Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 161044

Area Forecast Discussion
National Weather Service Des Moines IA
444 AM CST Sat Dec 16 2017

.SHORT TERM.../Today through Tonight/
Issued at 311 AM CST Sat Dec 16 2017

A surface low pressure trough will settle across Iowa later today,
with a modest temperature difference across the boundary keeping
northern Iowa a bit cooler than central and southern Iowa.
Nevertheless, temperatures will climb into the low to mid-50s from
around the I-80 corridor southward with ample sunshine for much of
the day. Tonight a low pressure center will move quickly
northeastward from Texas into Kansas and Missouri, pinching and
eventually absorbing the preceding trough. This will make for
light or calm winds at times allowing for good radiational cooling
overnight, but this may be mitigated by expanding and thickening
cirrus overhead which should also limit fog potential.

.LONG TERM.../Sunday through Friday/
Issued at 311 AM CST Sat Dec 16 2017

The low pressure center coming up from Missouri on Sunday morning
will translate quickly east northeastward, with an associated
cloud and precipitation field possibly clipping southeast Iowa.
Any measurable precipitation will likely be limited to our far
southeastern counties, roughly southeast of a line from Lamoni to
Oskaloosa. Within this area low-level temperatures do support the
potential for some mixed precipitation early Sunday morning, but
more light rain by midday. In any event, accumulations will be
very light, assuming the precipitation does not slide southeast of
our forecast area entirely, and no real snow or ice accumulation
is anticipated. Have lowered forecast maximum temperatures for
Sunday in our southeastern counties to account for expected low
clouds/light precipitation and associated cooling effects.

From Sunday night through Wednesday the forecast is dry and
overall quiet. On Monday southwesterly low-level flow will once
again promote warming, with temperatures approaching or exceeding
50 degrees across some southern and western portions of Iowa. A
weak trough will then cross the area early Tuesday, but bring very
little cold air advection before southwesterly flow returns on
Wednesday. This will make the first half of the work week
unseasonably warm and generally pleasant especially for this time
of the year.

By Wednesday night a rather potent mid/upper level trough will be
swinging down across North Dakota and Montana, generating ahead of
it a broad and intensifying cyclone in the lee of the Rockies,
centered from eastern Colorado into the Texas panhandle. This is
shaping up to be, quite possibly, the first real snowfall event of
the winter for most of Iowa. It is a classic set-up with an
inverted trough stretching northeastward from the surface low up
across Iowa by Thursday morning, with a very large cloud and
precipitation shield along the northwestern hemisphere of the
trough covering much of Kansas, Nebraska, Minnesota, Wisconsin and
Iowa at various times Thursday. The GFs/EC/GEM have shown some
inconsistencies in timing with this system, which may affect low-
level temperatures at the onset of precipitation when the
trough/front finally moves through, and thus initial precipitation
type. However, cooling behind the boundary will likely support
mostly snow, with accumulations a likelihood at least across parts
of our forecast area, and perhaps all of it by Thursday night. In
short, while there is still some uncertainty, it appears we may
have our first legitimate winter weather for the end of the work

In terms of amounts/impacts, difficult to project given the
current degree of discrepancies among models/runs. However, there
is a good signal for a pretty strong pressure gradient behind the
surface boundary, leading to gusty winds while some of the
precipitation is falling. If it is all snow, this may result in
lowered visibilities and possibly some minor blowing/drifting of
snow, though this is less likely given the antecedent conditions
and apparently limited potential snowfall amounts. In any event,
we will be closely monitoring this system during the week as the
first snow of the season sometimes has inordinately high impacts
on driving, and as Thursday/Friday may see an increased number of
vehicles traveling ahead of the holiday weekend.


.AVIATION.../For the 12Z TAFS through 12Z Sunday morning/
Issued at 444 AM CST Sat Dec 16 2017

Mainly VFR conditions are expected through the TAF period. Modest
southwest breezes this morning will become light and variable and
shift to north then northeast at times later today and tonight.
There is a possibility of patchy fog/stratocumulus producing
spotty MVFR or lower conditions toward daybreak Sunday, but right
at or just beyond the end of the current TAF period.





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