Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 212105

Area Forecast Discussion
National Weather Service La Crosse WI
305 PM CST Wed Feb 21 2018

.SHORT TERM...(This evening through Thursday night)
Issued at 305 PM CST Wed Feb 21 2018

At 3 PM, a 1041 mb high was centered over southeast Minnesota.
Despite the mostly to partly sunny skies, temperatures only ranged
from the teens to lower 20s.

For tonight, the high pressure system will drift northeast into
the upper Great Lakes. Mid and high clouds will move into the
region in its wake. As low level warm air advection moves into
areas along and south of Interstate 90 toward sunrise,
temperatures will become steady and then slowly climb.

From late Thursday morning into Thursday night, a shortwave
trough, currently over the eastern Pacific just west of southern
California, will move through the region. This system will
initially encounter very dry air below 700 mb, but this layer will
quickly saturate as the lifted increases ahead of the wave. There
will be initially enough ice aloft for snow. However, during the
late afternoon and evening, there will likely be a loss of ice as
a mid level dry slot moves into the area. As this occurs, the
snow will likely change over light freezing rain. We may also have
to contend with some isolated thunder as elevated CAPES move
through the region. This would shift the precipitation back over
to snow or maybe even some sleet briefly. 2 to 3 inches of snow
look likely north of Interstate 90 and up to an inch elsewhere.
Icing will be up to a tenth of an inch south of Interstate 94 and
a light glaze elsewhere. If this trend continues, a Winter Weather
Advisory will be needed.

.LONG TERM...(Friday through Wednesday)
Issued at 305 PM CST Wed Feb 21 2018

From Saturday afternoon through Saturday night, another low
pressure system will move northeast through the region. While the
operational GFS and ECMWF are rather similar with the track of
their surface low, there remains some differences on how much
warm air is able to move into the region. The ECMWF ensemble has
850 mb temperatures ranging from -6 to 7C at La Crosse. This will
have an impact upon the precipitation types and amounts. With this
said there remains a lot of uncertainty with this system.

From Tuesday afternoon into Wednesday, both the GFS and Canadian
suggest that a front will become stalled south of Interstate 90.
As waves move along this boundary, expect some precipitation to
occur. Type is still uncertain, so just stayed with a mention of a
wintry mix.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1149 AM CST Wed Feb 21 2018

VFR through the period, then expected to deteriorate Thursday

High pressure will hold across the area this afternoon into
tonight, providing quiet/VFR conditions for the KRST/KLSE TAF
sites. A trough of low pressure will then lift toward the Upper
Mississippi River Thursday morning, pushing in thickening cloud
cover in from the southwest. There could be some light snow
moving in across northeast IA/southeast MN by later in the
morning, but opted to keep out of the TAFs for now. Otherwise,
look for gradual dropping of cloud base heights, below 10kft after
07z and below 5kft after 14z -but remaining VFR.

Conditions are expected to deteriorate Thursday afternoon,
especially west of the Mississippi River, as the likelihood of
light snow increases with the trough moving through.
More detail will be provided with the next TAF issuance this




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