Severe Storm Outlook Narrative (AC)
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000
ACUS03 KWNS 190717
SWODY3
SPC AC 190716

Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0216 AM CDT Sat Aug 19 2017

Valid 211200Z - 221200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS MONDAY INTO
MONDAY NIGHT ACROSS PARTS OF THE CENTRAL PLAINS AND LOWER/MIDDLE
MISSOURI VALLEY INTO THE UPPER MIDWEST...

...SUMMARY...
Strong thunderstorms may impact a corridor from the central Plains
into the Upper Midwest Monday into Monday night, accompanied by at
least some risk for severe weather.

...Synopsis...
The stronger mid-latitude westerlies likely will remain confined to
the northern tier of the U.S and southern Canada through this
period.  However, models indicate that amplification commencing
within this regime, across the northeastern Pacific into western
Canada on Sunday, will continue and translate downstream, with
significant upper troughing likely to evolve and dig across the
central Canadian/U.S. border area by late Monday night.  This is
expected to be accompanied by the southward advancement of a cold
front into and through much of Upper Midwest, lower Missouri Valley
and central Plains, and perhaps the initiation of cyclogenesis along
the front across the Great Lakes region by 12Z Tuesday.

...North central Plains into the Upper Midwest...
The possibility for considerable lower/mid tropospheric warm
advection driven convection to be ongoing at 12Z Monday near/east of
the middle Missouri Valley is one of a number of uncertainties
concerning convective potential for this period.  Also, relatively
cool mid-level air and stronger mid/upper flow appear likely to lag
behind the surface front.  However, models suggest that seasonably
moist low-level conditions may still contribute to moderate CAPE
with daytime heating.  And a belt of 30-40 kt westerly lower/mid
tropospheric flow may be sufficient to support organized convective
development.

Within an increasingly cyclonic regime aloft, to the south of the
main short wave trough, models indicate a possible consolidation of
forcing associated with one short wave perturbation emerging from
the southwestern monsoonal regime, and a mid-latitude impulse
digging southeast of the northern Rockies.  This seems to provide
the most obvious support for an appreciable evolving cluster of
thunderstorms.  However, this appears most likely to take place
across and east of the middle to lower Missouri Valley well after
dark, when boundary layer instability may be in the process of
waning.

While severe potential is evident for Monday into Monday night,
mostly in the form of evolving storms clusters accompanied by a risk
for potentially damaging wind gusts, this appears too conditional
for anything greater that 5 percent severe probabilities at this
time.

..Kerr.. 08/19/2017

$$



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