Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 130924
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
324 AM CST Sat Jan 13 2018

.SHORT TERM.../Today through Tonight/
Issued at 318 AM CST Sat Jan 13 2018

Confidence: High

Main concern this period will be the continued cold. 1044 mb High
centered over the Dakotas with cold air continuing to spill
southeast today. Clouds continue tracking southeast through Iowa
early this morning from an upper level short wave moving southeast
through the region at 07z. The clouds will track south of the state
by mid to late afternoon with only a few lingering areas of high
clouds across the south during that time. Meanwhile the north will
have more sunshine today with the south catching up during the
afternoon. Despite the sun the airmass remains rather cold.  Though
the wind chills will have improved by late morning with the Wind
Chill Advisory expiring at 10 am...afternoon highs will once again
be held back by the airmass.  The high will be settling into
northeast Iowa by 00z with H850 temperatures ranging from -17c
northeast to -13c over the southwest.  Winds will be lighter this
afternoon so even with highs ranging only from zero to 5 above
north...and in the lower teens southeast...it won`t feel as cold in
the afternoon hours. Wind chills will generally range from 5 above
in the south to about -10 in the north. Still...any outdoor
activities should be approached with some caution.  Already early
this morning GOES 16 mid level water vapor imagery vividly shows the
next upstream wave diving southeast from British Columbia/Alberta
Canada toward the US Canadian border.  This is progged by all the
models to reach southwest Iowa by 09-12z Sunday with frontogenetical
forcing increasing across a strong thermal gradient near H850.  An
area of light snow will break out over the High Plains and then move
east southeast with time reaching the far western portion of our
forecast area by that time.  Clouds will precede the light
snow...and should be nearly regionwide by 12z with the focus for any
light snow mainly over the southwest.  A light dusting of snow may
occur by sunrise in that area. South southeast winds of 10 to 20 mph
overnight Saturday night into Sunday morning will also allow
overnight temperatures to warm up.  Despite the increasing winds
there should be little impact with any snowfall accumulations due to
the very light amounts expected.  The only other concern tonight
will be colder wind chills over the northeast by Sunday morning.  A
small area bounded by Highway 20 and Interstate 35 may see wind
chills drop to near 20 below from 06z to 13z Sunday...though winds
may be so light that wind chill headlines may not be needed. Shift
later today will make a determination for the overnight hours.

.LONG TERM.../Sunday through Friday/
Issued at 318 AM CST Sat Jan 13 2018

Summary...A pair of waves within the northwest flow and an arctic
front associated with a digging upper level trough will bring about
chances for light snowfall Sunday and then brutal cold and wind
chills Monday through Wednesday AM. Relief comes to end the week as
warmer air filters back in with above freezing temperatures
potentially state wide by Friday and the weekend.

Sunday through Tuesday...This will be the active and most painful
part of the forecast. First up, a wave will slide predominantly
SW of the area Sunday morning, but will produce light snowfall
around a half inch across SW Iowa. The tendency has been to push
the wave progressively further SW in previous runs, but has
appeared to stabilize its path with the most recent 00z runs and
06z GFS. Temps will be on the mild side during the day as the
surface high slides east and southerly winds/WAA boost temps into
the teens to 20s. Sunday evening and overnight the next slug of
forcing comes in two pieces, another wave riding the NW flow to
affect SW Iowa, and an upper level trough digging into Minnesota
and Wisconsin. Phasing of the two is not great, which may result
in separation of precipitation for parts of central Iowa. Moisture
availability is overall underwhelming, and should result in
around an inch or less in most locations except the NE where the
stronger forcing and a deeper DGZ may produce around 2 inches.
With wind gusts to around 30 mph behind the front, have put in
patchy blowing snow in some areas. With the blowing snow risk
predominantly overnight, overall travel impacts should be low for
the majority of the population.

The associated cold front with the upper trough/low passage will
quickly filter arctic air back in after the one day reprieve,
dropping temperatures into the single digits for highs Mon/Tue and
lows into the negatives Tue/Wed morning. Breezy winds will drop wind
chills into the -20 to near -40 deg F range for much of the state.
Wind chill headlines, advisory and warnings, will most certainly be
required as the time approaches.

Wednesday through Friday...Milder temperatures return as the upper
level NW flow breaks down and returns predominantly zonal flow to
weak ridging to the central US. Welcome temperatures in the 30s to
low 40s will return state wide by Friday.

&&

.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1137 PM CST Fri Jan 12 2018

Widespread VFR ceilings will persist overnight, mainly across the
southwest half of Iowa with gradual clearing on Saturday morning.
Visibilities will be unrestricted for the duration of the forecast
with north to northwest winds through Saturday. Winds will become
variable on Saturday evening as warm advection begins later
Saturday night.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Wind Chill Advisory until 10 AM CST this morning for IAZ004>007-
015>017-023>028-033>039-047>050.

&&

$$

SHORT TERM...REV
LONG TERM...Curtis
AVIATION...Cogil



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