Area Forecast Discussion
Issued by NWS La Crosse, WI

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839
FXUS63 KARX 150430
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1030 PM CST Sat Jan 14 2017

.UPDATE...
Issued at 1030 PM CST Sat Jan 14 20178

All quiet on the weather front with high pressure overhead...but
with one pesky issue - fog. Clearing skies/light winds are a
given through the night with said high pressure and no gradient
nearby, but guidance has been insistent on fog development since
this time yesterday. Hard to ignore that possibility given
visibility already dipping in some spots (LSE briefly down to 2SM
already), and given some melting/contribution to the shallow
boundary layer earlier today, don`t see why we won`t see
additional thickening of fog in some areas. Even looks like a good
river valley fog setup with very light flow up through 850mb, and
have trended the forecast heavily in that direction. Not sure how
widespread it will end up, but I won`t be shocked to see some
pockets of dense fog before sunrise, with some nice riming
potential given the cold temps (some nice photos possibly in the
morning?).

&&

.SHORT TERM...(This evening through Sunday)
Issued at 315 PM CST Sat Jan 14 2017

Quiet weather will continue through the rest of the weekend as
surface high pressure very slowly begins to shift eastward. Some
high level moisture ahead of the approaching system from the
southwest will spread some high clouds back in from the southwest
by later Sunday. Otherwise, temps will climb back to near mid
January averages on Sunday.

.LONG TERM...(Sunday night through Saturday)
Issued at 315 PM CST Sat Jan 14 2017

Focus during this time remains squarely on Sun night-Tues system
likely to bring an icy wintry mix to the region with impacts to
roads/travel.

Model consensus is beginning to tighten with respect to the large
scale as a strong upper wave lifts northeastward into the plains and
Midwest Mon into Tues and a surface low tracks towards the western
Great Lakes by early Tuesday. Overall, 14.12Z models continue to
show less interaction with an upper wave dropping southeast from the
northern plains, favoring a more progressive solution. Strong
northward moisture transport will commence Sun Night with good agreement
in light precip associated with broad warm advection and isentropic
ascent spreading from south to north Sun into Mon, not reaching
north central WI until afternoon. The peak in precip is likely to
be Mon afternoon/evening as the upper wave approaches with deep
forcing and plentiful moisture. Some precip associated with the
mid-level deformation zone and also the surface low could linger
Mon night into Tuesday before the system pushes east.

As for ptype, some differences remain, with the NAM more aggressive
in bringing warmer temps aloft into the region and the GFS still the
coolest. Ultimately, ptypes will depend on 1)whether deep saturation is
maintained to promote cloud ice formation, 2)strength of the warm
nose aloft, and 3) surface temps. The current consensus guidance
suggests more of a liquid event overall. The best chances for snow
would look to be over parts of SE MN into NC WI, especially if the
cooler GFS solution pans out. Currently think that precip may
initially begin as snow/sleet as it spreads northward Monday
morning before transitioning to more freezing rain/rain, depending
on sfc temps, which are expected to rise into the low to mid 30s.
The wintry mix will continue Mon night into Tues.

All in all, no significant changes were made to the forecast with
an icy wintry mix still likely to impact the area late Sun Night
into Tues. The current forecast would give light snow accumulations
to parts of SE MN into NC WI. With more liquid precip, icing right
now appears to be the biggest concern with somewhere around a
tenth of an inch of ice accumulation possible through early Tues.
Once again, surface temps will play a big role in the degree of
icing, while cold roads may have an impact as well. It still
looks like an advisory will be needed late Sun night possibly into
Tues with potential impacts to travel from icy conditions. However,
details of the forecast still need to be ironed out with mixed
precip events being very challenging to nail down.

After this system passes by, mild weather will dominate the rest of
the week as zonal flow transitions to southwest flow aloft by late
in the week. Highs by Thur/Fri may be well into the 40s in some
areas. Will have to monitor for any hydro issues, such as ice jam
potential.  This will depend on how warm we get late next week as
well as how much rain falls over the next few days. Generally it
looks dry into late week before precip potential increases within
south flow aloft towards next weekend.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1030 PM CST Sat Jan 14 2017

High pressure will work through the region into Sunday evening
with overall light winds and skies mainly clear. However, it
appears increasingly likely we will see a period of some fog
development the rest of tonight, with already lower temperature/
dew point spreads and light winds in place (plus a little bit of
melting during the day on Saturday). Not really sure just how
widespread fog will end up, but there is the potential for periods
of IFR conditions at both RST and LSE with even a low risk for a
period of dense fog. Any fog that does develop will leave a
coating of ice on all surfaces given the cold temperatures in
place (classic riming ice setup). Any fog should clear by mid
morning, leaving clear skies through early evening, but with
another chance that some fog starts to develop again by late
evening.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

$$

UPDATE...Lawrence
SHORT TERM...JM
LONG TERM...JM
AVIATION...Lawrence



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