Area Forecast Discussion
Issued by NWS La Crosse, WI

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355
FXUS63 KARX 130319
AFDARX

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

.UPDATED...
Issued at 920 PM CST Tue Dec 12 2017

Short-term/meso models showing signs of a slight shift to the
southwest with the main frontogenetic snow band late tonight/early
Wed morning - along with a bit earlier onset time. Have adjusted
snow chances/amounts to reflect these trends, and have added Jackson
county WI to the advisory. Expect further refinement to the forecast
overnight - as the setup of the band becomes more evident. Highest
amounts likely in a narrow corridor, and confidence still a bit
shaky just where this will ultimately lay out.

.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 00Z TAFS through 00Z Wednesday evening)
Issued at 515 PM CST Tue Dec 12 2017

Cigs: generally looking at VFR bkn cigs through the night, with a
lowering into MVFR post a sfc front Wed morning. The mvfr should
hang around into the Wed evening.

WX/vsby: variety of pcpn possible from tonight through Wed
afternoon. Warm air advection, frontogenetic forcing and an upper
level shortwave trough will all play roles - as will sfc temps and
ice in cloud. As it looks now, a narrow snow band capable of several
inches will setup northeast of the TAF sites, closer to KLSE.
Elsewhere, showery-mostly light intensity pcpn is expected - mostly
snow overnight/Wed morning, with some loss of ice in cloud bringing
a freezing drizzle potential late morning/early afternoon - then
back to snow. Snow or ice accums look minimal at this time, but
obvious impacts if icing is realized. What falls, when and how much
is not clear cut at this time and will continue to monitor and
adjust forecast as needed.

Winds: south will swing northwest and increase with the passage of
the sfc front Wed morning. Despite expected low clouds, bufkit
soundings suggesting gusts of 30-35 kts for KRST. Tight pressure
gradient won`t start to relax until overnight Wed.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory from 4 AM to 6 PM CST Wednesday for
     WIZ017-029-034-043-044.

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

$$

UPDATED.....Rieck
SHORT TERM...JM
LONG TERM...JM
AVIATION.....RIECK



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