Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 042104
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
304 PM CST Mon Dec 4 2017

.SHORT TERM...(Late Afternoon through Tonight)
Issued at 304 PM CST Mon Dec 4 2017

High Impact Weather Potential: First things first, can`t rule out
an isolated severe storm through 7 pm as the front arrives.
Thereafter, wind remains the big story through the night, but
also watching the potential for a little accumulating snow for a
time this evening, especially north of a line from about Charles
City, IA to Black River Falls, WI.

A true tale of two seasons ongoing across the region at the
moment, with very mild air drawn northward into our area ahead of
an approaching strong cold front, just pushing into parts of
southeast MN and northeast IA as of 20Z. That feature is of course
tied to a strengthening surface cyclone expected to lift north
over western Lake Superior this evening, sending much colder air
in our direction along with a rapidly tightening pressure
gradient. Temps in many areas have approached (or will approach
shortly) record values well into the 50s and lower 60s for
December 4th (wow!), with a broken line of convection now
developing along the incoming sharp wind shift, aided by 500-750
J/kg MUCAPE pooled ahead of the front. As this line progresses
east, would anticipate it continuing to develop through 6-7pm, and
with strong 0-2km shear in place, can`t rule out a few stronger to
severe storms, especially east of the Mississippi River, though
we`ll have to overcome some residual surface-based inhibition as
well as a notable warm layer up around 800mb. RAP soundings
suggest we just may do this across far southern areas, and do have
a small concern for low-topped supercell development so definitely
something to watch closely.

That severe threat will end quickly just after 00Z as the cold
front clears the area, with steepening low level lapse rates in
strong cold advection through the evening promoting mixing up to
850-800mb. Solid area of 40-50 knots through the mixed layer
continues to suggest many areas will see surface gusts of 45-50
mph, especially for a few hour period just behind the front. Can`t
rule out a few gusts to 55 mph, especially west of the
Mississippi River in the tightest gradient but overall, just a
windy period through the evening and night, with speeds gradually
subsiding later tonight with slowly weakening mixed layer flow.

Also of interest is the potential for a few hour period of snow to
impact western and northern areas this evening, associated with
deformation forcing as the upper shortwave swings across the area.
Recent trends suggest this feature may be a little more robust
farther east into our area, with the potential for a half-inch to
maybe one inch of accumulation north of a Charles City, IA to Black
River Falls, WI line. This should be short-lived, but it`s possible
snow could fall heavily for a brief period, with some messy travel
possible while snow is ongoing given the gusty winds as well. At the
moment, don`t see the need for a Winter Weather Advisory just yet
but will hit the potential impacts a little harder in graphical
form/social media.

.LONG TERM...(Tuesday through Monday)
Issued at 304 PM CST Mon Dec 4 2017

Steady as she goes in the temperature department right on through
the weekend, with our well-discussed and rather stable western North
America ridging pattern teleconnecting well to broad upper troughing
centered over the Great Lakes. That of course opens the doors to
persistent shots of colder air working down across the area, as well
as the likelihood for periodic shortwaves to also traverse the flow.
As is typical is such setups, the timing and placement of each wave
remains rather low confidence beyond about 2 days, with already
plenty of change noted in the position or even existence of said
waves from just 24 hours ago.

Given such uncertainty, a blended approach remains best with regard
to any precipitation chances, but overall the pattern will not be
very conducive to any big chances for significant wintry
precipitation the next 7 days. With that said, a few stronger waves
could certainly put down some light snow accumulation, with one
target toward later Thursday night or Friday with consensus guidance
hinting at a stronger wave crossing the area with a modest
frontogenetic response somewhere nearby. Outside of that, periodic
flurries or light snow showers could occur with just about any
shortwave, but with the larger message being steered toward an
extended stretch of near to below normal temperatures for early and
mid December (a HUGE change from the past 2 weeks).

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1145 AM CST Mon Dec 4 2017

Cloudy conditions with low VFR/MVFR ceilings will be the rule
through much of the period. Rain/isolated thunderstorms from
earlier this morning have moved east and it should be mostly dry
through tonight. The exceptions being a brief chance for rain
showers at KLSE late this afternoon and light snow at KRST late
this evening. Introduced TEMPO groups for these precipitation
chances. In addition, cannot completely rule out another rumble of
thunder or two at KLSE with any afternoon showers, but not enough
confidence to add at this time.

Bigger concern through the period will be strong winds. Breezy
southerly winds with gusts in the 20 to 30 kt range will gradually
strengthen through the afternoon and then shift markedly to the
west after a cold frontal passage. Behind the front, expect
frequent gusts between 30 and 40 kts. Westerly wind will weaken a
bit by Tuesday morning, but remain strong the latter half of the
period with continued gusts around 30 kts.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Wind Advisory from 6 PM this evening to 6 AM CST Tuesday for
     WIZ017-029-032>034-041>044-053>055-061.

MN...Wind Advisory until 4 AM CST Tuesday for MNZ079-086>088-094>096.

IA...Wind Advisory until 4 AM CST Tuesday for IAZ008>011-018-019-029-
     030.

&&

$$

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



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