Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 200442

Area Forecast Discussion
National Weather Service La Crosse WI
1142 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 06Z TAFS through 06Z Wednesday night)
Issued at 1142 PM CDT Tue Sep 19 2017

Watching a large line of thunderstorms from near KFSD to KROX,
moving northeast around 40 kts. For the most part, these storms
are expected to weaken as they move eastward overnight. However,
latest high resolution guidance suggests there could be a brief
period of showers/storms at TAF airfields, so added VCTS/CBs at
KRST from 20.10Z to 20.13Z and at KLSE from 20.12Z to 20.15Z.
Ahead of these storms, expect strong 40 to 50 kt southerly winds
in the 1500 to 2000 ft agl layer. With sustained surface winds
from 10 to 20 kts, expect low-level wind shear through the night,
diminishing after sunrise Wednesday morning.

In the wake of any thunderstorms Wednesday morning, ceilings
expected to drop to MVFR at times. May not be continuous for
hours on end, but for now will keep lower cloud bases until
early-mid afternoon. VFR conditions will return to end the period.
Surface winds will gradually shift to the west-southwest by
afternoon as a cold front moves across the region.




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