Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 121733
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1133 AM CST Tue Dec 12 2017

.SHORT TERM.../Today through Tonight/
Issued at 338 AM CST Tue Dec 12 2017

Forecast area in final gasp of cold advection as thermal trof passes
to the east of the state early this morning.  Will see neutral
advection through mid morning with increasing warm advection as the
day continues.  Low and mid clouds associated with the warm
advection area will slide into the northwest this morning and expand
across the forecast area during the day.  The increased clouds will
be accompanied by backing surface winds as the northwest flow
becomes westerly by later this afternoon.  Temperatures will be
colder than Monday, especially in the east closest to the departing
thermal trof with highs remaining in the upper 20s with readings
close to 40 degrees in the far west.

The warm advection will persist into tonight ahead of an approaching
shortwave.  The thermal ridge will cross into the state tonight with
modest southwest winds overnight.  Given some scattered clouds
overnight, proximity of the thermal ridge and the modest winds, lows
tonight should remain warmer than blended guidance would suggest and
added several degrees to the overnight lows.  There remains little
threat for precipitation until after the frontal passage on
Wednesday morning.

.LONG TERM.../Wednesday through Monday/
Issued at 338 AM CST Tue Dec 12 2017

Bottom Line Up Front...
Hazardous winds are becoming increasingly likely Wednesday
morning and afternoon. There are slight precipitation chances
Wednesday and again Thursday night, and potentially this upcoming
weekend... nothing appears to pose a significant hazardous threat
at this time.

Setup...
09z Tue water vapor imagery picking up on a well-defined upper
low coming onshore over southwestern British Columbia.
Concurrently, another deep upper low is pushing northeastward into
the Gulf of Alaska towards Anchorage. Over the western Conus, the
highly- amplified, ridge/rex block setup that has been in place
for the past week or so is slated to flatten out and retreat back
into the Pacific Ocean. The regression of the high will allow the
British Columbia low to drop further south in latitude as it
progresses to the east. The 00z runs show little spread with the
track of this low, so consensus appears to be a great track with
this system as it heads into the upper Midwest on Wednesday.

In between the British Columbia low and the Anchorage low, is a
very subtle impulse/shortwave that is captured with initialization
for the 00z runs. This impulse is slated to follow a similar
track to the British Columbia low, arriving near Iowa Thursday
night. Given its nearing proximity, would think models should
start to be able to resolve it better in today`s runs. The 00z GFS
seems to be the strongest solution, and has been trending in a
strengthening direction the past few runs... even going as far as
including a 1014mb sfc reflection low in its 00z run. Long-range
models are then hemming and hawing about trying to bring another
wave through next weekend. This wave is traced back well off the
map, so at this point, do not see any target of opportunity that
is worth the time to detail.

Winds...
Signals are pointing to another day of very strong winds
Wednesday morning into the afternoon...in fact, these winds may be
notably stronger winds than were felt across Iowa yesterday
(Monday). By 15z, the 1000mb sfc reflection low from the British
Columbia upper low should be over northeastern Iowa. Behind this
sfc low, a sfc pressure gradient of around 3mb/100 km sets
in...accompanied with broad low-level subsidence and 3-hr pressure
rises of around 4mb. The pressure gradient is tightest across
northern Iowa, suggesting the strongest winds will be located
there...though most of the state should be very windy.
Transport/mixed layer winds look to be from the northwest in the
38 to 53 kt range from around 10 am to 7 pm CST (initial
suggestion for Wind Advisory time), with the peak winds occurring
during the afternoon commute.

Precipitation...
Wednesday`s shot of wintry precip looks to be a fairly quick-
hitter and light. Lack of low-level saturation biggest inhibitor
as a clearly-defined dry slot will make it to the MN/IA border
through the 12-15z range. 15z to 18z or so looks to be the best
chance for lift to be phased with low-level moisture, primarily
across northern Iowa. CAA beginning to take hold over Iowa in the
18z-21z timeframe, which obviously is not good for isentropic
ascent potential and should shut precip off rather quickly towards
the afternoon commute.

As mentioned above, the Thursday night system is still being
resolved. At this time, it appears western Iowa has the greatest
chance of experiencing a light wintry mix. Non-GFS solutions are
much less organized and further west with this system, so confidence
in sensible impacts is somewhat low at this time. Confidence is very
low in the possible system for next weekend, so not worth detailing
at this time.

Temperatures...
Temperatures this week will be "roller-coastery" with the waves
passing through the upper Midwest. Friday is some interesting as
the long-range models have diverged with placement of a longwave
trough following the Thursday system. The GFS has kept Iowa in the
cold and has been trending ever so slightly colder with each run.
Meanwhile, the ECMWF has kept the trough further east and Iowa
into return flow for Friday... even trending ever so slightly
warmer with each of its runs. So, if the GFS is right, it will
struggle to reach 30 on Friday...but if the ECMWF is right, it
will be in the mid 40s. Initial lean is towards the ECMWF as it
does not strengthen/amplify the passing shortwave as much as the
GFS... which seems to be more of an outlier at this time.

Looking ahead into next week, long-range models have been
consistently trying to establish a cut-off low over the western
CONUS. If this low is positioned correctly, Iowa could be far
enough south to be on the warm side of the pattern...implying
several days of temperatures well into the 40s and even 50s.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1132 AM CST Tue Dec 12 2017

Borderline MVFR/VFR stratus is in place near the NE/SD/MN borders
at 17z. This is expected to drift into IA later this afternoon,
but likely be VFR by that time. Low pressure will then near the
MN/IA border by 12z which will bring increasing winds, especially
beyond the valid period. The system will also bring MVFR stratus
and mixed precip to northern TAF sites 15-18z.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...Cogil
LONG TERM...Kotenberg
AVIATION...Small



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