Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 212344

Area Forecast Discussion
National Weather Service Des Moines IA
644 PM CDT Tue Aug 21 2018

...Updated Aviation Discussion...

.DISCUSSION.../Tonight through Tuesday/
Issued at 345 PM CDT Tue Aug 21 2018

Iowa remained within broad cyclonic surface flow on the back side
of the low pressure system that has moved over the Great Lakes
Region. This allowed cloudy skies to linger for a decent amount of
time this afternoon. However visible GOES 16 imagery has been
showing gradual clearing this afternoon across the north, and
clear skies across the southern portion of the CWA. Clearing will
continue with upper-level ridge sitting off to the west, promoting
AVA and enough subsidence to give us clearer conditions
overnight. This will continue into Wednesday afternoon developing
surface high pressure. This will hinder any POPs. Flow will be
northwesterly from near surface through most of the troposphere
over the next 24 hours, which will keep temperatures seasonably
cool Wednesday afternoon.

Attention turns to Thursday as a short-wave trough propagates
across the northern Plains into the upper Midwest. GFS and NAM
are more robust than the ECMWF with forcing available for POPs as
the short- wave moves though. For now, will leave around 30 POPs
for most of the CWA Thursday with WAA. Late Thursday night into
Friday morning POPs increase as surface low associated with short-
wave trough moves closer to the area, enhancing convergence. At
this time thunderstorms will have more potential to be robust. NAM
and GFS are in favor of SBCAPE in excess of 2000 J/kg Friday
morning into afternoon along with 35 to 40 knots of bulk shear.
For this reason, there is threat of severe weather late Thursday
night through Friday. However, challenge will be with any debris
from Thursday`s rain showers that may prevent boundary layer
destabilization for Friday, certainly limiting severe chances if
this is the case.

After Friday, upper-level flow will remain west-southwesterly
through the weekend. GFS favors a few embedded short-wave troughs
through the weekend which will introduce isolated rain and
thunderstorm chances Sunday through early Monday morning. Monday
afternoon through Tuesday upper-level flow remains west-
southwesterly, long term guidance does not favor any forcing which
will keep conditions dry. Temperatures will gradually warm back up
near and perhaps just above seasonal normals. Beyond GFS and ECMWF
solutions diverge considerably, with GFS favoring POPs along a
short-wave disturbance late Tuesday next week into Wednesday.


.AVIATION.../For the 00Z TAFS through 00Z Wednesday evening/
Issued at 643 PM CDT Tue Aug 21 2018

VFR conditions are forecast throughout the TAF period. Only
concern is with current breezy winds to subside later tonight.
High pressure will continue to build into the region and provide
clear skies with light winds by tomorrow.





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