Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 071750
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1150 AM CST Thu Dec 7 2017

.SHORT TERM...(Today and tonight)
Issued at 307 AM CST Thu Dec 7 2017

A quiet period is expected today and tonight. The mid-level trough
axis has swung through the area with increasing subsidence helping
to dry the column and gradually put an end to the light snow and
flurries. Satellite imagery/obs indicated some clearing early this
morning to the west in Minnesota and would expect some of this
clearing to continue to work eastward through the morning,
although we may see some mid/high clouds through the day as a
secondary trough axis rotates through. Winds will be weaker today
with only around 20 kts winds atop a rather shallow diurnal mixed
layer. It will another chilly day with highs mostly in the 20s.
Increasing clouds are expected tonight within a broad warm
advection regime, with maybe a flurry or two.

.LONG TERM...(Friday through Wednesday)
Issued at 307 AM CST Thu Dec 7 2017

Main focus during this period is late Friday/Friday evening with
increasing chances for a period of accumulating snowfall.

There continues to be good agreement among models with a strong
upper shortwave diving south out of Canada on Friday, with
additional upper support from a left exit region of an upper jet. As
the fast-moving wave drops south, a period of strong mid to upper QG
forcing will drive a band of snow southeastward across the region.
Right now consensus places the highest chances along and east of
the Mississippi, centered on Friday evening. A period of moderate
to strong deep lift within the snow growth region will lead to a
several hour window for snow Friday evening. Even with a very dry
environment, a fluffy inch or two of snow is possible, locally 3,
especially near and east of the Mississippi, before subsidence
behind the wave quickly puts an end to the snow later on Friday
night. In addition, a period of gusty winds may follow in the wake
of the wave with a strengthening pressure gradient behind the
surface low and strong cold advection, especially west of the
Mississippi. The strongest winds may end up west of the
accumulating snow, but something worth watching.

The weekend itself is shaping up to be quiet as high pressure builds
in behind the departing upper wave. Strong low-level warm advection
Saturday night with a push of Pacific air will allow temps to
recover from the 20s on Saturday into the 30s or even maybe low 40s
in spots on Sunday.

The northwest flow pattern will continue early next week, keeping
temps near or below average. Despite some differences with timing
and placement, both the 07.00Z GFS/ECMWF indicate a series of
upper waves will continue to rotate through the flow into early
next week, with chances for light snow at times. There is a
general consensus of Sunday night into Monday and again late
Tuesday and Tuesday night for the higher chances, although
confidence is not high at this range.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1150 AM CST Thu Dec 7 2017

VFR conditions are expected through the afternoon and evening
hours, with winds in the 7-12 kt range switching from the
northwest this afternoon to southwesterly this evening. Guidance
then suggests MVFR ceilings in the 1500 to 2500 ft agl layer
moving in at some point during the overnight hours. While the RAP
is the fastest (and somewhat of an outlier compared to other
models), it seems to have had the best handle on clouds this
morning, so have trended forecast toward this guidance, bringing
in the cloud deck at 6Z and 7Z for RST and LSE, respectively.
Should other guidance prove to be better, the deck would be moving
in somewhere in the 9-12Z time period. Also expect some flurries
with these clouds, but not anticipating any visibility
restrictions. Could see ceilings lifting a bit with daytime
heating on Friday, but not confident enough to bring either TAF
site back to VFR levels.

&&

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

$$

SHORT TERM...JM
LONG TERM...JM
AVIATION...CA



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