Area Forecast Discussion
Issued by NWS La Crosse, WI

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474
FXUS63 KARX 211738
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1138 AM CST Sun Jan 21 2018

.SHORT TERM...(Today)
Issued at 357 AM CST Sun Jan 21 2018

Main fcst concerns today are fog, -DZ/-RA chances and the
continued mild temperatures.

Data analysis at 06z had deepening low pressure near the TX/OK
panhandles with a broad trough/developing front NE into WI. Plenty
of mid/high clouds continued to stream over the region, helping keep
early morning temps on the mild side once again. Even with the high
clouds, light winds, a shallow nocturnal inversion and snowmelt from
Sat were producing fog across much of the area early this morning,
some of it locally dense. Increasing southerly low level flow ahead
of the low was spreading some lower clouds/moisture northward across
IA/IL.

The southerly low level flow ahead of the system moving into the
plains will continue to spread the lower level moisture/clouds
northward today. With this moisture coming north over frozen and at
least partly snow covered ground, the BR/FG looks to thicken/become
more widespread thru 8-9am, before warming today allows for some
dissipation and improvement of visibilities. Either way, looks like
some thinner fog to persist much of the day, perhaps all way into
tonight. The lower clouds, and moist/foggy boundary layer to temper
highs some from those of Sat, but with the warm start and
increasing low level warm advection, blend of guidance highs today
in the mid 30s to lower 40s appear quite reasonable.

The system coming across the plains continues to offer a slowing
trend, with the deeper moisture and any precip chances spreading
into the south end of fcst area to now arrive more into the late
morning and afternoon hours. This when BL temps should be above
freezing, and limiting the -FZDZ chances this morning. Now appears
any precip today will fall as liquid.

.LONG TERM...(Tonight through Saturday)
Issued at 357 AM CST Sun Jan 21 2018

21.00Z model suite in good overall agreement bringing a strong
500 hPa closed low from southeast CO into northern IL tonight
through Monday. At the surface, the low tracks from central KS to
the WI/IL border. The system is very wet with model consensus
liquid QPF from 0.75 to 1.0 inches. The initial slug of
precipitation moves into the southern forecast area late this
evening and then gradually lifts northward through Monday before
the southwest to northeast oriented deformation band takes over
Monday afternoon into Monday night.

Still some questions concerning potential dry slots and lack of
cloud ice, but model thermal profiles are in reasonable agreement
showing very warm air aloft encompassing most of the forecast area
tonight and then dropping below freezing from west to east through
Monday. At the surface, temperatures tonight through Monday
afternoon are generally at or slightly above freezing, helping
limit freezing rain and potential ice accumulations. The main
exception being the higher terrain areas of southeast MN,
northeast IA, and adjacent portions of southwest WI where
temperatures near or slightly below freezing could result in a
wintry mix and light icing. As cloud ice increases and the
atmospheric column cools, this mix is expected to change over to
all snow by mid-Monday morning for these areas. For areas farther
south and east, precipitation type should be mainly rain into
Monday afternoon given warmer surface temperatures. The colder air
finally surges eastward Monday evening into Monday night,
bringing a wintry mix and eventually all snow to the rest of the
forecast area.

All said and done, snow amounts will be highest across southeast MN
into north-central WI with 6-12 inches expected. To the southeast,
there will be a very sharp gradient. A degree or two warmer or
cooler along with any slight shifts in the highest QPF could
result in big changes in snow amounts within the transition area.
Snow amounts across northeast IA into far southwest WI look to be
less than an inch. As for ice, the highest amounts are expected
across southeast MN where up to two-tenths of an inch are
possible. Elsewhere, a light glaze up to a tenth is possible.

Still some spread in the latest SREF plumes for snow amounts (likely
due to small differences in cloud ice and the temperature profile),
but operational runs have consistently hit the northwest forecast
area with 6 or more inches of snow. This combined with the potential
for higher ice amounts at the onset of the event and some
blowing/drifting snow, will upgrade the entire current Winter Storm
Watch to a Winter Storm Warning. Winter Weather Advisories will
likely be needed for a tier or two of counties farther southeast,
but will allow later shifts to address this issue. Travel will be
hazardous in areas that see heavy snow, including during the Monday
morning and evening commutes. Travel will worsen from west to east
through Monday into Monday night as rain/wintry mix eventually
changes over to snow.

Tuesday through Friday look dry as surface high pressure dominates
the regional weather pattern. Seasonable temperatures Tuesday/
Wednesday will give way to significant warming to end the work
week given strong 500 hPa ridging across the area. Highs by Friday
into the 40s for most areas looks reasonable at this time. Next
best chance for precipitation will be Saturday as another large
trough moves into the plains. Model differences preclude much
detail at this time.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1138 AM CST Sun Jan 21 2018

Complex aviation forecast ahead over the next 24 hours as a winter
storm approaches the region. Early this afternoon, LIFR fog has
persisted at KLSE with little mixing in the valley. Expect some
increase in visibility through the afternoon as the gradient
tightens, but confidence is somewhat low.

Otherwise, a period of VFR conditions is possible into this
afternoon at KRST before low-level moisture surges northward later
this afternoon/evening. Ceilings/visibilities should fall back
into the MFVR and IFR ranges this evening with precipitation
spreading northward, and then to LIFR at least KRST as precip
increases Monday morning. Precipitation could initially be a
wintry mix at KRST before trending towards all snow on Monday.
KLSE will likely be near the transition zone so have kept all rain
for now, but snow/sleet and accompanying visibility reductions
are possible. Even some thunder could not be ruled out Monday
morning.

Winds will also become gusty from the east on Monday morning,
especially at KRST. Could see some LLWS, mainly at KLSE, as well
as winds around 1 km increase to 50-60 kts.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Storm Warning from 3 AM Monday to midnight CST Monday
     night for WIZ017-029-032>034.

MN...Winter Storm Warning from 3 AM Monday to midnight CST Monday
     night for MNZ079-088.

     Winter Storm Warning from 9 PM this evening to midnight CST
     Monday night for MNZ086-087-094-095.

IA...None.
&&

$$

SHORT TERM...RRS
LONG TERM....Rogers
AVIATION...JM



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