Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 140431

Area Forecast Discussion
National Weather Service Des Moines IA
1031 PM CST Wed Dec 13 2017

Issued at 529 PM CST Wed Dec 13 2017

Will expire wind advisory at 6 pm as forecast. May see a few gusts
in the evening just above 45...but overall the trends continue to
lower and expect between 6 and 7 pm most areas will be below
criteria. Breezy through late evening...then winds begin to relax.


.SHORT TERM.../Tonight through Thursday/
Issued at 248 PM CST Wed Dec 13 2017

We will be in strong cold advection tonight and Thursday though
winds will diminish through the evening. Cloud cover will save
temps from really dropping but we are still looking at teens
northeast to the mid 20s southwest. On Thursday into Thursday
evening, a shortwave is progged to drop through the flow. Sounding
do show a fair amount of low level moisture in place but the bulk
of the forcing will travel to our west and south. Still there
should be weak forcing across Iowa sufficient enough for some
light snow or flurries late afternoon into the evening hours.
Thursdays highs will only be in the mid 20s to mid 30s so it will
be pretty cold though winds will be much less than today.

.LONG TERM.../Thursday night through Wednesday/
Issued at 248 PM CST Wed Dec 13 2017

Timing differences between models on how fast an upper trough
pushes through the region on Thursday night and Friday morning.
Precip is sparse with this system and the general consensus is for
precip to remain to the south and west but the GFS and Canadian...
and to some extent the Euro (up in the northeast) all hint at some
moisture and weak forcing across eastern Iowa to at have some
flurries. While confidence is low, there is enough support for a
mention of flurries.

Friday through Saturday night there will be a weak upper ridge
over the region while at the surface a low develops over ok/ne
with a frontal boundary extending up into Iowa. For the most part
we will be warmer and dry.

Questions arise Saturday night/Sunday as an upper low drops out of
the Pacific Northwest. The GFS takes this low and digs it all the
way into New Mexico while the Euro puts the low over Utah with
precip developing into far NW IA into southern MN. Later on
Sunday a weaker shortwave ejects out of the base of this western
low spreading precip across the Southern Plains and into southern
Iowa in the form of light rain. At the same time across northern
Iowa forcing is departing the area for a chance of light snow in
that area.

The discrepancy in the models for the weekend carries through
into next week with the handling of the upper low. The GFS lifts
the low into the Southern Plains by Tuesday while the Euro quickly
drives the low east by late Monday. In either scenario the precip
with the system is too far south to impact Iowa much however the
Euro would certainly give far southern Iowa at least a slight
chance for something.

By mid week the upper ridge re-establishes itself over the western
United States putting a trough over the Great Lakes and keeping
the Upper Midwest in deep northwest flow. Timing of any
disturbance in the flow that far out is futile since model
consensus is poor and it is inherently difficult to do so anyway
in that flow pattern.


.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 1031 PM CST Wed Dec 13 2017

Main concerns will be clouds. Winds continue to diminish and will
finally drop below 12kts by 09-12z. Weak ridge aloft has allowed
cigs to lift over northeast sections and KMCW/KALO will see
improvement to VFR. Farther west at KFOD/KDSM/KOTM where MVFR
conditions will linger through 08-11z. As upstream wave propagates
toward Iowa from 18-00z...cigs will again lower to MVFR across
north sites/northwest areas...but lower confidence for KDSM/KOTM
where drier air may have a chance to dilute deck with time. /rev




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