Severe Storm Outlook Narrative (AC)
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792
ACUS03 KWNS 180737
SWODY3
SPC AC 180737

Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0237 AM CDT Sun Aug 18 2019

Valid 201200Z - 211200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER
ILLINOIS...EASTERN MISSOURI AND WESTERN INDIANA...AND ACROSS MUCH OF
NEBRASKA...

...SUMMARY...
Severe storms with damaging winds and hail are possible over
Illinois and parts of surrounding states, and over much of Nebraska.

...Synopsis...
A shortwave trough will pivot southeastward across MB and ON, with
30-40 kt midlevel winds across the Upper MS Valley and Great Lakes.
Heights will gradually lower well ahead of this feature, from the
mid MO Valley across the OH Valley and toward the Mid Atlantic by
Wed morning. Cool temperatures aloft for this time of year with -10
C at 500 mb will extend as far south as KS, MO, and KY. Meanwhile, a
moist and very unstable air mass will remain over a large part of
the central Plains, Midwest, and OH Valley, conditionally favoring
severe storms.

...IL...eastern MO...western IN...
Extreme instability is forecast to develop with 700-500 lapse rates
around 8 C/km, and mid 70s F surface dewpoints. Meanwhile,
west-northwest mean deep-layer winds will increase to 30-35 kt, with
lightly veering winds in the low-levels. Storms may be ongoing early
in the day over parts of IA, but will expand in coverage into IL by
midday as the air mass will already be uncapped. A forward
propagating MCS is likely, capable of destructive winds. Models are
not in complete agreement with this scenario due to weak forcing,
but such extreme instability with steep lapse rates, high
precipitable water and good flow aloft will conditionally favor
damaging winds, perhaps widespread. Further categorical upgrades are
possible in later outlooks.

...Much of NE...
Surface winds will back to southeasterly during the day, which will
maintain 70s F dewpoints. Capping will be an issue over eastern
portions of the state, but western parts of the state as well as
eastern WY will become uncapped by late afternoon, where initiation
will occur. Large to extreme instability with weak but veering winds
with height suggest brief supercells with eventual mergers into
bowing segments. Both large hail and damaging winds will be
possible, and a brief tornado cannot be ruled out in the early
stages of development. Additional storms are expected eastward
across the state by evening with low-level warm advection.

...KY and OH eastward into MD...
Southwesterly low-level winds will maintain a high theta-e air mass
from the OH Valley to the Mid Atlantic, beneath cooling temperatures
aloft. Heating will result in an uncapped air mass by afternoon,
with a weak surface trough providing a focus for scattered storms
within the theta-e plume. Shear will remain weak especially over
eastern areas, but pulse and/or multicell storms are possible,
producing locally damaging winds. If later forecasts confidently
suggest more concentrated areas of storms, parts of the region could
be upgraded to a Slight Risk for wind.

..Jewell.. 08/18/2019

$$



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