Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 110550
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1150 PM CST Wed Jan 10 2018

.UPDATE...
Issued at 937 PM CST Wed Jan 10 2018

Have adjusted max snow axis and made some minor headline changes
to account for persistent south and east shift in heavy snow axis
as depicted by 18z and new 00z guidance. Confidence remains low
however even as the event nears with a fairly wide range in model
spread. The latest NMM core suite of the NAM, NAMNest and HRW-
NMM are all fairly similar placing the max axis along a
Greenfield/Boone/Charles City line. However the unsettling 02z RAP
and HRRR runs are even more aggressive than that, going to the
southeast Des moines metro with the axis. Thus the evening update
is hopefully headed in the right direction with a few warning
counties added to better fit the 4 inch snow amount as mentioned
below and attempt to blend into adjacent offices for a consistent
WWA map. Although amounts have diminished on the NW side of the
warning, 2 to 4 inch amounts may still occur with considerable
blowing and drifting and significant travel impacts. Further
changes in amounts and headlines with the midnight packages may
very well occur too, hopefully with better model consensus.

&&

.SHORT TERM.../Tonight through Thursday/
Issued at 410 PM CST Wed Jan 10 2018

The primary forecast challenge remains the winter storm that
will impact the area late tonight into Thursday. Areas of fog
will continue over northern Iowa through this evening with then
the lowest visibilities due to fog will be situated just ahead of
the surface cold front as it moves southeast into Iowa.
Visibilities may drop near to below one quarter mile at times. As
of now, an SPS likely will be able to handle the situation if
visibilities do drop.

An area of low pressure continues to organize and deepen over
southeast Colorado this afternoon as a vigorous short wave/PV
anomaly moves across Arizona and New Mexico. This system will
eject northeast tonight through Texas and Oklahoma and be over
eastern Kansas by 12z Thursday. A surface low pressure will
accompany the system as it lifts north through eastern Iowa. A
deformation band of snow will develop along the western edge of
the low pressure track late tonight into Thursday morning. This
deformation band will have strong forcing through the dendritic
layer with higher snow ratios of 12-15 to 1 expected and snowfall
rates exceeding 1 inch per hour at times. Expecting a band of 4 to
6 inches of snow, which could potentially become higher should
the deformation band maintain a steady position. General trends
continues to be a more easterly from near Mason City southwest
through Audubon. In addition, a strong push of Arctic air will
arrive as a cold front sweeps through the state. This boundary
will bring strong northwest winds of 20 to 30 mph with gusts
approaching 35 to 40 mph. A strong inversion above 900 mb will
limit the overall mixing potential with peak mixed layer winds in
the low to mid 30s kts. The strong cold advection will enhance the
momentum transfer of winds so much of this should be realized at
the surface. A brief period of stronger pressure gradient and
higher pressure rises will move east through Iowa between 09-18z
and may enhance winds further. At this time, expect significant
blowing and drifting of snow that may create significant travel
impact but conditions should remain below blizzard criteria. Have
upgraded a large portion of the watch to Winter Storm warning
based on expected impacts and generally outlined an area with 4+
inches of snow combined with the winds. The snow will diminish
during the afternoon though low level temperatures will drop
within the dendritic layer by the afternoon and may keep light
snow going.

Finally, warm temperatures in the 30s to 40s ahead of the
precipitation may cause wet conditions initially as precipitation
begins as rain or a wintry mix before transitioning to snow in
addition to snow melting until pavement temperatures lower. The
arrival of the Arctic air with temperatures falling into the teens
by mid morning will cause flash freezing of any wet surfaces and
may cause very icy conditions. Due to the flash freeze potential
and expected wintry mix over central Iowa, have added a Winter
Weather Advisory along the southeast periphery of the warning. An
additional expansion to the southeast maybe required should an
icing potential develop. Finally, wind chills will fall to near 20
below zero over northern Iowa by the afternoon then expand south
to near Highway 30 overnight. Likely will need wind chill
headlines once the current headlines expire.

.LONG TERM.../Thursday night through Wednesday/
Issued at 410 PM CST Wed Jan 10 2018

Cold temperatures will persist through much of the forecast period
as strong northerly flow continues through at least Tuesday. This
will lead to several periods with wind chills dropping to 20 below
zero or less each night. Two strong short waves to watch arrive
Friday night and Sunday afternoon and night. Light snow is
possible with the Friday night system and at this time looks to
have minimal impact. The Sunday/Sunday night system may have a
bigger impact as a stronger system arrives. This system will
feature a deep dendritic layer from near the surface to around 13
kft. This system may produce a widespread 1 to 3 inches of snow.
In addition gusty northwest winds may produce additional impacts.

&&

.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 1150 PM CST Wed Jan 10 2018

VFR conditions are in place ahead of an approaching cold front
which has reached a KADU-KFXY line at 06z. Just to the west
conditions dropped to MVFR and then quickly at least IFR and this
progression will continue into the early morning hours as the
front advances eastward and precipitation expands into IA from the
SW. The precipitation should initially be rain but transition to
a wintry mix before switching to all snow area wide into midday.
This will be followed by gusty NW winds to 25-35kts and
considerable blowing snow in areas with appreciable accumulations.
At least MVFR conditions due to ceilings and/or blowing snow
should persist through at least 00z before a transition to VFR
late.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 AM to 6 PM CST Thursday for
IAZ027-028-038-039-048-049-059>061-071>074-081>084-092>095.

Winter Storm Warning from 3 AM to 6 PM CST Thursday for
IAZ005>007-015>017-023>026-033>037-044>047-057-058-070.

Winter Weather Advisory from 3 AM to 6 PM CST Thursday for
IAZ004.

&&

$$

UPDATE...Small
SHORT TERM...Donavon
LONG TERM...Donavon
AVIATION...Small



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