Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 111200
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
550 AM CST Sun Dec 11 2016

.UPDATED...
Issued at 550 AM CST Sun Dec 11 2016

Snow continues to track east/northeast early this morning...with the
heavier snow band still generally along and north of I-90 - but
quickly moving toward I-94. Expect this trend to persist as RAP/NAM
move the main frontogentic band into northern/northeast WI before
noon.

Was able to cancel part of the Winter Storm Warning over northeast
IA with the exit of any signficant snowfall. Will continue to cut
the various warnings/adv early as trends allow.


.SHORT TERM...(Today through Tuesday)
Issued at 305 AM CST Sun Dec 11 2016

Concerns in the short term: 1) exiting snow & headlines, 2) blowing
snow tonight-Monday, 3) snow/blowing snow chances Mon night.

1) Ongoing snow: snow was falling across the area early this
morning, but the heavier snows were concentrated along the I-90
corridor, in the main region of frontogenetic lift. A shortwave
trough was driving east into the Dakotas (per latest satellite water
vapor imagery) and will work to enhance/continue the snow in this
band early this morning...but lifting/bumping it east northeast by
mid to late morning.

An additional 3 to 5 inches looks likely with the main band, less
south of there. The bulk of the accumulation should be done by noon.

As the system exits, the saturation shallows out and the cloud layer
could lose its ice. All the favorable low level thermodynamics have
shifted east by the this time though, so not sure there would be any
lift for a drizzle/freezing drizzle threat...IF there was not ice in
the cloud, which isn`t for sure. Going to leave out of the forecast
for now, but will monitor.

Headlines: The I-90 corridor already upgraded to a warning based on
the additional snow totals. To the south, amounts are going to be
less, but radar and models trends also point to the snow ending in
these areas early this morning. Considering downgrading to the an
advisory, but it would only be for a few hours and might lend for
more confusion in "messaging" rather than clarity.

Will work at adjusting end times (earlier) and canceling the various
headlines through the morning.

2) Blowing snow: several inches of very fluffy snow is ready to blow
and drift, and tightening pressure gradient tonight into Monday
morning will help do that. Winds not looking too strong tonight,
mostly 10 to 20 mph with gusts into the 20s - strongest in the usual
open areas west of the Mississippi River and along ridge tops.
Generally sustained near 20 mph starts causing problems, but expect
at least some patchy to areas of blowing snow overnight in those
wind prone areas. Don`t think an additional Winter Weather Adv will
be needed for drifting/blowing snow, but motorists should anticipate
an additional driving challenge to the already fallen snow. Rural
areas probably the most impacted.

3) Monday night snow chances: GFS/NAM/EC have all been in good
agreement with sliding a shortwave trough across the Upper
Mississippi River Valley Monday night. The bulk of the upper level
forcing holds north, but there is a bit of upper level energy
progged to spin across the local area, along with the system/s cold
front. Not very dynamic but does have some frontogenetic lift along
the boundary...and isentropic upglide on the 280:290 K sfcs precedes
it. Should be enough for some light snow/flurries. Minor accums
possible...up to 1/2 inch or so.

Pressure gradient set to tighten again for Monday night-Tue. Again,
doesn/t look overly wind at this time, but 10 to 20 mph with higher
gusts could result in areas of blowing snow in the usual wind prone
areas.

.LONG TERM...(Tuesday night through Saturday)
Issued at 305 PM CST Sat Dec 10 2016

Two concerns for this period: 1) how cold will it get, 2) storm for
the weekend.

1) Cold: No change in the GFS/EC with wanting to drop an arctic
airmass southward across the Upper Mississippi River Valley for the
middle part of the upcoming week. 850 mb temps progged to drop from
around -7 C at 00z tue to around -24 C by Wed evening. NAEFS 850 mb
temp anomalies continue to hold from -1 to -2. Highs in the single
digits above with lows below zero can be expected for Tue through
Friday morning.

With a fresh snow pack and the potential for some clearing -
especially at night - temperatures could be even colder than
currently forecast. Thu morning could be the coldest of the bunch,
currently looking like the best bet for clear/scattered skies during
the night time hours. If a good radiational scenario manifests, lows
could be 5 to 10 degrees colder than currently forecast. The cold
drainage areas of central WI would flirt with -20.

Then, winds will be a concern. Its not going to be a blustery
period, but even 10 to 15 mph will add a bite to the air. Wind
chills advisories could be needed Tue/Wed nights to account for
cold/wind combination.

2) Weekend Storm: the GFS and EC have been adamant in bringing a low
pressure system with several inches of snow across the region for
next weekend. Timing and positioning has been "all over the place",
but they have been persistent that parts of the region would get
snow.

Latest run have sped the system up...with both the EC and GFS in
better agreement with bringing a warm air advective wing of snow in
Friday...sliding the low just south of the area Sat morning. GFS has
been the stronger of the two, but the latest EC has trended toward
the strength of the GFS. There also appears to be more consensus
on timing. This solution would result in several inches of snow for
the local area, with some potential for a wintry mix in the south as
the warm sector could come into play. Meanwhile, winds also look
stronger with this storm compared to the current snow maker, and
blowing/drifting would be a concern in the areas where snow
dominates.

Overall, it has the ear marks of an impactful winter storm that
bears a really close watch.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 528 AM CST Sun Dec 11 2016

The meso models are showing that the snow will move out of the
area this morning. Currently have this occurring at KRST at 11.17z
and at KLSE at 11.18z. However would not be too surprised that
this occurs 2 to 3 hours earlier. Will continue to monitor it and
make changes as necessary. While there may be some pockets of VFR
ceilings, overall thinking that the ceilings will be primarily
MVFR through this time period. The winds will be light and
variable through this evening and then become northwest and
increase into the 10 to 20 knot range overnight tonight.

&&
.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Storm Warning until 6 PM CST this evening for WIZ054-055-
     061.

     Winter Storm Warning until 9 PM CST this evening for WIZ032>034-
     041>044-053.

     Winter Weather Advisory until 9 PM CST this evening for WIZ017-
     029.

MN...Winter Storm Warning until noon CST today for MNZ079-086>088-
095-     096.

     Winter Weather Advisory until noon CST today for MNZ094.

IA...Winter Storm Warning until 6 PM CST this evening for IAZ011-030.

&&

$$

UPDATED...Rieck SHORT TERM...Rieck LONG TERM....Rieck
AVIATION...Boyne



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