Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 172332

Area Forecast Discussion
National Weather Service Des Moines IA
632 PM CDT Tue Oct 17 2017

.SHORT TERM.../Tonight through Wednesday/
Issued at 320 PM CDT Tue Oct 17 2017

The short term forecast period is very quiet. Modest southerly
breezes will prevail tonight and much of tomorrow, with clear
skies and relatively mild temperatures. Late tomorrow a weak
trough will approach from the west and cross our northwestern
counties by the evening, but only a few clouds are expected and no
precipitation, with light winds shifting to southwest and little
to no cold air advection.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 320 PM CDT Tue Oct 17 2017

The weak surface trough moving across the area late Wednesday will
wash out over Iowa on Wednesday night, then rebound northward on
Friday. This will result in only slightly cooler temperatures,
perhaps five degrees below levels of the previous day, and light
and variable winds. From Thursday night through Friday a larger
low pressure trough will slowly approach from the Rockies,
resulting in a tighter pressure gradient and more robust southerly
winds, reaching speeds of 20 to 25 mph with higher gusts by Friday
afternoon. Increased forecast winds toward CONSMOS to reflect
this. Fire weather concerns should be mitigated by relative
humidity minima only in the 40s and fairly wet surface fuels in
many areas.

The aforementioned western U.S. trough will finally move across
Iowa over the weekend, bringing a swath of showers and
thunderstorms with it. There continue to be timing differences
between various model runs, as evidenced by the 12Z GFS and EC
which are about 6-9 hours apart with frontal passage on
Saturday/Saturday night. Have spread out POPs a bit temporally to
account for this uncertainty, but either way rain is likely
sometime Saturday and while severe weather is improbable, QPF
could end up pretty decent in spite of the progressive nature of
the system, due to anomalously high precipitable water values.
Even so, no significant impact is expected other than
rain/lightning for outdoor activities, and putting more water in
the ground.

By Sunday morning the trough will have moved off to the east, with
clearing skies and cooler weather then on tap. It appears a
reinforcing cold front will traverse the region on Monday or
Monday night, perhaps bringing a few light showers but more
notably even cooler temperatures for the middle of next week.
Forecast high temperatures next Tuesday are only in the 50s across
the forecast area, and long range models show the potential for
more reinforcing fronts later in the week, so we may be on the
cusp of the descent into true autumn weather.


.AVIATION.../For the 00Z TAFS through 00Z Wednesday evening/
Issued at 632 PM CDT Tue Oct 17 2017

Very favorable aviation conditions as ceilings and visibilities
remain mostly unrestricted. Surface winds will be from the south
to southwest.





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