Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 222113
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
313 PM CST Sun Jan 22 2017

.SHORT TERM.../Tonight through Monday/
Issued at 312 PM CST Sun Jan 22 2017

The situation has changed little over the weekend with low clouds
and fog trends the primary concerns. While the potent short wave
continues to plow through the Gulf Coast states, a weaker wave is
caught in our weak flow in MO. This is trying to brush southeast
IA with precip and weak forcing and a hint of frontogenetical
response but likely isn`t going to result in anything so the
forecast remains dry.

Stratus and fog continues to blanket IA this afternoon as well as
upstream into the Dakotas and MN. With 500m trajectories coming from
these same areas, expect little change into the night. The overall
flow is very weak with local soundings showing <50kts through the
depth of the column. Outside of some brief clearing potential west,
NCEP NMM/ARW, HRRR, RAP and SREF all suggest plenty of stratus into
the night. The bigger question will be how much fog develops. These
same models do not depict much in the way of additional fog
formation overnight, which makes some sense. Our low level flow is
out of the north rather than south vs this time yesterday,
visibilities are higher and the more uniform clouds should preclude
much radiational cooling. Thus have some fog wording in the
forecast, but confidence is not high enough for another advisory.
These clouds should also hold up temperatures so have lows above MOS
guidance.

.LONG TERM.../Monday night through Sunday/
Issued at 312 PM CST Sun Jan 22 2017

Winter storm potential shaping up for Tuesday into Wednesday
across northern Iowa was the main focus of the extended. Some
model consensus with storm track and timing for storm
Tuesday/Wednesday. The NAM/ECMWF and even Canadian are very
similar with the track of the surface low but differ with timing.
The NAM is the fastest while the Canadian lags behind roughly
12-18 hours from the ECMWF. As far as timing, the GFS and ECMWF
are in decent agreement with the surface low tracking across the
region but believe the GFS is too far north with the track. Thus,
leaned closer to the ECMWF/NAM blend for pops/qpf.

Surface low develops over the central Rockies and transitions
across the Plains Monday into early Tuesday before centering
over northeast Kansas by midday Tuesday. A decent slug of theta-e
advection combined with strong frontogenetical forcing b/t
600-75mb advances north-northeastward ahead of the surface low
late Tuesday morning into the afternoon hours. Good omega within
the DGZ from a line near Fort Dodge to Mason City west-northwest
through the afternoon hours into Tuesday evening. Deep saturation
per ECMWF forecast soundings looks to be b/t 15-21z time frame in
the aforementioned location. Cross section from southeast to
northwest Iowa suggests some negative EPV and possibly releasing
CSI over northern Iowa which would enhance the snowfall rates a
bit Tuesday afternoon into the evening. There`s also plenty of
large scale forcing Tuesday afternoon into the early evening,
with a lull in the activity late Tuesday night before the surface
low moves east and the wrap around snow develops in the CAA
regime overnight towards Wednesday morning. Confident to raise
pops/qpf and thus snow amounts across the northern portions of the
forecast area, roughly along and north of Highway 20 to the
Minnesota border. A band of 4-7 inches seems likely with the
higher amounts in the far northern portions of the forecast area
and issued a Winter Storm Watch Tuesday into Wednesday afternoon.

Strong surface pressure gradient develops on the backside of the
surface low Wednesday with the column remaining saturated for
much of the morning. Forcing isn`t as significant as Tuesday
afternoon/evening time frame, but enough to produce falling snow
within the blustery winds. Winds are not strong enough for
whiteout conditions, but certainly drifting and visibility
restrictions are highly likely. Lingering snow showers seem
possible per the ECMWF Wednesday night and added low chance pops
across the eastern half of the forecast area but may need to
increase if the ECMWF remains consistent with this solution.

Thursday into Sunday...Generally north to northwest flow aloft
develops during this time frame and puts the state in some colder
air through the end of the week. A couple weak shortwaves may
bring some light snow showers to the north, but timing between
extended models remain inconsistent and thus low confidence with
any mention past Thursday.

&&

.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1210 PM CST Sun Jan 22 2017

Widespread fog and stratus continues to affect central Iowa with
varied conditions ranging from MVFR to LIFR. Visibility trends
will slowly improve into the afternoon, but persistent low stratus
will likely result in little overall category change. Thus expect
IFR or less conditions through much of the valid period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Tuesday afternoon through Wednesday
afternoon for IAZ004>007-015>017-023>028-033>038.

&&

$$

SHORT TERM...Small
LONG TERM...Podrazik
AVIATION...Small



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