Area Forecast Discussion
Issued by NWS La Crosse, WI

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782
FXUS63 KARX 202105
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
305 PM CST Sat Jan 20 2018

.SHORT TERM...(This evening through Sunday)
Issued at 305 PM CST Sat Jan 20 2018

Water vapor imagery early this afternoon showed a plume of mid to
high level moisture streaming eastward within zonal flow ahead of an
upper trough over the Great Basin that will become the focus of the
forecast heading into late in the weekend. Another mild night is on
tap with temps only falling into the 20s along with a canopy of high
clouds and some low cloud/fog development as low-level moisture
begins to work northward ahead of the system approaching from the
west. As warm, moist air begins to nose northward on Sunday, some
freezing drizzle transitioning to drizzle is possible, especially
along and south of I-90, which could lead to a few slick spots on
roads.

.LONG TERM...(Sunday night through Saturday)
Issued at 305 PM CST Sat Jan 20 2018

A winter storm remains set to impact the Upper Midwest Sunday
night into Monday night, with model guidance continuing to come
into better agreement leading to increasing overall confidence.
However, with the local area likely to be in the messy transition
zone area, there is still some uncertainty as to impacts.

With the 20.12Z guidance there is better consensus that the
surface low will track from eastern Kansas Sunday evening
northeastward to central Lake Michigan by Monday evening.
Accompanying low to mid-level thermal fields are also more
consistent among models. With strong low-level warm/moist
advection developing Sunday night, temps likely won`t budge much
overnight, remaining in the upper 20s to mid 30s. Moisture will
not be an issue with this storm with a strong low-level Gulf
moisture tap. Consensus is for 0.75 to 1.00 inches of QPF over
most of the area from Sunday night through Monday night. The
initial slug of heavier precip should come in later Sunday night
into Monday morning along with a tongue of warm air aloft. This
should keep liquid the dominant precip type across much of
northeast Iowa and southwest Wisconsin. Cold enough mid-level
temps will likely keep the dominant precip type as snow across
parts of southeast Minnesota through north-central Wisconsin. In
between it still looks like a messy wintry mix of freezing
rain/rain/sleet. Up to a tenth or two of ice is possible depending
on surface temps.  Precip type could be impacted by intensity,
with some hints of embedded convective elements with the surge of
moisture advection. With high enough rates, would have to keep an
eye on a possible transition to snow during this time if the
strong lift can overcome the warm nose with some quick
accumulations around the morning commute. Regardless, for much of
the area, the Monday morning commute could be a difficult one.

Consensus is for the longer lasting heavy deformation zone snowfall
area to set up from south-central Minnesota northeastward through
north-central Wisconsin with the heaviest snow falling during the
day Monday, including parts of southeast Minnesota. Gusty
northerly winds by Monday afternoon could lead to some
blowing/drifting as well in southeast Minnesota. Snowfall amounts
over 6 inches are expected within this band. Given some
uncertainty with the southward extent of the heaviest snow, will
maintain the Winter Storm Watch as is for now but did delay the
onset times a few hours given a slightly slower evolution. To the
southeast of the heavy snow band closer to the surface low, as
drier mid-level air is advected northward, precip intensity will
wane and be more of a light rain/drizzle Monday afternoon/evening
with loss of cloud ice before perhaps some light snow moves in on
the backside of the system Monday evening.

The system will exit to the east on Tuesday with cooler but
seasonable highs and some decrease in clouds through the day.

There is some indication that a weak clipper system could pass
through the area Tuesday night into Wednesday, but not expecting
any precipitation at this time. Upper-level ridging will build in
for Thursday and Friday, allowing temperatures to rise well above
normal. Models are then in agreement with a deepening shortwave
ejecting off the Rockies towards the Mississippi River Valley for
late Friday into Saturday, producing chances for rain and snow.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1145 AM CST Sat Jan 20 2018

The main challenge for this forecast period will be timing the
arrival of IFR ceilings and visibilities this evening into
tonight. SREF ensemble guidance suggests this should occur between
21.00Z and 21.06Z. Visibilities could be reduced to IFR levels
closer to 21.00Z if fog can develop. Areas of dense fog can also
not be ruled out overnight. But with mid to high level clouds
overhead and nearly steady dew points, overall confidence in fog
is low at this time. Once IFR conditions commence, they should
persist into Sunday afternoon. Winds are expected to remain light
through this forecast period.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Storm Watch from late Sunday night through Monday evening
     for WIZ017-029-032>034.

MN...Winter Storm Watch from late Sunday night through Monday evening
     for MNZ079-088.

     Winter Storm Watch from Sunday evening through Monday evening
     for MNZ086-087-094-095.

IA...None.
&&

$$

SHORT TERM...JM
LONG TERM...JM/Hollan
AVIATION...Hollan



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