Area Forecast Discussion
Issued by NWS La Crosse, WI

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697
FXUS63 KARX 171128
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
529 AM CST Sat Feb 17 2018

.SHORT TERM...(Today through Sunday Night)
Issued at 242 AM CST Sat Feb 17 2018

High Impact Weather Potential: Light snowfall accumulation still on
track for today. A period of strong winds remains possible on
Sunday, but some questions about how strong. Could possibly be
advisory-level in some areas.

Much-advertised active weather regime begins today, with the first
in a series of shortwaves working out of the northern Plains in our
direction. Already seeing hints of top-down saturation ahead of that
feature to our west, and that trend should continue, with a quick-
hitting batch of mainly light snow for much if not all of the area,
mainly during the morning to early afternoon hours. Per multi-model
trends, still looking like a general inch of snow to fall before we
quickly shut down the warm advection ascent through the afternoon,
with a brief interval of low level ridging and mid level drying
leading to clearing skies for a while into this evening and tonight.
That setup with fresh snow and light winds should allow for temps to
tumble quickly and may actually have to watch for a little fog in
some spots as some later today melting of snow could occur today
with temps nearing freezing in many areas. Something to watch.

Next shortwave still on track to advance well to our north on
Sunday, with an associated surface low tracking from central
Minnesota into northern Wisconsin. As much-discussed in recent days,
that places nearly all of our local area in the "warm sector" for
this event, with an impressive wind signal aloft, featuring 850mb
flow of 60-70 knots! Some things about that - 1) we will have
trouble mixing too deep given the time of year, but could realize up
to 900mb per forecast soundings. 2) The overall omega pattern is for
upward motion during the period of strongest winds aloft - not
exactly favorable for maximized momentum transport. 3) Thicker clouds
may also be skirting the area, again not very favorable for a great
high wind event. With that all said, can`t rule out some advisory
level gusts for a few hours, centered on 18Z, especially ridge tops
and open areas, and have "upped" winds accordingly. Also some small
potential for light snow north of I-94, but increasingly that looks
to be just north of the area where lift/saturation are maximized.

.LONG TERM...(Monday through Friday)
Issued at 242 AM CST Sat Feb 17 2018

Of course all eyes are focused on Monday with digging western CONUS
energy (haven`t seen that much this winter season) and an eventual
eastern ejection of the remnants of that trough in our direction
late Tuesday. That setup continues to favor an active weather regime
across the local area, with a series of surface waves lifting north
toward the area along a very tight baroclinic zone. Unfortunately
for us, with such warmth in place just to the south, this setup
spells big-time precip type issues, with continued hints that we`ll
be fighting a sizable warm nose across at least a part of the area
while reinforcing low level colder/drier air via northerly flow from
the north. Overall forcing continues to look somewhat disjointed
through the event, with the strongest mid level frontogenetic
forcing laid out well to our northwest, and much of the area relying
on a bigger slug of warm advection on Monday for the best precip
production. We may then see a secondary surge of precip back north
for parts of the area sometime Tuesday with the approach of the
upper wave.

Suffice it to say for this early week system that confidence
continues to increase that we will see some type of impactful wintry
weather for at least parts of the area, but confidence in specifics
remains low, not helped in the least by continued large spread among
both GEFS and SREF members regarding precip types (i.e. small
wiggles in this system will have a big impact on eventual precip
type). Interesting to see the 00Z GEFS plumes favor more snow over
the northwest half of the CWA this go around, indicative of a more
suppressed low level cyclone track. From a pattern perspective, this
one matches a classic freezing rain setup, especially given so much
ice in the ground currently for colder low level surfaces, so
another one to watch closely. Could ultimately even see some thunder
over southeast parts of the area on Monday if the warmer solutions
aloft win out, with hints of up to 500 J/kg elevated CAPE showing
up. Spring isn`t far behind.

Beyond Tuesday, thankfully not much to discuss into mid and late
week as expansive low level ridging works through much of the
Midwest, with seasonable temperatures early on (maybe just slightly
below normal) trending back to something closer to late February
reality by late week. There are some small hints we may see some
precip chance by later Friday with another upper wave approaching
from the mean western CONUS trough, but lots of time to figure out
those details.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 529 AM CST Sat Feb 17 2018

The LLWS will abate by 14Z as winds aloft decrease and sfc winds
become more S-SW. A around of moisture and lift will spread a rather
quick shot of MVFR cigs and IFR vsbys in -SN across the area from
mainly mid morning into the early afternoon hours. This mainly 14-
18z at KRST and 15-19z 1530-1930Z at KLSE. Some MVFR clouds are
expected to linger for a few hours behind the -SN, but by mid to
late afternoon, good VFR conditions are expected to return as the
weak sfc-850mb trough axis passes. Westerly winds will spread across
the area behind the trough for the bulk of the afternoon/evening
hours, With winds turning back to the south by Sun morning, ahead of
the next low moving across SD.

&&

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

$$

SHORT TERM...Lawrence
LONG TERM....Lawrence
AVIATION.....RRS



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