Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 230505
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1105 PM CST Thu Feb 22 2018

.SHORT TERM...(This evening through Friday)
Issued at 214 PM CST Thu Feb 22 2018

Wintry mix still on schedule for late this afternoon through
tonight. Current GOES-East water vapor imagery shows a fairly
vigorous mid-level trough lifting out of WY/CO. Lift out ahead of
this wave was producing mainly light snow over NE with a wintry mix
being reported over parts of western/southwest IA. This mid-level
wave will continue lifting northeast, spreading precipitation into
our area later this afternoon into tonight. Latest indications point
toward mainly light snow north of a Charles City IA to Winona MN to
Black River Falls WI line. NAM shows an elevated warm nose of +2 to
*6C (RAP showing cooler +1 to +4C) surging north into the area by
later this evening/late tonight which will likely change
precipitation to rain/freezing rain/sleet mix for areas south of the
aforementioned mainly snow line. Right now, still looking like 2 to
locally 4 inches of snow likely for areas along and north of that
aforementioned line with perhaps a light glaze on the tail end of
the precipitation as cloud ice is lost toward morning. For areas
further south, could shape up to be a real mess with a light glaze
to 1/10 inch of ice accumulation on exposed objects and any
untreated roadways and walkways. The wave and associated
precipitation finally exits east of the area by mid-Friday morning.
Bottom line, will keep Winter Weather Advisory headline/timing going
as is.

.LONG TERM...(Friday night through Thursday)
Issued at 214 PM CST Thu Feb 22 2018

Water vapor satellite this afternoon shows a large system over the
Rockies with long wave troughing covering much of the western
portion of the country. The 22.12Z models are in good agreement
that this will impact the region over the weekend as the whole
system progresses east and a strong short wave trough in the base
of the long wave trough deepens and becomes a cut off low over the
Midwest. Plenty of differences though on the speed of which this
will occur and how strong the system becomes. The GFS continues to
be one of the quicker and stronger runs with the cut off low
forming over the Missouri River Valley Saturday evening and then
moving northeast across the area Saturday night as it continues to
take on a negative tilt. The NAM is similar with timing but has
the cut off low much farther north and does not become as
negatively tilted. The ECMWF is very similar with timing and
placement when compared to the GFS, and while also taking on a
negative tilt with the system, it does not form a cut off low.
Despite these differences, all the models have ample forcing to
produce precipitation across the area Saturday afternoon and
night so the problem becomes what type of precipitation will fall.

As the system deepens, it will draw mid level dry air northward
into the area which will create ice in the cloud problems. It will
also draw in warm air which could further complicate issues if
this overlaps areas where ice is still present in the clouds. As
it currently looks, the potential loss of ice in the clouds looks
to be a bigger factor than the warm air and will be the main
player in determining the precipitation types. Still lots of
uncertainty in how this will play out and as of now, have the
highest confidence of ice remaining in the clouds from about
north-central Iowa across parts of southeast Minnesota into
north-central Wisconsin. South of there it becomes much more
questionable on how long ice will remain in the clouds which leads
to the possibility of some freezing rain/drizzle and some minor
ice accumulations before becoming all rain where surface
temperatures reach the middle to upper 30s. With this layout, the
potential exists for snowfall of 6 inches or more over parts of
southeast Minnesota into north-central Wisconsin. Low confidence
in these numbers as the spread in the models is quite large. For
KRST, the 22.15Z SREF has a low of 0.5 inches, up to 8 inches with
the mean around 3 inches. The 22.12Z GFS ensembles are similar
with a 0 on the low end, 13 inches on the high end, a mean of
about 2 inches and the operational run with about 6 inches. After
coordination with neighboring offices, will issue a winter storm
watch from Mitchell County up through Wabasha County for late
Saturday afternoon through the night and for Clark and Taylor
Counties for Saturday night.

Once this system moves past, the western long wave trough looks to
reload early next week as another system comes in off the Pacific.
Differences between the ECMWF and the GFS on how this system
evolves into the middle of next week with the GFS bringing it
northeast into the region while the ECMWF keeps it primarily to
the south. Will have some precipitation chances in for this system
for both Wednesday and Thursday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1105 PM CST Thu Feb 22 2018

Cigs: mostly LIFR/IFR overnight. Should see some increase Fri
morning, with gradually breakout into sct conditions by/during the
afternoon.

WX/vsby: snow, freezing rain/drizzle and/or wintery mix into the
overnight, with most of the pcpn done by 12z. Warm air a loft and
ice in cloud making pcpn type tricky, but trends favoring more snow
for KRST and more freezing rain for KLSE. Both could flip to -fzdz
for 1-2 hours before it ends. vsbys mostly 1-2sm in the snow,
although some mvfr possible at the outset.

Winds: winds will go from the east to the west Fri morning as the
low pressure system responsible for the wintry mix slides northeast
across the region.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory til 6 AM CST
     Friday for WIZ032-033-041-053>055-061.

     Winter Weather Advisory til 9 AM CST
     Friday for WIZ017-029-034-042>044.

     Winter Storm Watch from Saturday evening through late Saturday
     night for WIZ017-029.

MN...Winter Weather Advisory til 6 AM CST Friday for MNZ086-087-
094-095.

     Winter Weather Advisory til 6 AM CST
     Friday for MNZ079-088-096.

     Winter Storm Watch from Saturday afternoon through late Saturday
     night for MNZ079-086-087-094.

IA...Winter Weather Advisory til 6 AM CST Friday for IAZ008-009-
018-019.

     Winter Weather Advisory til 6 AM CST
     Friday for IAZ010-011-029-030.

     Winter Storm Watch from Saturday afternoon through late Saturday
     night for IAZ008.

&&

$$

SHORT TERM...DAS
LONG TERM...04
AVIATION...Rieck



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