Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 191856

Area Forecast Discussion
National Weather Service La Crosse WI
156 PM CDT Tue Sep 19 2017

.SHORT TERM...(This evening through Wednesday)
Issued at 156 PM CDT Tue Sep 19 2017

After a bout with some morning stratus and fog, today turning out to
be quite pleasant. Easterly outflow from high pressure over the
Great Lakes keeping our area mostly sunny this afternoon with
temperatures as of 2 pm in the 70s.

For tonight, low pressure and a cold front will move east out of the
Northern Plains into the Upper Mississippi River Valley. Our area
will be on the eastern fringes of better moisture
transport/isentropic lift axis tonight. As a result, expecting some
increase altocumulus along with an isolated shower/thunderstorm
overnight. Based on this, pushed a 20-30 pop eastward across the
area tonight.

The low and cold front push through the bulk of our area by
afternoon which will limit amount of time for better instability to
build-up. Main forcing for deeper/stronger convection through the
morning hours will be across northern MN into northern WI as
mid/upper level PV-anomaly swings across that area and then eastern
WI during the afternoon with convergence along the surface frontal
boundary. Based on this, kept pops in the 20-50 percent range,
highest chances northeast of I-94 in WI. Otherwise, look for highs
in the middle 70s to the middle 80s.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 156 PM CDT Tue Sep 19 2017

Good consensus between the 19.12Z NAM and GFS that the cold front
will be east of the area by early Wednesday evening. Other than
maybe some lingering showers over the far eastern sections of the
forecast area, expect dry conditions with high pressure quickly
building in over the Upper Midwest. This high will shift east of
the area Thursday and with the return flow on the back side of
this, look for warmer and more moisture to start returning into
the region. The GFS suggests that 925 mb temperatures will quickly
climb into the lower to mid 20C range for Thursday afternoon and
be in the mid 20C for Friday. Forecast soundings for both days
suggest we will be able to mix deep enough to tap into this warm
air and allow surface temperatures to warm well into the 80s.
Friday looks to be the warmest day with some lower 90s possible
for portions of the area.

The moisture return will also set up the possibility of some
showers and storms. The front that goes through Wednesday looks
to return north as a warm front Thursday night. Most of the models
pass the front through with little to no activity along it, but
the GFS is a bit more aggressive with development in the low
level moisture transport axis. For now, will limit the rain
chances with this returning front to the northern sections of the
area where saturation has a better chance of occurring.

For Friday through the weekend, the rain chances will be tied to
how the upper level flow evolves in regards to the tropical
systems over the Atlantic. Jose is still expected to be meandering
off the coast while Maria will be moving north off the southeast
coast. The upper level ridge will continue to expand north from
the Gulf Coast and become oriented southwest to northeast to be on
the northwest side of the tropical systems. This will effectively
hold the long wave ridge back over the Rockies and keep the short
wave troughs coming out of it northwest of the local area. This
should allow the rain associated with these features to stay just
off to the north and west with primarily dry conditions
anticipated through Saturday night. The upper level ridging is
expected to start weakening Sunday while possibly starting to
shift east. However, there is not a consensus among the models on
this signal as the 19.00Z ECMWF is the most aggressive with the
break down of the ridge and allowing the trough to move into the
Upper Midwest. The GFS, while shifting the ridge a little to the
east, keeps it stronger and in place to the northwest of Maria
which holds the long wave trough back over the lee of the Rockies.
Despite these differences, for now, both models suggest the
surface cold front associated with the long wave troughing, will
be able to move into the area and provide on and off rain chances
from Sunday afternoon through Tuesday.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1203 PM CDT Tue Sep 19 2017

Morning stratus finally lifting/dissipating across portions of
northeast IA and southeast MN, leaving VFR conditions for the rest
of this afternoon into this evening at both KRST and KLSE. Low
pressure and a cold front will be pushing out of the Northern
Plains into the Upper Mississippi River Valley region by Wednesday
morning. Lift out ahead of this low and front is expected to
produce some altocumulus overnight and a slight chance of an
isolated shra/ts. With chances so low due to expected isolated
nature of this convection, opted to keep out of the TAFs for now.
MVFR cloud expected to move into the TAF sites after 14z as lower
level moisture is pulled northward along the cold frontal
boundary. Otherwise, models showing bulk of any shra/ts activity
associated with the low and front remaining north of the TAF
sites, so continued dry through 18z.

Of note will be a bout of low-level wind shear due to increasing
southerly winds around 2kft in the 35-45kt range tonight into
early Tuesday morning ahead of the approaching low and front.




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