Area Forecast Discussion
Issued by NWS Des Moines, IA

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471
FXUS63 KDMX 192115
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
315 PM CST Fri Jan 19 2018

.SHORT TERM.../Tonight through Saturday/
Issued at 308 PM CST Fri Jan 19 2018

Main concerns in the short term is fog potential Saturday morning
and temperatures. At the surface early this afternoon, low pressure
is moving over Ontario this afternoon with a cold front draped to
the southwest of the low into Minnesota back into north central
Nebraska. Despite high clouds as viewed on the GOES-East Day Cloud
Phase Distinction RGB, temperatures are well into the 40s across
central Iowa as winds from the southwest deliver warmer air to the
state. Possible refreeze on roads/bridges will return tonight into
tomorrow morning with the additional snow melt today. Soundings from
the NAM/GFS show that the inversion this afternoon remains intact
through Saturday. While the cold front will drop into the state
tonight and stall, the winds remain quite light. With soundings,
especially the NAM, showing low level saturation, fog development is
possible later tonight into Saturday morning. At this time, have
included patchy fog over northern Iowa where confidence is highest

NAM cross sections maintain low level moisture across central Iowa
below 900mb through the day while the GFS/ECMWF are not as robust.
Still think moisture in the lowest 1 to 2 km will begin to increase
over southern Iowa late in the afternoon. Drizzle may also start to
develop toward the Missouri border as well. Highs tomorrow will be
lower today due to the cloud cover as well as 850mb temperatures
falling from the low teens today to the upper single digits. This
yielded highs in the upper 30s over the northeast part of the
forecast area with low 40s over the rest of central Iowa.

.LONG TERM.../Saturday night through Friday/
Issued at 308 PM CST Fri Jan 19 2018

./Saturday Night through Tuesday Night/...Confidence Medium

The main story in the extended remains the winter storm from
Sunday into Monday night with details of storm track/evolution
still leading to lower forecast confidence at this time.

Still plenty of challenges with the upcoming system mainly due to
run to run model consistency...along with lack of a consensus
between GFS/Euro/NAM solutions regarding storm evolution and track.
The overall picture is about the same...but the axis of maximum
qpf may still shift a bit going forward. Quick look at incoming
model data so far and the 12z NAM is more in line with the 00z/12z
Euro in that the track of the sfc/mid level features remains farther
south than the GFS. The 12z GFS continues to be in lock step with
the other models until 00z Monday...then begins to speed up the
500 mb low tracking slightly more northeast than the others. There
still remains secondary sfc low development as the system begins
to occlude Monday morning. This in itself is shifting the axis of
maximum qpf slightly either north/south depending on the model.
Due to continued uncertainty...a blended approach to qpf amounts
will likely win the day for now which will result in a slightly
southward shift across northwest/northern Iowa. This brings the
heavier snowfall into our far northwest...with a sharp gradient in
expected snowfall across Emmet County and areas south. The good
news is that thermally...the models have remained generally consistent
in pattern which should result in a similar percentage distribution
of ptype with changes in qpf resulting mainly in changes to amounts.

Similar to the past few days...warm air advection and isentropic
lift beginning Saturday evening will saturate the lower layers
enough to induced drizzle/light freezing drizzle into early Sunday
morning.  Then warmer air will overtake the entire area with by mid
to late afternoon.  As the low tracks into the region between 03 and
12z Monday cold air will begin to wrap back into northern/northwest
Iowa and change the precipitation back over to a wintry mix/snow
through the day Monday.  Again...this transition will be extremely
sensitive to model track and timing of the expected surface occlusion.
By far the worst travel day will be Monday. As the sfc low occludes
there has been at least one consistent signal in the models showing
a bent back occlusion and trowal forming as the system wraps up.
The bent back occlusion will support a period of very strong
winds immediately behind the low Monday from the western half of
Iowa into Nebraska during the day and possibly extending farther
east into the evening and overnight hours as the low pulls northeast
of the region and into Michigan. Current GFS/Euro H850 wind forecasts
behind the system show to 40 to 45 kts aloft...which will likely
mix to the sfc during the event. Will maintain blowing snow behind
the system on Monday and will need to watch visibility for any
upgrades to headlines...as some areas in the far north/northwest
may see blizzard conditions in areas of heaviest snowfall. Farther
south...with the higher amounts of precipitation expected...the
transition back to snow on Monday will mean that for a while a
slightly greater amount of freezing rain is now forecast. Winds
behind the system may cause problems with powerlines if ice
loading is heavy enough. All of these areas have not been added
to the watch yet due to forecast uncertainty. The forecast again
hinges on eventual track...which may change...so confidence is
still less than desired at this point. Once the system passes
winds will gradually lessen along with an influx of colder air
Monday night into Tuesday. Lows Monday night will be in the teens
to mid 20s with highs Tuesday in the 20s to 30s.

./Wednesday through Friday/...Confidence Medium to High

Little sensible weather is anticipated for the remainder of the
week.  Highs will gradually warm from Wednesday into Friday as more
zonal flow returns to the region by that time.  Highs will recover
into the 30s to 40s regionwide.

&&

.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1144 AM CST Fri Jan 19 2018

Primary concerns this period are fog potential tonight into
tomorrow morning and low level wind shear (LLWS). High confidence
that VFR conditions will persist through the remainder of today
into this evening. Stronger winds aloft with lighter surface winds
will continue to foster LLWS conditions and have expanded to all
terminals. Other item of note is fog has been introduced Saturday
morning and have kept in MVFR range at this time, though it is
possible IFR conditions may occur as well as adjustments to
timing in future issuances.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Sunday evening through Monday afternoon
for IAZ004>007-015.

&&

$$

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



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