Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 072335

Area Forecast Discussion
National Weather Service La Crosse WI
535 PM CST Wed Dec 7 2016

.SHORT TERM...(Tonight through Thursday)
Issued at 222 PM CST Wed Dec 7 2016

Cold cyclonic flow over western Great Lakes has dropped temperatures
below normal for a change, producing this cold air stratus and
occasional flurries or periods of snow. Winds also creating blustery
conditions with single digit wind chills - quite a change from
earlier this fall.

Still seeing a little bit of drifting snow with patchy slick spots
on more rural areas, otherwise weather fairly nil in short term.
Minor short wave may keep clouds around well into Thursday with more
occasional light snow. Temperatures should be fairly similar tonight
with winds staying up.

.LONG TERM...(Thursday Night through Wednesday)
Issued at 222 PM CST Wed Dec 7 2016

Certainly more to look at in later forecasts with snow threat over
the weekend AND even colder air pushing in next week.

Looks like by Friday upper low has shifted far enough east that
surface ridge finally builds in. Could see more sun and break in
snow flurry activity finally.

Main focus for weekend will be watching approach of short wave
trough in westerlies that comes out of the northern Rockies by
Saturday morning. Confidence HAD been growing for a snow event for
our area as this waves passes through Saturday night into Sunday.
But flip-flop in how US guidance handles strength of this wave, plus
big change in 07.12z ECMWF came across lowers that confidence down a
few notches sorry to say.

While last few GFS runs have been dropping 5-10 inches of snow
across our area by Sunday, 07.12z NAM is stronger now but only goes
out through 00z Sunday. ECMWF delays wave by at least 12 hours and
deepens it as it passes just south of the area. Will not get caught
up in details that far out and remain using a blend with more weight
on last few GFS runs that have been most consistent with each other.

07.12z NAM/GFS/Canadian have many prime ingredients for very
efficient snow event Saturday afternoon into Sunday morning. This
includes decent isentropic lift ahead of wave, deep and strong
quasi-geostrophic forcing to aid lift, strong lift through
dendritic zone according to model soundings, and what could be
elongated period along frontogenetic zone to add to amounts. IF
this all plays out we could see warning-level snows in parts of
the area. Although a bit early for snowfall amounts, looking at
high-end advisory or low end warning event perhaps? Suppose next
24-48 hours will provide more answers.

Bottom line for forecast is snow chances were raised even more for
Saturday night especially. Small snow chances have also been added
for parts of Sunday and Sunday night based on how different 07.12z
ECMWF looks. Trends for this solution will also have to be monitored.

Other story in later periods is next shot of arctic air due early
next week. We are talking Days 6 and 7 but medium range temperatures
depict sub zero conditions for 24-36 hours perhaps with very cold
wind chills. Depending on how much snow we get this weekend could
have an impact on this airmass as well. 850/925mb temperatures
suggest 1 to 2 standard deviations anomalies so have tried to
lower temperatures next week. This could be another impact period
with wind chills and bears watching.


.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 535 PM CST Wed Dec 7 2016

MVFR conditions will prevail over the next 24 hours with
widespread stratus/stratocumulus in place as cold air remains in
place. The approach of an upper wave should deliver a round of
very fluffy snowfall for both LSE and RST roughly 07Z through 17Z,
with the potential for some light accumulation at both sites.
Given the fluffy nature of the snow, don`t envision any major
impacts, though continued gusty conditions for RST may result in
some additional visibility reduction in blowing snow (confidence
not very high on that just yet). For the moment, have held
conditions MVFR during the expected snow later tonight, but a
period of IFR visibility is certainly possible during any heavier
snow showers.




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