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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KDMX 082353
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
553 PM CST Sun Dec 8 2019

.UPDATE...
Issued at 548 PM CST Sun Dec 8 2019

Have issued Dense Fog Advisory for northwestern to far northern
tier of counties... specifically highlighting the potential for
freezing fog. Confidence high that ongoing 1/4 mile visibilities
will continue and expand until precipitation arrives... which
looks to be around 09z or so.

A non-zero chance may need Winter Weather Advisory after 09z
across northern counties for mixed precip... transitioning to
snow. Timing of mixed precip is during AM commute. Icing, followed
by a snow may amplify adverse travel impacts. Will monitor
closely over next few hours.

&&

.DISCUSSION.../Tonight through Sunday/
Issued at 312 PM CST Sun Dec 8 2019

The primary weather highlights through the period will be fog
this evening, wintry mix to light snow and wind tomorrow, and
then a surge of colder temperatures Tue into Wed. This will be
followed by a return to near normal temperatures and several minor
precip windows as we translate from the mean MS Valley trough
early this week to faster, lower amplitude Pacific flow into next
weekend.

At onset the 20z surface analysis shows a weak front/pressure
trough SW-NE through the area with stratus oozing in from the
north and south. The front wont move much into the evening, but
will be reinforced by Plains cyclogenesis ahead of the
approaching MT PV anomaly. This may lead to areas of fog north
and south this evening with some separation possible. Would not be
surprised to see a Dense Fog Advisory needed, but will hold off
for now until geography and magnitude confidence increases.

Precip wise, patchy drizzle may form overnight either side of the
boundary and could be of the freezing variety north until mixing
with and then changing entirely to snow into the morning as
moisture from the bottom up and top down begins to erode an
appreciable 1-2km dry layer. Much of the QG forcing will remain
to our north ahead of the aforementioned anomaly, but models are
in good agreement with a trailing lobe of lift across northern IA
from the predawn hours into at least mid morning which will be
immediately followed by increasing NW winds. High resolution model
simulations suggest the window for snow at any particular site
may not last more than 1-3 hours, but could be moderate+ and
quickly accumulate around an inch of snow north. The question is
whether a Winter Weather Advisory is warranted. Road surface
temperatures are currently above freezing well into MN even though
air temps are below freezing due to the blanket of stratus, so am
unsure how impactful freezing drizzle will be to travel. Snow
amounts will not approach advisory criteria, but will likely occur
during the morning commute. So at this point, rather than issue a
mess of two marginal headlines, have opted to leave decisions for
later shifts which will have a better handle on trends and
potential impacts.

The snow should end by midday tomorrow with wind the primary
story. Sustained speeds will be 25 mph+ with gusts around 35 mph
and temperatures falling area wide by afternoon heading toward
lows in the single digits and teens Mon and Tue Nights, and only a
minor rebound during the day Tue and Wed. Some below zero temps
are possible north. These temps are certainly below normal, but
not to an extreme degree with sensible cooling more a reflection
of our recent mild stretch. There could be a few periods of
flurries Mon into Tue with the cool down resulting in stratus at
<= -12C temps, but moisture will be of marginal depth so it has
been omitted for the time being. Any occurrence would likely be
brief, insignificant and difficult time.

The cold won`t last long however with near normal temperatures
back in the forecast by Thu. Heading into that, have added a
chance for light snow north late Tue Night as a short wave
traverses the baroclinic zone and start of warm advection before
heights begin rising in earnest. Another potential window would be
around Wed Night when there is decent lift, but as of now
moisture seems insufficient to introduce PoPs. After this the fast
Pacific flow buckles, and as is often the case, is not handled
well by the models with uncertainties regarding how and when
individual short waves will intensify. Thus the forecast has a
fairly broad-brushed period of intermittent low end PoPs from
Friday into the weekend. This low confidence, low amplitude flow
will keep seasonal temps going through the period.

&&

.AVIATION.../For the 00Z TAFS through 00Z Monday evening/
Issued at 548 PM CST Sun Dec 8 2019

High confidence in IFR to LIFR TAFs across Iowa through tonight
and into tomorrow. Will be watching KMCW for potential for dense
fog to cause LIFR before current 10z forecast... At this time it
looks like vsbys reductions should just miss the site. Gusty
northwest winds develop Monday afternoon, which may lead to
blowing snow. CIGs to gradually improve.



&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 3 AM CST Monday for IAZ004>007-015-016-
023-024.

&&

$$

UPDATE...Kotenberg
DISCUSSION...Small
AVIATION...Kotenberg