Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 171748
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1148 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 18Z TAFS through 18Z Sunday afternoon)
Issued at 1148 AM CST Sat Feb 17 2018

Back edge of snow stretches from near KAEL to KEAU and will move
eastward through early afternoon. IFR visibility and MVFR ceilings
will continue in light snow through 17.19Z-17.20Z. MVFR ceilings
may then linger for another or two before a return to VFR
conditions late this afternoon and through the rest of the period.

Light west-southwest wind will be the rule through this afternoon
becoming light/variable or light from the south overnight. Then
expect big changes as winds become strong from the south by mid
Sunday morning. Sustained surface winds will increase to between
15 and 25 kts with frequent gusts at KRST topping 30 kts. Winds
aloft will also be very strong, increasing to between 45 and 55
kts around 2000 ft agl. As a result, introduced low-level wind
shear at both TAF airfields to end the period.

&&

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

$$

SHORT TERM...Lawrence
LONG TERM....Lawrence
AVIATION...Rogers



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