Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 132334
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
534 PM CST Sat Jan 13 2018

...Updated Aviation Discussion...

.SHORT TERM.../Tonight through Sunday/
Issued at 330 PM CST Sat Jan 13 2018

High pressure remains entrenched across most of Iowa and along the
Upper Mississippi River valley today, with cold but calm and quiet
conditions. Stratus clouds have stubbornly persisted in our
south/southwest, and do not appear as though they are going to
clear out as the high slides off to the east tonight. By Sunday
morning weak warm air advection aloft will not only allow for
gradual expansion of these clouds across our forecast area, but
also some light snow in our southwest. Little to no accumulation
or impacts are expected, but this will make overnight temperature
trends tricky and have raised forecast lows a couple degrees in
anticipation of these issues. Meanwhile in our northeastern
counties light/calm winds and clear skies will cause temperatures
to plummet again overnight, with wind chills approaching -20 in a
few counties toward sunrise Sunday. However, given the light winds
and marginal/uncertain nature of the wind chill values, will not
issue an advisory at this time.

On Sunday warm air advection will steadily increase ahead of an
approaching storm system, promoting expansion/deepening of clouds
and perhaps some continuing flurries but no snow of any impact, at
least through the morning. Meanwhile a robust PV anomaly will be
swinging down into the Upper Midwest over Minnesota and the
Dakotas, accompanied by a sharp surface cold front. Ahead of this
front a ribbon of low-level forcing and saturation will promote a
quickly moving swath of snowfall which should reach our
northwestern counties by the late afternoon. This will be
discussed in more detail in the LONG TERM section immediately
below.

.LONG TERM.../Sunday night through Saturday/
Issued at 330 PM CST Sat Jan 13 2018

The robust storm system discussed in the SHORT TERM section above
will impact our forecast area on Sunday night. By Sunday evening
a quickly moving swath of snow will be crossing central Iowa, and
will exit our southeastern counties around midnight. The snow will
only last a few hours in most locations, but forecast soundings
indicate a sufficient dendritic growth zone depth and forcing that
high ratios could result in a quick couple inches of snowfall,
especially across our northeastern and eastern counties closer to
the parent system. This may lead to a period of hazardous travel
conditions that will translate quickly across Iowa, however, road
temperatures are pretty cold and winds during the period of
snowfall will be modest, so combined with a relatively low travel
time frame the impacts may be mitigated.

As the sharp cold front associated with the storm system sweeps
across the region on Sunday night, winds will shift to northwest
and increase substantially. By the pre-dawn hours Monday it is
likely we will see speeds of 20 to 25 mph with gusts to near 35
mph at times, which will cause blowing snow issues especially in
flatter/open areas. However, there is considerable uncertainty
regarding the magnitude and duration of these effects, given both
the low snowfall amounts forecast and the relatively meager
antecedent snowpack, as well as the fact that the strong winds
will come after the snow has ended, and not while it is falling.
Certainly expect some blowing snow and travel impacts, but these
may be shallow and relatively brief and concentrated during the
very low travel hours before sunrise. This threat will be closely
monitored tonight and tomorrow as a headline may be needed to
account for the hazardous travel. Meanwhile much colder air will
surge into Iowa behind the front, sending temperatures plummeting
concurrent with the strong winds. This will result in wind chills
falling rapidly Sunday night and bottoming out from -10 to -15
south to -30 to -35 north on Monday morning. The result would be
wind chill advisories and perhaps a warning, but it has yet to be
determined how these hazards may combine with lingering blowing
snow/travel hazards and what effect that will have on the
resulting headlines. All in all Sunday night and Monday will be a
bad time to be outside or traveling if it can be helped.

Bitter cold will persist from Sunday night through Tuesday night,
with temperatures falling below zero during the mornings and only
peaking in the single digits above on Tuesday. This will
necessitate another round of wind chill headlines and it will
remain hazardous outside due to the cold. There may also be a few
additional flurries around Monday night/Tuesday, but these would
have no real impact. Finally, from Wednesday through the end of
the week we will see a return of a pronounced ridging pattern
aloft and south southwesterly flow at the surface, leading to
pronounced warming. By Thursday high temperatures will range in
the upper 20s to mid 30s, and on Friday and Saturday 40s are
likely across southern Iowa, finally providing some relief and
melting most of the snow in place. Long range models continue to
signal another storm system toward the latter half of next
weekend, with the potential for precipitation, but confidence in
any details is very low at this time and it remains beyond the
seven day forecast period.

&&

.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 530 PM CST Sat Jan 13 2018

VFR conditions are expected tonight into the early portion of
Sunday morning. First shortwave to impact southern Iowa with
mainly MVFR ceilings by 16z Sunday or possibly sooner. Low
confidence in mentioning any visibility restrictions as the snow
with the first system remains in Missouri. The next shortwave to
cut across Minnesota and northern Iowa beginning Sunday late
afternoon will bring MVFR ceilings and visibility at FOD, MCW, and
ALO. Have MVFR mentioned at DSM as well but the snow looks to move
in past 00z Monday.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...Lee
LONG TERM...Lee
AVIATION...Podrazik



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