Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 050526
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1126 PM CST Sun Dec 4 2016

.UPDATE...
Issued at 546 PM CST Sun Dec 4 2016

Local radar and observations indicate all the significant snow has
moved out of central Wisconsin. There could be some remaining
flurries or drizzle for a couple hours this evening. Temperatures
are near freezing, so the drizzle could put a very light coating
of glaze on any untreated surfaces. Not expecting this to last
long enough to pose any significant problems. Will let the winter
weather advisory expire at 6 p.m. as scheduled.

&&

.SHORT TERM...(Tonight through Tuesday)
Issued at 300 PM CST Sun Dec 4 2016

Concerns are 2-fold:
1) Dense fog and/or low stratus tonight
2) Precipitation chances Monday night-Tuesday

Snow has made steady progress east, but probably won`t clear
Adams/Juneau WI counties until 6 pm. Opted to extend to advisory for
those counties, letting it expire elsewhere. Expect some impacts
through the evening for travel with many roads snow covered, slushy
and slick.

The challenge for the rest of tonight is the potential for fog
and/or stratus development. Current cloud mass is progged to shift
northeast through the nighttime hours, although it could hang tough
in the I-94 corridor. RAP more pessimistic, compared to the
NAM12/GFS, with slower clearing. Some high clouds then start to
spill in from the west near 12z ahead of a shortwave trough over the
Northern Plains. Meanwhile, winds stay fairly stiff through the
night - both at and just above the surface. That said, the fresh wet
snow pack will be a great moisture source, and hrrr/rap/nam bufkit
soundings keep a saturated near sfc layer through the night.
Certainly suggestive of either thick fog or low stratus. Winds would
usually point to more stratus, but could be an advective type fog
scenario.

Confidence not high on which way it will play out, but too much
consensus to dismiss the potential. Will go with areas of fog for
now...especially west of the Mississippi where the clearing is more
likely. If fog manifests, it could become dense - which would result
in a dense fog advisory. Something to watch.

That next shortwave swings across the region Monday night-Tue with
the bulk of its forcing holding to the north. It will drag a sfc
front through the local area though and that looks to have a
relatively strong, but very narrow band of frontogenetic forcing.
Saturation could be an issue, at least with pcpn type potential.
North-south running x-section keeps the deepest saturation generally
north of I-94. To the south it quickly shallows out, and loses any
ice in the cloud. Pcpn type would be liquid - and depending on sfc
temps - freezing rain/drizzle would be possible. Whatever falls
looks to be short lived - but a quick 1/2 inch of snow in the north,
and that freezing potential in the south - allude that it won/t
necessarily be an impact-free frontal passage.

.LONG TERM...(Wednesday through Sunday)
Issued at 300 PM CST Sun Dec 4 2016

Temperatures are going to be the big story for the later half of the
week. Significantly colder air will funnel southward across the
Upper Mississippi River Valley post the Tue system. The GFS and EC
drop 850 mb from around +2 C at 00z Tue to -16 C by 00z Fri. NAEFS
850 mb temp anomalies are around -1 to -2. The cold air starts to
moderate a bit by the end of the weekend as the next potential snow
maker moves in from the west. Highs look to hover near 20 Wed-Sat.

The weekend is showing some promise for accumulating snows as both
the GFS and EC want to drive an upper level shortwave trough across
the northern plains and then through the local area sat/sun.
Previous GFS run was more robust/quicker compared to the EC, and
would have favored more accumulations. 12z run came in weaker
though, more like the EC. Something to watch for now.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1126 PM CST Sun Dec 4 2016

The clearing has taken longer to advance eastward than previously
anticipated but is now showing signs of moving east. Once the
clouds clear out, there is still the concern of fog forming
overnight and there have been persistent MVFR visibilities over
portions of southeast Minnesota and northeast Iowa both under the
clouds and where it had cleared out. Some guidance remains very
aggressive in the fog formation overnight suggesting LIFR
conditions would be possible but not confident in this as the
winds stay out of the southwes around 10 knots. Plan to show MVFR
visibility reductions for both airports. The fog should lift by
mid morning for VFR conditions through the afternoon. The next
short wave trough will approach Monday evening along with a cold
front. The 05.00Z NAM continues to slow very little precipitation
along the front with this concentrated farther north near the
surface low. The 04.12Z ECMWF and 04.21Z SREF probabilities look
very similar to the NAM with the 05.00Z GFS showing the most
precipitation along the front. For now, will side with the drier
models and not show any precipitation in either forecast through
06.06Z.

&&

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

$$

UPDATE...04
SHORT TERM...Rieck
LONG TERM....Rieck
AVIATION...04



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