Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 200443

Area Forecast Discussion
National Weather Service La Crosse WI
1145 PM CDT Mon Sep 19 2016

.SHORT TERM...(This evening through Tuesday)
Issued at 316 PM CDT Mon Sep 19 2016

Main forecast concerns through Tuesday: Thunderstorm chances
across extreme southern Wisconsin early this evening. Focus then
turns to valley fog late tonight into Tuesday morning.

Ongoing severe storms are moving across portions of southwest
Wisconsin at mid afternoon. Cape and shear profiles would support
the possibility of large hail and damaging winds. These storm will
quickly exit by 5 pm. Scattered showers further north...along
and north of interstate...will subside tonight as a shortwave
exits to the east.

High pressure builds in tonight into Tuesday morning providing
mostly clear skies and light winds. This will set the stage for
valley fog and fog across central Wisconsin Cranberry Country. The
fog may be locally dense with visibilities reduced to around 1/4

The high will exit to the east on Tuesday. Meanwhile  trough dives
into the western Conus as surface low pressure deepens over the

.LONG TERM...(Tuesday night through Monday)
Issued at 316 PM CDT Mon Sep 19 2016

Main forecast concerns through Thursday: Flooding possible. Several
rounds of thunderstorms could produce heavy rainfall which may
lead to flash flooding and river flooding later in the week.

A warm front is expected to lift into the region late Tuesday
afternoon into Tuesday night. An anomalously moist airmass pushes
into the area with precipitable water values increasing to 1.5 to
1.9 inches. This is nearly +3 standard deviations above climatology .
Also, the warm cloud depth increases to 3.5 to 4 KM. So, the
environment is primed for heavy rain. With saturated soil across the
area flash flooding could become a concern. The low level jet kicks
in Tuesday evening with thunderstorms erupting across portions  of
southeast Minnesota, northeast Iowa, and possibly western
Wisconsin. The thunderstorms look to be ongoing Wednesday morning
then we could see the activity taper off. The big question is what
impact will the cold pool from the storms have on the position of
the warm front. The warm front could be driven south, taking the
axis of heavy rain further south too across Iowa into southern
Wisconsin. In addition to the heavy rain threat a few thunderstorms
could produce damaging winds. Large hail cannot be ruled out

Another round of heavy rainfall is looking likely Wednesday
afternoon into Wednesday night with the heavy rain environment in
place and another shortwave pushing across the area. Deterministic
models suggest the warm front lifts northward taking the heavy rain
threat along and north of Interstate 90. This is probably too far
north. Worst case scenario this second round of heavy rain tracks
over the same areas that were hit Tuesday night into early Wednesday
morning.  This could result in a serious flooding situation. Will
continue to monitor the situation closely. A flash flood watch will
likely be needed for Tuesday night through Thursday. The heavy rain
threat looks to finally subside by Thursday afternoon as ridging
builds in ahead of a vigorous trough diving into the southwest

Chances for showers and thunderstorms continue through the end of
the work week into the weekend. This rainfall looks to mainly be
north of where the heaviest rainfall from earlier in the week.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1145 PM CDT Mon Sep 19 2016

Very challenging fog forecast for KLSE Tue morning. T/Td spread of
only 2 degrees at 03z along with light southeast wind excellent
precursors for 1/4sm fg. Clear skies will keep radiational cooling
going, but Tds are starting to drop a bit as drier air moves in from
the north. NAM12/GFS bufkit soundings supporting the fog cause with
a deep light wind layer. Might have pulled the trigger on 1/4sm
hours ago if it wasn`t for the HRRR/RAP soundings. Both continue to
bring up winds overnight..10+ kts by 300 ft persisting til 12z or
so. This could be enough stirring to keep the main Mississippi river
channel fog away from the airport. Maybe bkn003 instead? Perhaps the
T chases the Td thru the overnight? Latest visual into the valley
already has low stratus layer over the river, but with a ragged top -
indicating some winds. Pockets of dense fog already developed in
some nearby coulees. Tough, tough call. Will likely add some tempo
mention for some fog/bkn stratus with confidence oscillating from
"yes-no" for the fog based on those modeled wind differences.

Overnight crew will monitor closely and update as needed.

The rest of Tue looks VFR with some high thin clouds moving through.
Threat for shra/ts ramp up later in the evening as the low level jet
impinges on a low level frontal boundary and interacts with a weak
ripple of shortwave energy.


.HYDROLOGY...Tuesday night Through Thursday
Issued at 316 PM CDT Mon Sep 19 2016

Flash flooding and river flooding is possible through the period.
Heavy rainfall in excess of 3 inches is possible. The exact location
of the heaviest rain is uncertain at this time. There is
increasing confidence that southeast Minnesota, portions of
northeast Iowa, and western Wisconsin could be impacted. Two main
rounds of heavy rainfall are expected. The first will be late
Tuesday night into Wednesday morning with another Wednesday
afternoon into Wednesday night. The worst case scenario would be
these two rounds of rain occur over the same areas resulting in
serious flooding. A flash flood watch will likely be issued over
the next 24 hours for parts or all of the local area. Continue to
monitor later forecasts closely and consider making preparations
for potential flooding.




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