Area Forecast Discussion
Issued by NWS Des Moines, IA

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598
FXUS63 KDMX 110005
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
605 PM CST Sat Dec 10 2016

...Updated Aviation Discussion...

.SHORT TERM.../Tonight through Sunday/
Issued at 328 PM CST Sat Dec 10 2016

Initial band of snow has lifted into northeast Iowa this afternoon.
General snowfall amounts thus far have ranged from 1-3 inches and
expect a few local 4 inch amounts by 6 pm. A wedge of drier air has
moved into central and western Iowa where cloud heights have
increased and snow has ended for now. The next shortwave remains
upstream and will enter the area this evening. A band of strong
theta-e advection will precede the kinematic forcing and will result
in snow redeveloping across the area. Latest radar trends near Sioux
Falls show snow is expanding again across southeast South Dakota.
The renewed snow band is expected to become moderate to heavy at
times with visibilities dropping to one half to one quarter mile in
falling snow. The latest guidance has developed stronger forcing in
the -8C to -10C along with forcing in the dendritic growth zone so
may transition back and forth between good dendrites and smaller
plates. Given the snowfall from the initial snow wave and potential
for additional snow overnight, have issued a winter storm warning
north for heavy snowfall amounts through 18z Sunday with total
amounts in the 6 to 9 inch range and potential for locally higher.
Currently have the gradient between Highway 20 and Highway 30 though
some uncertainty in exact cutoff of the higher amounts due to the
transition to drier air.

The wind will increase overnight into the 15 to 20 mph with higher
gusts and with the fluffy consistency of the snowfall, these winds
will be strong enough to create blowing snow. In addition, vehicles
will be able to kick up this snow easily and create a snow fog
immediately behind them and near zero visibilities for short period.

The snow will end from west to east on Sunday with the bulk of the
accumulating snow done by noon. Warmer air will move into southern
Iowa with readings eclipsing 32 in that region. Elsewhere highs will
be in the upper 20s to low 30s with conditions improving during the
afternoon.

.LONG TERM.../Sunday night through Saturday/
Issued at 328 PM CST Sat Dec 10 2016

Weak surface ridging will build into the state on Sunday night
after the departure of the current system. Temperatures are
expected to cool nicely with clearing skies and light winds.
Another weak system will rapidly approach on Monday with warm
advection during the day and reading climbing above freezing in
the south. While the system is generally moisture starved, forcing
may be sufficient enough for a few flurries/snow showers in
northern Iowa late in the day before the system races to the east.
Much colder air will surge back into the state Monday night on
strong northwest winds producing some pockets of blowing snow in
northern Iowa along with wind chills dropping well below zero
over the north overnight. The cold air will continue to deepen
across the state through the middle of the week especially with
the fresh snowpack with readings remaining well below normal.
There is some concern with wind chill readings approaching/exceeding
advisory criteria, especially Wednesday night/Thursday morning.

Next concern will focus on returning warm advection and moisture
transport into the state as western trof develops late in the work
week. Medium range models are both indicating widespread snow
developing across the state on Friday and continuing to some
extent into Saturday. Concern with this system lies with the
strong return of warmth aloft and the possibility of mixed
precipitation in the southern half of the state on Friday
night/Saturday morning. Still plenty of time to ascertain how this
system will evolve but have continued to raise pops during this
timeframe given the relatively good agreement with the models.
Another surge of cold air looks likely on the backside of this
system into the later half of next weekend.

&&

.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 603 PM CST Sat Dec 10 2016

Expecting IFR to potentially LIFR ceilings and visibility late
tonight into the overnight hours for FOD, MCW and ALO. DSM and
OTM look to be only impacted by MVFR ceilings tonight with IFR
ceilings moving Sunday morning. Low stratus will stick around much
of the day tomorrow.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until noon CST Sunday for IAZ023-033>037-
048>050.

Winter Storm Warning until noon CST Sunday for IAZ004>007-
015>017-024>028-038-039.

&&

$$

SHORT TERM...Donavon
LONG TERM...Cogil
AVIATION...Podrazik



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