsin and vicinity...
Insolation and ascent associated with a well-timed, approaching
mid-level shortwave will foster development of scattered
thunderstorms in the afternoon (especially across northern Minnesota
and into northwestern Wisconsin.  More isolated activity may develop
farther south toward the Twin Cities area.  Although surface flow
should be veered to west/northwesterly across the region, warming
surface temperature and 70s F dewpoints will aid in moderate to
strong destabilization (around 3000-4000 J/kg MUCAPE).  Deep shear
favors updraft organization, with scattered hail and wind-producing
thunderstorms.  Any remnant low-level boundaries from early morning
convection may augment low-level shear in the region and result in
brief updraft rotation as well.  With time, this activity should
spread eastward toward the Upper Peninsula of Michigan, although a
loss of daytime heating should result in a lessening of the severe
threat with eastward extent.

...Southern Nebraska/northern Kansas eastward to the Quad Cities
area...
A surface boundary should migrate into the region from the north
over the course of the afternoon, with strong destabilization
expected along and ahead of this feature.  Scattered, loosely
organized convection should develop across the region during peak
heating hours and pose a damaging wind and hail threat - with
perhaps some greater organization into forward-propagating linear
segments possible in eastern Iowa/Missouri where deep shear should
be somewhat stronger.  Farther west, higher-based activity should
unfold given greater boundary layer mixing (near 100F surface
temperatures and 50s F dewpoints), fostering more of a damaging
downburst wind threat.  This threat should wane some after dark with
the onset of nocturnal boundary layer cooling.

...Central/southern Arizona...
20-25 knot northeasterly mid-level flow, along with moist
low/mid-level profiles and modest destabilization, should foster
development of scattered thunderstorms initially along higher
terrain.  These storms will have a tendency to propagate into lower
elevations with time, posing an isolated threat for damaging wind
gusts into the afternoon and evening hours.

..Cook.. 07/21/2017

$$



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