Area Forecast Discussion
Issued by NWS La Crosse, WI

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

000
FXUS63 KARX 042349
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
549 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 00Z TAFS through 00Z Tuesday evening)
Issued at 549 PM CST Mon Dec 4 2017

Through 7 PM, a line of showers and scattered storms located along
and ahead of a cold front will move east through western
Wisconsin. With this line already east of the TAF sites, the
showers and storm were not added to the TAFs. In the wake of this
front, ceilings will become MVFR. These MVFR have already moved
into KRST and they will move into KLSE around 06.01z.

Overall, the HRRR is handling the deformation band the best. It
shows that some light snow may fall at KRST between 06.04z and
06.06z. As a result, included a MVFR visibility at that time. By
06.09z, the deformation will be pulling out of the TAFs and the
ceilings will become VFR.

Strong west winds will be seen at the TAF sites into Tuesday.
The strongest winds will be from 06.03z and 06.13z when the wind
gusts will be in the 35 to 45 knot range.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Wind Advisory until 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...Boyne



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.