Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 070537
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1137 PM CST Wed Dec 6 2017

...Updated for 06z Aviation Discussion...

.SHORT TERM.../Tonight through Thursday/
Issued at 238 PM CST Wed Dec 6 2017

A highly amplified will continue through the short term forecast
period with a deep upper level low over the Hudson Bay and cyclonic
flow encompassing much of the eastern two-thirds of the conus.
Several weak shortwaves were noted on water vapor imagery rotating
around the Hudson Bay low, with one such wave dropping south across
the northern Plains and upper Great Lakes region. Some light precip
will be possible across north central Iowa this evening and into the
early overnight hours as the wave passing through. Model soundings
indicate weak lift extending into the DGZ, which should be enough to
generate and sustain a few light snow showers and/or flurries. Given
the dry atmospheric conditions and not expecting much, if any,
snowfall accumulation.

A reinforcing shot of cold air will keep winds elevated through much
of the overnight into tomorrow morning. With lows in the single
digits and teens expected, much of the forecast area may wake up to
wind chill values hovering a few degrees above and below zero. Winds
will subside compared to today/tonight as an area of high pressure
settles into the central Plains and the local surface pressure
gradient weakens. Subsidence on the backside of the departing
shortwave will lead to clearing skies and sunshine for much of the
area. Despite the sunshine we will still likely see one of our
coolest days so far this season given 850mb temps of -12c to -15c
across the state of Iowa.

.LONG TERM.../Thursday night through Wednesday/
Issued at 238 PM CST Wed Dec 6 2017

Little change expected in northwest flow pattern through the period.
This will result in continued passage of surface cold fronts as well
as upper level systems tracking south southeast from Canada bringing
light precipitation mainly in the form of light snow/flurries.  The
first in a another series of potent...but somewhat moisture limited
shortwaves...will quickly drop southeast out of Canada and through
the Plains/Great Lakes areas Friday.  This will bring quick shot of
snow showers/flurries to the region Friday morning with the passage
of the system.  At this time little if any moisture appears to
accompany the wave along though it will feature some warm air
advection.  This should help moderate highs Friday with lower to mid
30s again likely over the region by afternoon.  A second and fast
moving Clipper will fly south southeast Friday night across
Wisconsin and eastern Minnesota.  The models are probably not yet
handling this wave with high degree of certainty due to its rapid
displacement during a 24 hour time period...but indications are that
the low will track to our northeast/east and the better dynamics and
lift will occur from northeast Minnesota southeast across most of
Wisconsin then into lower Michigan just ahead of the H850 low
circulation.  The rather potent system has the potential to blanket
those areas with 2 to 3 inches of light fluffy snow from Minnesota
into Wisconsin along with gusty winds. On the back side of the storm
and immediately following a strong cold front Friday night...our
forecast area will be in line for a burst of strong northwest winds.
Strong subsidence with strong cold air advection will push surface
winds to 20 to 25 mph overnight with gusts likely to reach 35 to 40
mph or higher. Will need to evaluate the forecast for headlines in
the next 24 hours...but signal is there for another windy system.
There are some differences between the Euro/GFS with the GFS being
slightly stronger with wind fields. The winds may briefly decouple
near sunrise only to mix quickly into the morning hours Saturday.
Much colder air will follow with lows Saturday morning in the teens
to lower 20s/wind chills 5 to 10 F across the region and highs only
reaching the mid 20s northeast to the lower 30s in the southwest.
The system tracking southeast Friday night and Saturday will briefly
drop H850 temperatures down to -10C central to -18C northeast during
the morning hours Saturday. There may be some flurries Friday night
into early Saturday otherwise sunshine will be mixed with clouds
during the daytime hours as strong subsidence sets into Iowa.

./Saturday Night through Wednesday/...Confidence Low to Medium

Each Canadian Clipper system tracking southeast continues to
reinforce the cold air across the region and enhance the upper level
trough.  There will be a brief respite later on Saturday as a
renewed push of warmer air moves into the Northern Plains with yet
another clipper tracking toward the Great Lakes. Temperatures will
briefly recover into the 30s/40s Sunday as southwest flow returns to
the region. The weakening system will eventually move into lower
Michigan with a trailing cold front across Iowa. Though moisture
starved...some light snow or flurries/sprinkles are possible with
the frontal passage. Highs will be cooler Monday but still above
normal for December. With northwest flow still maintained through
midweek...another fast moving clipper will move south toward Iowa
either Tuesday or Wednesday though timing/placement remains suspect.
The lowering of confidence in the extended period is primary due to
3 factors:  the models have a hard time handling fast moving flow;
northwest flow is inherently difficult to model; and model bias in
flow is again showing up with the GFS faster with storm and pattern
movements.  Thus...though the overall upper level pattern still
suggests northwest flow with potential shortwave energy propagating
south/southeast with time...much less confidence continues with the
overall pattern.

&&

.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 1135 PM CST Wed Dec 6 2017

Most active portion of this TAF period comes during the first few
hours as lingering flurries and subsequent MVFR CIGS/vsbys
gradually depart our forecast area to the east. KOTM to be most
impacted over the next few hours. IFR not likely.

Strong winds to continue as well for the next few hours, and will
gradually decline and level off for the rest of the day after
sunrise. After sunset, winds should be under 10 kts at all TAF
sites.

&&

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

$$

SHORT TERM...Martin
LONG TERM...REV
AVIATION...Kotenberg



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