Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 051740

Area Forecast Discussion
National Weather Service La Crosse WI
1140 AM CST Mon Dec 5 2016

.UPDATE...Dense Fog Improving
Issued at 1140 AM CST Mon Dec 5 2016

With the visibilities improving, plan on allowing the Dense Fog
Advisory expire at noon and potentially replace it briefly with a
Special Weather Statement.

UPDATE Issued at 909 AM CST Mon Dec 5 2016

Widespread dense fog developed across northeast Iowa and parts of
southwest Wisconsin overnight. With the meso models and soundings
showing that it will likely continue through noon...decided to
issue a Dense Fog Advisory until noon for these areas.

Also considered one in Clark and Taylor counties in central
Wisconsin, but the law enforcement reports from these counties
made it sound that it was more areal in nature, so just issued
another Special Weather Statement until noon.

With temperatures near freezing, some slick spots may develop on
area roads due to the moisture deposited from this fog freezing.


.SHORT TERM...(Today and tonight)
Issued at 316 AM CST Mon Dec 5 2016

Some fog has developed within the moist airmass following
yesterday`s snowfall, especially as clearing began to occur. To
this point, based on observations, significant visibility
reductions have been patchy over the area, especially with
southwesterly winds in the 5-10 kt range. However, will have to
keep an eye on possible expansion of the fog over the next several
hours, as some short term model guidance indicates. Any fog,
especially with temps near freezing, may contribute to some slick
spots on roads for the morning commute.

The next system of interest is evident early this morning on water
vapor imagery across the northern Rockies. The upper low will move
into the Dakotas today into tonight with a broad surface low
organizing and deepening across northwest Minnesota tonight. Low-
level warm advection will result in one more mild day for the
area. Some of the high-res models attempt to break out light
precip across SE Minnesota into west-central Wisconsin by later in
the afternoon associated with the warm advection, but model
soundings indicate quite a lot of dry air to overcome, so
confidence not high on this initial precip. Otherwise, a
relatively brief period of stronger forcing associated with the
low-mid level frontogenesis will pass across the area tonight into
early Tuesday bringing the potential for a quick shot of light
precip. As for precip type, if saturation occurs quick enough when
surface temps remain mild, some areas may begin as light rain.
However, with much of the forcing initially going into saturating
the column, it may be cold enough for mainly light snow by the
time any precip occurs (or drizzle/freezing drizzle if the moist
layer is not deep enough for ice growth). All in all, any precip
should be light with minimal accumulation.

.LONG TERM...(Tuesday through Sunday)
Issued at 316 AM CST Mon Dec 5 2016

The upper low will lift northward into southern Ontario on
Tuesday with the light precip exiting western Wisconsin. Another
upper level shortwave trough is expected to rotate across the
central plains into the Mid-Mississippi Valley Wednesday into
Thursday. As has been the trend over the past few days, this
solution would keep the area generally dry through mid-week with
perhaps a bit of light snow/flurries by Thursday under cyclonic
flow aloft.

However, the main weather story this week will be the colder air
that will take up residence for much of the week. Following
tonight`s frontal passage, low-level cold advection will keep temps
from rising much at all on Tuesday from morning lows near 30. The
cold air will go nowhere through the rest of the work week, with
highs relegated to the upper teens and 20s through Friday and lows
in the teens. Wind chills down at least into the single digits
will be possible at times through mid and late week as winds
remain breezy. This will be quite the change after a very mild
November and start to December.

Temps will try to slowly rebound over the weekend as the upper flow
becomes more zonal. There has been a consistent model signal
suggesting an upper wave will progress out of the plains eastward
this weekend. The 05.00Z deterministic models vary with the
placement and intensity of the system with the ECMWF weaker and
quicker than the GEM/GFS. Either solution would bring the
potential of at least some snow, so the system will continue to
bear watching.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1140 AM CST Mon Dec 5 2016

With the ceilings and visibilities improving here at the office,
expecting that the same will happen at KLSE within the next 1 to
2 hours. Once this occurs...KLSE will be VFR for the remainder of
the afternoon.

A cold front will move east across the area tonight. As this
occurs, the ceilings will become IFR/MVFR and the visibilities
will become MVFR. These reductions will be mainly due to fog.
There continues to be mainly a drizzle/rain signal along this
boundary. Maybe some snow if we can get some ice crystal seeding.
This should move through KRST between 06.07z and 06.11z, and KLSE
between 06.08z to 06.12z.

Once this front moves through, the ceilings and visibilities will
gradually climb through the morning.





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