Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 080533

Area Forecast Discussion
National Weather Service La Crosse WI
1133 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 06Z TAFS through 06Z Thursday night)
Issued at 1133 PM CST Wed Dec 7 2016

A band of deeper moisture will sweep across the region through
mid morning Thursday, resulting in an uptick in mainly light snow
showers for LSE and RST. Overall, conditions are expected to
remain MVFR at worst, though a few brief periods of IFR visibility
are possible under any briefly heavier snowfall. Any accumulation
will be well under an inch for the terminal sites and snow will
be very light and fluffy. Look for winds to remain gusty from the
west/northwest, with gusts either side of 25 knots common through
the afternoon before diminishing tonight.




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