Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 202035

Area Forecast Discussion
National Weather Service La Crosse WI
235 PM CST Mon Feb 20 2017

.SHORT TERM...(This evening through Wednesday night)
Issued at 235 PM CST Mon Feb 20 2017

Local and regional radars show the first round of showers has been
progressing steadily to the east/northeast across the area this
afternoon. This was ahead of the weak cold front that extended
from northwest Minnesota into central Kansas along with the long
wave trough that was just behind the cold front. The long wave
trough and the embedded short wave troughs in it, will progress
east overnight and should be just about out of the area by 12Z
Tuesday. The cold front will clear the area quicker than the upper
level trough and with the rain remaining ahead of the front, this
should end starting this evening west of the Mississippi River
and then by late tonight for the eastern sections of the forecast
area. There is still a good round of forcing to come through ahead
of the upper level trough with the 20.12Z GFS showing a period of
moderate pv advection in the 500-300 mb layer. Since the cold
front is rather weak, there is not much frontogenesis with it, but
what there is does look to be well timed to come through with the
pv advection. With the anonymously high precipitable water in
place ahead of the front, on the order of 1 to 1.5 inches, there
will be more than enough forcing to keep the showers going until
the cold front moves through. The areal coverage of thunder has
dropped off but there continues to be an axis of 100-200 J/Kg MU
CAPE that should come through ahead of the front to support at
least a small chance of some thunder being heard through the early
evening hours.

Once this system goes by, the upper level flow will be nearly
zonal through Wednesday. A couple of short wave troughs look to
come out of the Pacific Northwest and move across the Upper
Midwest in this flow. The first, Tuesday night, looks to stay far
enough north to not have much impact on the local area. The
second comes through Wednesday and Wednesday night. With the
zonal flow, there will not be much chance for moisture to return
back to the area, but it may have just enough to work with to
produce a little bit of precipitation across the northern sections
of the area Wednesday afternoon. With not much of a change in
airmass ahead of this system, what precipitation it does produce,
should fall as some light rain. Of more importance with this
system, is that once the surface low zips by, it will help to drag
the baroclinic zone farther south, which will play into the next
system for the end of the week.

The string of well above normal temperatures will continue through
Wednesday as well. Highs both Tuesday and Wednesday will be well
into the 50s with 60s pretty common. In the southwest flow ahead
of the surface low for Wednesday and with some sunshine expected
across the south, concerned that some spots across the south could
even flirt with or top the 70 degree mark.

.LONG TERM...(Thursday through Monday)
Issued at 235 PM CST Mon Feb 20 2017

Attention remains focused on the large system that forms over the
southern Rockies by the middle of the week and then moves
east/northeast across the region to close out the week. The 20.12Z
ECMWF and GFS are very similar in the evolution of this system
with it strengthening in the lee of the Rockies and an upper level
low forming Thursday night and moving across the region Friday
and Friday night. However, there are some subtle differences
between the two that could have significant impacts on the
outcome. The ECMWF is slightly farther north with the upper level
low tracking over northern Iowa into southern Wisconsin while the
GFS takes the low across southern Iowa into northern Illinois. At
the surface, the two are actually fairly close on the track of
the area of low pressure going from northern Missouri into lower
Michigan. This will place the low level baroclinic zone across the
area and this should be the main controlling element in the
precipitation type. An examination of the GFS bufkit forecast
soundings would suggest primarily a rain event across the southern
sections Thursday night into parts of Friday before enough
cooling occurs for a change to all snow. Farther north, it should
be cold enough for all snow for the entire event. These same
forecast soundings are also showing the possibility of a loss of
ice in the clouds for a period late Thursday night into Friday
morning which looks to be about the time the surface low should be
passing through the area.

While there may be decent agreement between the ECMWF and the
operational GFS, there remains a huge amount of spread in the GFS
ensembles on both the QPF and snow amounts, with the biggest
spread in the snowfall amounts. This is likely indicative of the
spread in the surface low track and the location of the warm air.
The system on Wednesday will play into how far south the
baroclinic zone moves, but concerned that it may ooze south more
than surge to the south and that there is a lot of anonymously
warm air that has to be displaced in a short period of time.
Definitely a system that bears watching but way too early to
determine what the impacts will be for the local area.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1130 AM CST Mon Feb 20 2017

Periods of showers will spread eastward through this evening,
impacting both KRST/KLSE. IFR/MVFR ceilings/visibilities can be
expected through the afternoon with the plentiful low-level
moisture and rainfall. Isolated thunder is possible this
afternoon, but coverage/timing of any thunder precludes mention in
the TAF for now. As a surface trough moves through tonight, winds
will shift from southerly to westerly. Clearing skies are expected
as the trough clears tonight. With the moist ground and potential
for clearing skies, will have to watch for the possibility of fog
development, especially at KRST, but confidence is not high enough
to mention right now.


Issued at 235 PM CST Mon Feb 20 2017

Widespread rainfall amounts of 0.5 to 1 inch are expected across the
area through tonight. The rain combined with 1) continuing warm
temperatures that will cause additional snow melt (mainly northern
and central Wisconsin) and 2) saturated or frozen ground will create
increased runoff, leading to rises on many rivers. The most
significant rises are expected along the Black, Yellow, and
Wisconsin Rivers, whose headwaters are co-located with the remaining
snow pack. Minor to possibly moderate river flooding will be
possible at some locations along these rivers. Minor flooding may
also occur along portions of the Trempealeau and Kickapoo Rivers.
Any ongoing river issues may be exacerbated by the potential winter
storm late this week.


Issued at 235 PM CST Mon Feb 20 2017

High temperatures will remain near record territory through
Wednesday at La Crosse and Rochester. Lows will also be near
record high minimums at times through mid-week. The warmest day in
this stretch is looking to be Wednesday. Good warming should occur
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 could top the 70 degree mark that day.

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

                          Record Highs

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

Tue (21st)    64/1930        Tue (21st)     60/1930
Wed (22nd)    62/1984        Wed (22nd)     59/1930

                         Record High Lows

Mon (20th)    41/1930        Mon (20th)     34/1954
Tue (21st)    48/1930        Tue (21st)     39/1930
Wed (22nd)    39/1930        Wed (22nd)     42/1930




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