Area Forecast Discussion
Issued by NWS La Crosse, WI

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460
FXUS63 KARX 212337
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
537 PM CST Tue Feb 21 2017

.SHORT TERM...(This evening through Wednesday Night)
Issued at 306 PM CST Tue Feb 21 2017

The first part of the forecast looks to be fairly quiet. Water
vapor satellite shows a couple of short wave troughs over the
Pacific Northwest that will move quickly east in the zonal flow
that is in place across the northern part of the country. These
short wave troughs look to combine into one and pass over the
Upper Midwest as a positive tilt open wave Wednesday afternoon and
night. Decent forcing from this system but not a lot of moisture
to work with, so would expect any precipitation to be along and
north of the track of the surface low. Will maintain a small
chance of rain for Wednesday afternoon across the north and then
continue that into the evening hours. Ahead of the surface low
there will be excellent warming in the low level southwest flow.
Some guidance still pushing parts of the southern and eastern
sections in the lower 70s for highs. For now, do not plan to go
quite that warm but will continue with mid to upper 60s for much
of southeast portion of the area.

.LONG TERM...(Thursday through Tuesday)
Issued at 306 PM CST Tue Feb 21 2017

Attention then turns to the stronger system coming in for the end
of the week. The 21.12Z guidance has not added any confidence to
the forecast with the GFS showing a pretty good northward shift in
the track of the system. This was not unexpected as it also
shifted the system for Wednesday farther north. This then does not
let the baroclinic zone to get as far south causing the end of the
week system to track farther north. Would not be surprised to see
the ECMWF show a northward shift as well. Looking at its ensemble
members, all the spread in the surface low position is on the
north side of the operational run. So for now, the forecast was
adjusted to a bit more of a northward track and also started to
incorporate the possibility of a dry slot working in ahead of the
upper level low.

As the short wave trough crosses the central Rockies, an upper
level low should take shape Thursday afternoon. This system then
works to the east/northeast with a neutral to slight positive
tilt. This will cut down on some of the forcing with the GFS
showing weak to moderate pv advection in the 500-300 mb layer
ahead of the system for Thursday night and Friday. Some potential
for some additional forcing from the jet dynamics, but models are
split on whether there will be a couple jet structure or not.
Strong frontogenesis will occur on the leading edge of the system
in the warm air advection where there will also be 5 to 8 ubar/s
of up glide on the 290K isentropic surface. As the surface low
then moves past Friday, a trowal looks to form on this isentropic
surface to help maintain the up glide, especially over the
northern sections of the forecast area. The one thing that is
really going to play havoc with this system is the dry slot. All
the models show this dry slot surging north ahead of the upper
level low and coming in over much of the eastern two-thirds of the
forecast area. This will not have much of an impact where
temperatures stay warm and rain looks to be the dominant
precipitation type. However, deeper into the colder air, this dry
slot will take out the ice aloft which could lead to more
precipitation type concerns and the potential for some ice
accumulations. The northward shift also pushes the better snow
accumulations somewhat out of the area so confidence on the
overall outcome remains rather low. At this point, will continue
to message the potential winter storm for the area but do not plan
to issue a winter storm watch. Messaging right now will focus on
snow accumulations of 4 to 7 inches generally north of Interstate
90 with some glazing also possible if the freezing rain would
form.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 537 PM CST Tue Feb 21 2017

VFR conditions will be the rule at KRST/KLSE through the period.
With only periods of high clouds, any ceilings likely to be 15
kft or higher. Greater impact to aviation operations will be low-
level wind shear. Expect an increasing low-level jet this evening
into the early overnight hours with southwest winds between 35 and
40 kts around 2000 ft agl. Winds aloft will then diminish after
22.07-22.09Z. Surface winds through the period will be from the
south-southwest, generally 12 kts or less.

&&

.HYDROLOGY...
Issued at 306 PM CST Tue Feb 21 2017

Several factors are contributing to elevated water levels on area
rivers: 1) Widespread rainfall amounts of 0.5 to 1.0 inch of rain
occurred on Monday, 2) High and low temperatures averaging 20 to 30
degrees above normal since Friday have led to significant melting of
snow and ice, and 3) Ground conditions are saturated and/or frozen,
leading to more efficient runoff. The most significant impacts will
remain in western and central Wisconsin. The Black River is
currently experience the most issues. River levels have already
risen above moderate flood stage at Black River Falls and are
forecast to do so at Galesville as well. In addition, an ice jam was
reported early this morning north of Hatfield which has caused minor
flooding along the river bank. Flooding is also expected along the
Yellow River in central Wisconsin. With dry weather expected into
Thursday and much of the snow pack already eroded, rapid river rises
will be less likely for the next couple days. However, a winter
storm is forecast to impact the region late Thursday into Saturday,
with model consensus showing a large swath of +1 in. QPF will be
possible. This forecast precipitation is expected to keep river
levels elevated through the weekend and into next week.

&&

.CLIMATE...
Issued at 306 PM CST Tue Feb 21 2017

Low temperatures tonight will be near record high minimums for
both La Crosse and Rochester. Good warming should occur Wednesday
ahead of the surface low that will be moving across the area with
southwest winds and the potential to mix up to about 900 mb. Some
locations across southwest and central Wisconsin could top the 70
degree mark. The all time February high temperature looks to be
in jeopardy for La Crosse. This is 65, which just occurred Sunday,
tying the mark set back on February 12, 1882 and February 15,
1921.

Below are the records for La Crosse and Rochester for Wednesday
February 22nd:

                          Record Highs
                          ------------

             La Crosse                     Rochester
            -----------                   -----------

Wed (22nd)    62/1984        Wed (22nd)     59/1930

                         Record High Lows
                         ----------------

Wed (22nd)    39/1930        Wed (22nd)     42/1930

&&

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

$$

SHORT TERM...04
LONG TERM...04
AVIATION...Rogers
HYDROLOGY...Hollan
CLIMATE...04



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