Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 150542
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
1142 PM CST Sat Jan 14 2017

.SHORT TERM.../Tonight through Sunday/
Issued at 336 PM CST Sat Jan 14 2017

Mid-level clouds moved across southern Iowa this morning
and afternoon as high pressure and drier air moved into the northern
part of the state. This drier air will remain overnight as the
trough of low pressure over Baja California begins to lift to the
northeast. While weak lift will begin to spread toward far southern
Iowa late tonight, the aforementioned dry air will preclude
precipitation before daybreak Sunday. Low temperatures tonight will
range from the single digits over northern Iowa where skies will be
mainly clear to the low 20s over southern Iowa as clouds spread
northward.

.LONG TERM.../Sunday night through Saturday/
Issued at 336 PM CST Sat Jan 14 2017

There has been little change from previous thinking with a
significant winter storm and ice accretions anticipated from late
Sunday into at least Monday. At onset Sunday deep moisture will
begin spreading slowly northward into IA from MO with the last
remnants of our surface high and 1-2km dry layer finally losing the
battle. What forcing there is seems weak at best with weak mid level
kinematic forcing. The H85/H7 QG forcing seems just neutral but
there is some theta-e/warm advection aiding weak lift and precip
generation. The precip will take its time moving northward only
advancing from the MO border to maybe halfway through the state by
00z Monday. This lift will only slowly increase through the night
before much stronger forcing from the current Baja closed low
reaches the central Plains.

Initially the warm layer aloft will be 1-2C or less leading to
snow or a wintry mix, but the persistent warm advection will
likely boost those values to 3C+ flipping our precip type to
freezing rain. This will unfortunately also coincide with the max
lift from the aforementioned low with much stronger and deeper
forcing into Monday. The persistent warm advection and latent heat
release from a warm layer possibly in excess of 5-6C depending on
model choice should warm surface temps and dewpoints sufficiently
to flip our precip type to mainly rain across the southeast two-
thirds of the forecast area later in the day. Confidence in how
this plays out in sensible weather impacts is still somewhat low
however as the warm layer degree and road vs surface temperatures
are all uncertain. The GFS warm layer is much less aggressive
suggesting more of a mix, and much like that occurred last Tuesday
the degree of road temperature recovery vs temp/dewpoints is
questionable. However the most favored solution leans toward the
warmer ECMWF/NAM warm layer and more significant freezing rain
impacts so with the event now starting around 24 hours away have
opted to issue winter weather headlines in the form of an Ice
Storm Warning west and a Winter Weather Advisory elsewhere. There
will certainly be some mixed precip potential in the warning, but
felt it best to highlight that area where ice accums are expected
to reach 0.3 in spots. With a mix and lower ice accums elsewhere,
felt it best to keep it simpler with just a Winter Weather
Advisory with varied precip types north and the possibility of an
inch or two north on the back side of the system.

The GFS lingers deformation zone precip into early Tue more so than
the ECMWF and with moisture exceeding 1km in the following cool
advection some light snow or freezing drizzle may linger into
Tuesday. The good news is that any ice impacts will not last long
with dry weather and warming temperatures into midweek as upper
level ridging builds into the central CONUS. This will warm us to
the point that highs may reach 50F in spots by Thursday. Eventually
southwest flow returns again toward the end of next week starting
precip chances again, but with temps sufficient to keep precip type
only briefly snow, if at all and mainly during the night.

&&

.AVIATION.../For the 06Z TAFS through 06Z Sunday night/
Issued at 1141 PM CST Sat Jan 14 2017

Patchy fog is developing over the north and have increased the
risk of MVFR/IFR cigs and vsby at KMCW with MVFR at KALO and KFOD
through 13z. The main concern thereafter will be the approaching
storm with chances lowering cigs...especially aft 21z in the south
and aft 03z in the north with MVFR to IFR expected south sites by
06z Monday. -SNPL changing to -FRZA will increase over the area
from south to north aft 23-01z slowly spreading north through the
remainder of the period. Winds remain light overnight then become
light ESE through 06z. /rev

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 9 PM Sunday to midnight CST Monday
night for IAZ024>028-035>039-047>050-060>062.

Ice Storm Warning from 9 PM Sunday to midnight CST Monday night
for IAZ023-033-034-044>046-057>059.

Winter Weather Advisory from 3 PM Sunday to 6 PM CST Monday for
IAZ073>075-084>086-096-097.

Winter Weather Advisory from midnight Sunday night to midnight
CST Monday night for IAZ004>007-015>017.

Ice Storm Warning from 3 PM Sunday to 6 PM CST Monday for
IAZ070>072-081>083-092>095.

&&

$$

SHORT TERM...Ansorge
LONG TERM...Small
AVIATION...REV


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