Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 152006

Area Forecast Discussion
National Weather Service La Crosse WI
206 PM CST Fri Dec 15 2017

.SHORT TERM...(Tonight thru Saturday night)
Issued at 206 PM CST Fri Dec 15 2017

Main short-term fcst concerns this period are -SN chances/amounts
tonight then warmer temps Sat.

Data analysis at 18z had a weak ridge of high pressure from western
Ont to IL with a developing trough/warm front from western ND to
western IA. Even with the ridge axis, plenty of clouds remained
across MN/IA/WI with a few flurries continuing in parts of WI and
eastern MN. Mid day temps across the region varied from some single
digits in NE MN, the teens across northern WI, to the lower 40s
across SW IA where skies were more partly cloudy and there was SW
flow behind the warm front for mixing.

Model runs of 15.12z initialized quite well and offer very similar
solutions as hgts rise and ridging aloft builds into/over the Upper
Midwest tonight thru Sat night. Short-term fcst confidences is good
this cycle.

For the short term, a NW to SE band of 925-700mb warm advection/FGEN
spreads east across the area tonight. Initially a dry airmass above
850-800mb to overcome before precip would occur, so the early
portion of the lift works to saturate the column. Higher PW (around
1/2 inch) catches up a bit later too, with the bulk of the precip/
-SN tonight confined to locations near/NE of a Lake City MN to
KLSE to Richland Center WI line. The consensus remains strong for
the bulk of any accumulating -SN to be near/NE of I-94. Have
trended -SN chances near/NE of I-94 toward 70-90% as the band
moves east across the area, with maximum snow amounts of around an
inch or so across Clark/Taylor ctys. Forcing/lift breaks down/exits
rather quickly Sat morning as a weak sfc-925mb low/wave lifts
toward northern IA/MN-IA border and the stronger low level warm
advection shifts into eastern IA/northern IL. Some low level warm
advection does continue across the area Sat/Sat night as the sfc-
850mb low/trough remains nearly stationary. Rather tight 925-850mb
temp gradient progged over the area Sat/Sat night, with a gradient
in the cloud cover as well. Drier sfc-700mb air to spread into
the south 1/3 to 1/2 of the fcst area Sat/Sat night, for potential
of more sunshine south on Sat. May need a flurry mention north
Sat night, but will leave that out of the grids for now. Increasing
high level moisture/clouds ahead of troughing coming out of the
northern Rockies is set to spread across the area Sat afternoon/night,
helping to limit the sunshine and temper highs/lows. Blend of the
guidance highs/lows, generally above normal, for Sat/Sat night
appear well trended.

.LONG TERM...(Sunday through Friday)
Issued at 206 PM CST Fri Dec 15 2017

For Sunday thru Monday night: main fcst concerns this period are
seasonably mild temperatures.

15.12z models offer improving agreement on a weakening trough to
come across the northern plains and a weakening piece of energy to
lift out of eastern OK/KS into the OH valley Sun. The weakening
northern plains trough to weaken as it crosses the region Sun
night, to be followed by pieces of northern stream energy to drop
toward the great lakes Mon/Mon night. Fcst confidence for Sun thru
Mon night is generally good this cycle.

Even with a couple of troughs and some shortwave energy to into
across the Upper Midwest this period, Sun-Mon night continues to
trend generally dry with above normal temps. Model soundings and X-
sections showing The northern plains trough produces little in the
way of lift as it crosses, with generally limited moisture in the
850-500mb layer Sun. Dry trend across most of the fcst area Sun/Sun
night is reasonable. Will have to watch the shortwave trough lifting
out of OK/KS Sun. If it trends a bit stronger or a bit further NW,
the SE end of the fcst area may be clipped with enough moisture/lift
for some mainly -RA chances. For now a small -RA chance across
mainly southern Grant cty looks OK. Northern stream energy dropping
toward the great lakes and lowering hgts Mon/Mon night brings with
it some 925-500mb moisture and some lift as hgts fall and the column
cools. This impacts mainly the N/NE end of the fcst area with some
small mainly -SN chances over the north end of the fcst area Mon
afternoon/evening looking OK. Weak low level cold advection Sun is
replaced with stronger warm advection on Mon. Clouds to temper highs
some Sun/Mon, but lower levels to remain on the warm side with the
blend of guidance highs mostly in the 30s each day looking good.

For Tuesday thru Friday (days 4 to 7): main fcst concern this period
are some seasonably mild temps Tue/Wed then winter storm potential

15.00z/15.12z medium range models in good agreement for rising hgts
and broad ridging aloft to build across the Upper Midwest Tue/Wed,
ahead of the next strong trough digging into the western CONUS.
Lesser agreement among models by Thu into Fri as the western trough
would eject into the plains (at least the northern) Thu, then
into/across the Upper Midwest Thu night/Fri. Trend is for more of a
split of the trough Thu/Fri, with the southern stream portion/energy
remaining over the SW CONUS late next week. Fcst confidence average
to good Tue/Wed, then average to below Thu/Fri.

500mb hgts start to rise Tue, but a weak bout of low level cold
advection spreads across the area Tue, followed quickly by a surge
of low level warm advection Wed. Trend is for plenty of sunshine and
deeper mixing Tue as ridging aloft builds in, then for mid/high
clouds ahead of the Rockies trough to spread across the region Wed.
In spite of the cold advection Tue, then clouds Wed, consensus highs
mostly in the 30s both days and some 5-10F above normal look
reasonable. Attention then turns to the ejecting western trough late
in the week and will it develop a significant round of snow some-
where across the Upper Midwest. As usual, plenty of detail
differences as usual in the day 6/7 time-frame, impacting how, when,
where the trough and its shortwave energy move across the mid/upper
MS valley late next week. Present trend of the splitting trough and
a large portion of the energy remaining over the SW CONUS thru day 7
produces a much weaker system, and much lighter snowfall, to move
thru the region sometime Thu/Thu night/Fri. Given the model/ensemble
variability and lesser confidence, stayed with the blended/
consensus fcst grids for Thu-Fri. Better consensus that amplifying
ridging along the west coast of NOAM later next week will send a
strong shot of cold/arctic air into the north-central CONUS behind
any departing low/trough for Fri and into next weekend.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1139 AM CST Fri Dec 15 2017

IFR conditions in stratus will continue at KRST through around 21Z.
Very light snow is expected to overspread the TAF sites from west to
east late this afternoon into this evening. Any visibility restrictions
in the snow should remain in the MVFR range. Better chances for
snow at KLSE this evening however it will be rather light but cannot
rule out a period of IFR conditions. The snow band is then expected
to lift north of Interstate 94 with conditions improving to VFR
at both TAF sites by late this evening.




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