Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 202339

Area Forecast Discussion
National Weather Service Des Moines IA
639 PM CDT Sat Aug 20 2016

...Aviation Update...

.SHORT TERM.../Tonight/
Issued at 325 PM CDT Sat Aug 20 2016

Broad cumulus field across the forecast area this afternoon as upper
system and cool air aloft pass near the state. A few showers have
developed in the weak instability across the northern half of Iowa
and anticipate this to gradually diminish into the evening as
daytime heating subsides. The cloud field will also diminish into
the evening with skies becoming clear during the overnight. The
approaching surface ridge will also produce a weak wind field and
with the clear skies and dry airmass, temperatures are expected to
cool quickly overnight. Have dropped overnight lows just a bit in
the favored river valleys in the west and north.

.LONG TERM.../Sunday through Saturday/
Issued at 325 PM CDT Sat Aug 20 2016

The long term forecast period is essentially a two-hit wonder with
storm systems slated to affect the region around the middle of
next week and then again around next weekend.

From Sunday through Monday night the weather will remain quiet and
fairly cool as a mid-level trough departs to the east and is
replaced by weak ridging. At the surface this will mean a ridge
moving across around Sunday night followed by a return of
southerly flow, promoting steady airmass modification during the
early part of the week.

By Tuesday morning a much more humid airmass will be in place over
our region and a broad trough will be crossing the Rockies. This
trough will move slowly eastward, taking a couple of days to
finally clear our forecast area. This will result in likely a
couple of rounds of widespread rainfall, the first on Tuesday
night into Wednesday morning and the second potentially around
Wednesday evening. At the same time a cool front will be pushing
southeastward across the region and it remains to be seen whether
the first round of storms will push the boundary further southeast
and result in the second round being more across Missouri and
eastern Iowa. In any event, the atmosphere will be primed for
heavy rainfall especially with the initial round Tuesday night.
High PWATs, modest steering flow roughly parallel to the front and
moisture transport enhanced by a nocturnal low-level jet will
increase the possibility of hydrologic issues and this threat will
be closely monitored over the next few days. On Wednesday
afternoon and evening, if the surface front remains within our
forecast area there may be some threat of severe thunderstorms
however it will be conditional on the degree of destabilization
that can occur and the flow/shear parameters look pretty weak.

The front and storms will finally blow out to the southeast by
Thursday resulting in more cool and quiet weather to end the week.
Around next saturday another storm system will move overhead and
likely generate more showers and thunderstorms. However, at this
time it appears that this system will be less potent and with less
warming and destabilization ahead of it so the threat of hazardous
weather should be lower, though there is a lot of time for that to


.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 636 PM CDT Sat Aug 20 2016

A very low impact TAF period on deck here. The only concern will
reside at the beginning as lingering showers fade this evening,
primarily at KMCW with an outside shot at KFOD and KALO. With
peak heating in the rear window, cumulus field already clearing
significantly, signaling a small remaining window for showers.
Remaining TAF period highlighted by lighter northwest winds and
clearing skies.




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