Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 050916

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

.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 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 southwest 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




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