Area Forecast Discussion
Issued by NWS Des Moines, IA

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054
FXUS63 KDMX 101727
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
1127 AM CST Sat Dec 10 2016

.SHORT TERM.../Today through Tonight/
Issued at 301 AM CST Sat Dec 10 2016

A prolonged snowfall event is still on track for today and tonight,
mainly across northern Iowa. Short-range models are in good
agreement with each other and the previous forecast on the overall
placement and timing of precipitation, though QPF remains more
variable. The NAM continues to be hotter than other guidance
particularly overnight and have maintained a forecast more down the
middle, with total QPF of around half an inch up by Mason City. The
forcing with this system is somewhat nebulous at times and while
snow may be continuous in the north, accumulations should accelerate
and decelerate in waves. This will allow for some compaction between
bursts of accumulating snow and keep the overall amounts a bit lower
than one would expect given the total QPF. Forecast amounts for the
entire event now range up to around 7 or 8 inches up toward the
Minnesota border, but 12-hour amounts are not approaching 6 inches.
Combined with the relatively mitigated impacts due to timing of the
snowfall, relatively light winds, etc., we will maintain an advisory
at this time and not upgrade to a warning. In addition, confidence
is increasing in the location of the fairly sharp southern edge of
snowfall, resulting in seven counties along the edge of the advisory
being cancelled due to low expected accumulations over such a long
duration.

.LONG TERM.../Sunday through Friday/
Issued at 301 AM CST Sat Dec 10 2016

Sunday through Tuesday Night...Confidence Medium to High: Models
more consistent on the departure of first snow event and the
lessening of snowfall chances Sunday morning. Will continue high
PoP for snow early Sunday with additional light accumulations
mainly north/northeast/east. Over the southeast/south there may be
rain snow mixture for a while early on Sunday morning. Bufkit
soundings support a light mix across the south at that time. As
low departs and northwest winds winds briefly take over prior to
ridge axis arriving overnight with light winds through 12z Monday.
After a mild day across the area...the overnight lows will fall
into the teens south with single digits over the north in areas
that receive the higher snowfall totals. Monday temperatures will
recover as south winds ahead of the Arctic boundary provide some
respite from the cold for at least 12 hours. The Arctic boundary
is expected to arrive between 18-00z across the area with a push
of stronger northwest winds during the evening hours...along with
some isolated to scattered flurries. There is some disagreement
between the GFS and the Euro model regarding the strength of the
wave accompanying the front...which may lead to a little more
light snow with the arrival of the boundary. Though the airmass is
fairly dry the expectation is mainly for flurries...gusty winds
and potentially some blowing snow especially over the north. Given
the H850 winds are only about 30 to 35 kts at worst Monday night...
there may be only a brief period of poor travel from late evening
to predawn and mainly across the north where deeper snowpack is
anticipated. H850 temperatures then fall to about -8C in the south
to -14C in the north by 00z Wednesday resulting in both cold
Tuesday lows north just below zero to teens south...to highs only
at 5 north to 20 south by days end. With clear skies early Tuesday
night lows will quickly fall into the upper single digits to
lower teens south and just below zero north. Already by late
Tuesday evening a weak wave will be tracking east...spreading both
clouds and potentially some light snow into the region by Wednesday.

Wednesday through Friday...Confidence Medium: Wednesday through
Friday will remain cold but with the added wrinkle of more precipitation.
Confidence on amounts is low at this time...though the timing is
in general agreement between the GFS and Euro models...with a
hedge toward the slower Euro toward Friday with the arrival of a
stronger wave. In any case...clouds and cold temperatures are
expected. By Wednesday a push of Pacific moisture will ride the
westerlies across the Plains along with the help of a weak short
wave and cold front will result in a band of snow. Presently...an
inch or two of accumulation may accompany this feature given the
cold H850 temperatures but quick departure. Cloud cover and light
snow will keep highs in the single digits north to teens south.
Winds will pick up behind the boundary during the day Wednesday
and result in cold air advection...along with wind gusts to near
20 to 25 mph at times. The winds are lessen overnight but remain
near 5 to 10 mph. Along with H850 temperatures falling to - 17C
north to -14C south...overnight lows may reach 5 to 10 below north
and remain near 0 to 5 above south. Across the north there will
also be the threat of wind chills lower than 20 below early
Thursday morning. As the high slides east Thursday south winds
will return along with a slow moderation in temperatures. Highs
will warm back into the teens south and in the single digits
north. Overnight lows may again drop below zero north Thursday
night. Both the GFS and Euro are indicating a stronger system by
either late Thursday or into Friday. The GFS solution is slightly
faster for now...with a slower but similar evolution to the
southern stream short wave with the Euro. A panhandle low is
forecast to take shape and spread overrunning snows into the
region by Friday morning with the potential for accumulating snow
by days end. Highs will change little through Friday as the cold
airmass remains in place. The current forecast shows an open wave
with little deepening. Despite that limitation...snowfall ratios
may be on the high side due to the cold airmass. Will be monitoring
evolution of this and Wednesdays system along with cold air through
the week.

&&

.AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/
Issued at 1127 AM CST Sat Dec 10 2016

Widespread snow is expected this afternoon over the sites then
becoming focused over northern IA including KFOD/KMCW/KALO with
less activity near KDSM/KOTM. The snow will drop visibilities to
one half mile or less at times. The wind will be breezy therefore
some vsby reduction due to blowing snow may occur. Cigs will be
VFR at times over southern Iowa today otherwise generally MVFR
dropping to IFR overnight into Sunday morning at times.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 3 PM CST Sunday for IAZ006-007-016-
017-025>028-036>039-048>050.

Winter Weather Advisory until 3 PM CST Sunday for IAZ004-005-015-
023-024-033>035.

&&

$$

SHORT TERM...Lee
LONG TERM...REV
AVIATION...Donavon



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