Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 060534
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1134 PM CST Mon Dec 5 2016

.SHORT TERM...(Tonight through Tuesday)
Issued at 220 PM CST Mon Dec 5 2016

Concerns in the short term focus on the pcpn chances and type for
tonight-tuesday.

Forcing: Sfc/upper level low lifts northeast across the region and
into southern Canada Tuesday afternoon. The low slides its sfc cold
front across the local forecast area overnight/tue morning.
Relatively strong but narrow frontogenetic forcing with the
boundary. Good isentropic upglide in the 275-290 K layers overnight
too...just along and ahead of the front. Add in the push of the
upper level wave and there is plenty of lift for pcpn production.
That said, the models produce minor qpf, mostly in a narrow band.
Saturation is the biggest reason.

Saturation: a time-height north-south running x-section of RH and
temp shows the issues with saturation. The bulk of the saturation
holds under 900 mb this evening...deepening as the front approaches-
and more to the north. Meanwhile a mid layer of clouds is expected
to ride over the low cigs...but probably too wide a gap for a seeder-
feeder process until after 09z.

Bufkit soundings show the expected saturation well also...generally
shallow with no ice as the front approaches, saturating deeply
enough to tap into ice - and thus greater snow threat - as the front
nears - and then a ice/no ice question as the saturation shallows
post front but colder air moves in.

Precipitation: feel pretty confident that a band of precipitation
will work its way across the forecast area, mostly during the
overnight through early morning hours of Tue. Ptype looks like
drizzle/freezing drizzle initially, changing over to snow for a few
hours around the front, and then possible a wintry mix post. Most
of any QPF likely comes when ptype would be snow, but can`t rule
out minor icing.

Its certainly a tricky forecast tonight-tuesday, and adjustments may
be needed based on timing/saturation/temps.

.LONG TERM...(Wednesday through Monday)
Issued at 220 PM CST Mon Dec 5 2016

Main forecast concerns in this time period...

1) Coldest air of the season
2) Accumulating snows for the weekend?

1) Cold...much colder, but par for the season, air still on track to
move into the region for the later half of this week. The departing
upper level low/trough from Tue setups a northwesterly fetch for the
Upper Mississippi River Valley. Colder air funnels in from western
Canada...with 850 mb temps progged to drop from around +3 C at 850
mb at 00z Wed to -14 C by 00z Sat. NAEFS 850 mb temp anomalies hold
from -1 to -2 for the forecast area. Highs around 20 with lows in
the teens continue to look likely. Clouds look fairly plentiful, but
if we end up with some clearing one night, expect lows to bottom out
about 5 to 10 degrees colder - more so in the areas with the snow
pack.

In addition...there will be a wind element to the cold. Tight sfc
pressure gradient Wed-Thu should result in blustery northwest
winds...15 to 25 mph with higher gusts. Stronger in the open areas
west of the Mississippi. Resulting daytime wind chills in the single
digits would result. Going to be a raw couple days.

2) Models favoring becoming more zonal a loft for the upcoming
weekend...sliding a shortwave from the PAC NW to across the Upper
Mississippi River Valley. They disagree on strength and timing of
this wave. The GFS has been the stronger compared to the EC and has
flip flopped with the timing. The EC has remained relatively steady
with a weaker ripple, but still shows some variances run to run.
They both agree that some accumulating snows would be likely - just
not when or how much. Perhaps an inch, perhaps several. Too far out
to make that call, or refine the timing. Will continue with
consensus solution for now.

Prior to the weekend snow potential, the area will be under cyclonic
flow with an inverted - westward extending sfc trough slipping
across the local area on Thu. With such cold air a loft, the setup
favors flurries and/or periods of light snow. Any accumulation looks
minimal. Best chances would be in/north of the I-94 corridor. Don`t
expect any significant impacts at this time.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1134 PM CST Mon Dec 5 2016

The band of light precipitation has formed this evening in the
warm air advection zone ahead of the cold front that was working
across western Minnesota. So far the precipitation has been light
but as the forcing continues to increase as the short wave trough
approaches, expecting that the precipitation will get a little
heavier for awhile overnight and produce a period of IFR
ceilings with MVFR visibilities for both airports. Once the
precipitation ends the visibility will come back up to VFR and the
ceilings should follow a few hours later. However, the VFR
conditions are not expected to last very long. Cyclonic flow will
set up behind the cold front with plenty of low level moisture
trapped below the inversion. This should allow MVFR ceilings to
move back in during the afternoon and then remain through the
evening. The west winds will also become gusty behind the front
and these should continue into the evening as well as the good low
level lapse rates remain in place.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...Rieck
LONG TERM....Rieck
AVIATION...04


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