Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 140916

Area Forecast Discussion
National Weather Service La Crosse WI
316 AM CST Thu Dec 14 2017

.SHORT TERM...(Today and tonight)
Issued at 316 AM CST Thu Dec 14 2017

At 3 AM, subsidence has allowed a pocket of clear skies to
develop across the Upper Mississippi River Valley. The combination
of the light winds and clear skies have allowed temperatures to
rapidly cool overnight. Temperatures ranged from -3 at Black River
Falls Wisconsin to 17 at Prairie du Chien. In many locations,
this is the coldest night of the season. Prior to tonight, the
coldest temperature was -1F at the Black River Falls RAWS site
back on November 10th. The La Crosse ASOS was 10F. Prior to this
morning, its coldest temperature was 11F on December 12th.
Rochester still has a ways to fall to reach their coldest
temperature of the season (4F on December 12th).

As the clouds to the west and northwest move into the area later
on this morning and early afternoon, snow flurries will move back
into the area.

From late this afternoon into this evening, a trough of low
pressure will move through the region. Much of the lift
associated with this wave is located below a deep (up to 200 mb)
dendritic growth zone (DGZ). As a result, only expecting a
dusting to a couple of tenths of an inch. However, if we happen to
get the lift more into the DGZ, these snow totals could
potentially be higher.

.LONG TERM...(Friday through Wednesday)
Issued at 316 AM CST Thu Dec 14 2017

From Friday afternoon into Saturday morning, a 900-800 mb band of
strong frontogenesis will develop along and north of Interstate
94. This is response to an approaching short wave trough embedded
in the polar jet stream. This will result in a band of light snow
developing on Friday afternoon and this snow will then continue
through Friday night and then gradually dissipate on Saturday
morning as the wave moves off to the southeast. The Cobb data
generates up to 1 inch along this band which makes sense
considering the amount of moisture that it has to work with during
this time frame.

From Monday night into Tuesday, the models are in agreement that a
strong short wave will move east through the region. However,
there are uncertainties on saturation. The Canadian and GFS keep
the area dry. Meanwhile, the ECMWF would generate up to 5
hundredths of an inch. Soundings suggest that if this
precipitation did occur, it would be a wintry mix of rain and

As far as temperatures are concerned, it looks like a majority of
this time period will be influenced by maritime Pacific air
masses, so temperatures would likely average above-normal through
much of this time period.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1111 PM CST Wed Dec 13 2017

Cigs: big holes have opened up in the cloud cover, in an area of
subsidence post a departing upper level trough. Satellite trends
suggest the skc/sct conditions will hold for several hours into the
overnight, with low clouds returning between 09-11z. Might be VFR at
first, but models still suggestive that MVFR will move back in by
12z-ish or so, and then hold across the TAF sites into Friday.
Not completely sold that VFR conditions won`t hold longer into the
morning, but will follow trends for the moment.

WX/vsby: a few ripples in the upper level flow and/or low level
warming could result in scattered flurries, perhaps a few snow
showers, over the next couple days. Nothing that looks impactful,
nor "cut and dry" enough to add to the forecast. Friday night
showing more promise for an elongated light snow band that could
bring minor accumulations and some vsby restrictions, but models
favor keeping north of the TAF sites for the moment.

Winds: sfc pressure gradient has slackened this evening as low
pressure exited east, and winds will continue to drop off. Mostly
N/NW, then variable toward 12z. Winds should increase a bit from the
west by 18z Thu, mostly holding 10kts or less.




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