Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 121955
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
155 PM CST Tue Dec 12 2017

.SHORT TERM...(This evening through Wednesday)
Issued at 155 PM CST Tue Dec 12 2017

Primary forecast concerns will focus on snow/wintry mix chances once
again tonight and Wednesday with another northwest flow system.

Afternoon water vapor imagery indicated a strong shortwave trough
riding across the Canadian Rockies with deep northerly flow
downstream into the Midwest. Low-level warm advection was increasing
across the northern plains ahead of developing surface low pressure
across the Canadian prairies.

Strong low-level warm advection is expected this evening ahead of
the approaching upper waves. There are some differences among models
with how quickly the column saturates, with the NAM drier and the
GFS/RAP/ECMWF/GEM more aggressive with saturation farther south
across western Wisconsin into southeast Minnesota. Upstream
surface obs across northern Minnesota indicate some light snow
reports already, so wouldn`t be surprised to see a band of light
snow move across the area this evening in tandem with the
strongest warm advection.

The more significant precip is expected late tonight into Wednesday
as one upper wave slides west of the area and another follows
closely from northern Minnesota into Wisconsin on Wednesday. A
fairly intense 996 to 997 mb surface low is expected to reach to
reach the WI/IL border by 18Z.  With these systems, details are
often tricky to nail down, especially with potential for relatively narrow
bands of heavier precip and minor track/timing differences among
models. Right now the model consensus continues to suggest the
strongest frontogenetic forcing aided by the approaching upper
shortwave will be near and north of I-94. This area stands to see
the most precip late tonight into Wednesday with amounts tapering
off to the southwest, although will still have to monitor for some
slight shifts in track. The NAM is most bullish on QPF, with over
0.40 inches falling over parts of Taylor/Clark county, while
consensus favor about 1/4 to 1/3 of an inch. If cloud ice is lost,
some freezing drizzle is possible. Based on where confidence is
currently highest, will issue a Winter Weather Advisory for late
tonight and Wednesday for Taylor/Clark/Juneau/Adams Counties, but
will have to monitor trends through the evening and overnight.

Farther south and west, additional light precip is expected with the
upper forcing from the upper shortwaves crossing the area. It`s
possible this precip could fall as a mix of snow/freezing rain with
a loss of cloud possible west of the surface low track for a time on
Wednesday.

Winds will be another concern as the low passes off to the
southeast on Wednesday. Expect increasingly gusty northwest winds
with cold advection and 40 kt winds at the top of the mixed layer,
especially across southeast MN/northeast IA. Winds nearing
advisory thresholds are possible Wednesday afternoon over these
areas, which could lead to some blowing of any fallen snow.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 155 PM CST Tue Dec 12 2017

The persistent northwest flow pattern will continue through
Friday keeping temps below normal. Global models do indicate
several upper level shortwaves dropping across the region through
this time with periodic chances for light snow or flurries.
Amounts will be light, though.

During the weekend, the upper flow pattern relaxes allowing for a
rebound in temps to near or slightly above average this weekend into
early next week. Confidence in precipitation chances is much lower
with plenty of model variability still. Some of the deterministic
models and GEFS members indicate potential for warm advection snow
Friday night or Saturday ahead of an upper trough, although model
agreement is poor so have continued with only lower end pops.
Otherwise, kept a mainly dry forecast Sunday-Tuesday with no
strong signal for precip at this time within the progressive west
to northwest flow pattern.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday noon)
Issued at 1114 AM CST Tue Dec 12 2017

Generally good VFR this afternoon will give way to some MVFR or near
MVFR clouds this evening as lower moisture ahead of the next
approaching wave/low spreads in from the NW. This looking to rather
transient across the area this evening, as drier low level air SW of
the approaching low spreads across the area. Once the low passes
between roughly 12-14z Wed and a strong cold front pushes across the
area, MVFR clouds to return, along with brisk/gusty NW winds and
some -SHSN. Confidence pretty good on the winds and MVFR clouds
behind the front Wed morning, but not so much on the -SHSN.
Considerable variability among models/ensembles for -SN chances Wed
morning. Some keep the area dry or just flurries, while others
producing some snow accumulations at both KRST/LSE. Left precip
mentions Wed morning to VCSH for now, pending a tighter model
consensus or better signal which of the models may be more correct.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory from 4 AM to 6 PM CST Wednesday for
     WIZ017-029-043-044.

MN...None.
IA...None.
&&

$$

SHORT TERM...JM
LONG TERM...JM
AVIATION.....RRS



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