Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 220538
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1138 PM CST Sat Jan 21 2017

.SHORT TERM...(This evening through Sunday)
Issued at 312 PM CST Sat Jan 21 2017

Starting to sound like a broken record, but fog remains the main
forecast concern through the remainder of the weekend. A Dense Fog
Advisory continues through noon on Sunday for the entire area.
With little in the way of a surface pressure gradient tonight
between the weakening surface low to the north and another system
passing to the south, and a low-level saturated layer aided by
continued melting snow, just don`t expect much change in
conditions overnight into Sunday. In fact, some locations with
improved visibilities this afternoon may go back down again
tonight. Some improvement in visibility may occur through Sunday,
but fog trends will need to be monitored, for better or for worse.
Either way, like the past few days, temps won`t budge much with
lows tonight in the low to mid 30s and highs Sunday in the mid 30s
to low 40s.

.LONG TERM...(Sunday night through Friday)
Issued at 312 PM CST Sat Jan 21 2017

Weak surface ridging builds into the area Sunday night and Monday.
Although NAM soundings suggest the depth of moist layer will
decrease on Monday, it still may be tough to scour out the low
clouds/fog.

The time to watch for potential impactful weather is late Tuesday
into Wednesday. 21.12Z GFS/ECMWF continue to show an upper level
shortwave trough lifting into the Great Lakes aided by a 140 kt
300 mb jet. This scenario is supported by GEFS members as well.
However, the speed and placement of the upper wave and associated
surface low do vary quite a bit among models, not too
surprisingly given the strong downstream ridging. The GFS/ECMWF
both take the surface low track just south of the area. The GFS is
a bit faster and weaker with the system compared with the ECMWF,
which holds the wave back a bit with stronger forcing from the
upper wave/jet aiding in surface cyclogenesis. Sufficient to say,
it`s too early to tell how exactly the upper wave will evolve, but
right now would lean towards a slower solution indicated by the
ECMWF/NAM with the highest precip chances Tuesday night, location
depending on where the deformation zone winds up. Boundary layer
temps will determine the predominant precip type. Assuming a
surface low track to the south, overall a wet snow would be
favored during the bulk of the event, with maybe some rain mixed
in at times, depending on low-level thermal profiles. Several
inches of snow continues to look possible Tuesday night and
Wednesday over at least parts of the area, but given the spread
among possible solutions, including precip amounts/placement, it`s
too early to get into amounts at this point.

Beyond Wednesday, models are projecting the region to remain
under broad upper-level troughing into the weekend, with embedded
shortwaves rotating through. This will keep periodic, small
chances for light snow/flurries in the forecast through much of
this period, particularly on Thursday. Thermal profiles are
showing the precipitation generating layer around -10C and surface
temperatures below freezing, suggesting precipitation type should
be all snow. Steep low-level lapse rates could add a convective
element to the snow showers during the daytime hours.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1138 PM CST Sat Jan 21 2017

Improvement in flight category at both KLSE/KRST as of 22.0530Z
should deteriorate through the night as a large area of dense
fog/low ceilings across central MN drifts southeastward behind a
weak advancing surface trough. Will keep KLSE at 1SM or more
through the night, although KRST should eventually drop back to
1/4SM in dense fog. That said, hour-to-hour changes in visibility/
low ceilings have been difficult to predict and amendments are
likely. While ceilings are not likely to rise much through the
period, should finally see an improvement in visibility, especially
after 23.18Z as slightly drier low level air moves in with light
northwest flow. Light west-northwest wind will be the rule at KRST
while KLSE remains light and variable through much of the period.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Dense Fog Advisory until noon CST Sunday for WIZ017-029-032>034-
     041>044-053>055-061.

MN...Dense Fog Advisory until noon CST Sunday for MNZ079-086>088-
     094>096.

IA...Dense Fog Advisory until noon CST Sunday for IAZ008>011-018-019-
     029-030.

&&

$$

SHORT TERM...Lawrence/JM
LONG TERM...JM/MH
AVIATION...Rogers


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