Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 200859

Area Forecast Discussion
National Weather Service La Crosse WI
359 AM CDT Tue Sep 20 2016

.SHORT TERM...(Today and tonight)
Issued at 359 AM CDT Tue Sep 20 2016

At 3 AM...dense fog continues to be reported across much of
western Wisconsin. This fog is expected to linger through at least
830 AM and then start to quickly dissipate.

During this evening, the 925 mb and 850 mb moisture transport is
focused in southwest Minnesota. This will result in the
development of elevated supercells. With the 850 to 300 mb lapse
rates around 7.5 C/km, the main threat looks to be large hail
initially. These storms are then expected to congeal into a
mesoscale convective complex that will move east across the
forecast area during the late evening and overnight. With DCAPEs
climbing into the 1000 to 1250 J/kg, there will be the potential
for damaging winds along with the hail threat. The Storm
Prediction Center currently has the entire forecast area under a
marginal risk of severe weather. There was even discussion that
there maybe a potential for an upgrade to a slight risk. This was
dependent upon the track of this system becoming more certain.

With precipitable water values approaching 1.6 inches (which is 2
to 3 standard deviations above normal) and the warm cloud layer
depths around 4 km, the showers and storms will be highly
efficient rain producers. This could create a flooding threat if
the storms train across the same area. However with the GFS, NAM,
and even the ECMWF showing a progressive mesoscale convective
system. This would limit the possibility of this occurring. Also
another concern is that many of the CAM models do not even show a
mesoscale convective complex forming. They show scattered
convection that progresses east as the low level jet veers away
from the area.

.LONG TERM...(Wednesday through Monday)
Issued at 359 AM CDT Tue Sep 20 2016

On Wednesday morning, a majority of the deterministic models
continue to show heavy rain falling across the area. Looking at
the 850 mb moisture transport, I am concerned that these models
may be holding onto the rain a bit too long. This is supported by
the CAM models which suggest that it could dry out across most of
the area.

From Wednesday afternoon into Wednesday night, the models continue
to show that a short wave trough will move northeast out of the
Central Plains. As this occurs, the southerly winds will increase
across the area. This will allow the warm sector south of the
front to have dew points in the lower to mid 70s. With high
temperatures climbing into the lower 80s, the 0-1 km mixed-layer
CAPES will climb into the 2 to 3K range. While the deep shear will
found north of the front, there will be sufficient 0-3 km shear
for damaging winds. However soundings suggest that a strong CAP
may be in place which would limit the convection from even

Meanwhile along and north of the front, precipitable water values
will increasing to around 1.9 inches as moisture from hurricane
Paine and the Gulf of Mexico moves through the region. In
addition, the warm cloud layer depths will range from 3.5 to 4 km.
This will make the showers and storms very efficient rain
producers. While this has been a persistent feature over the last
couple of days, there still some uncertainty on where this front
may be located. The NAM would suggest that it will be near
Interstate 94. This boundary would then sag south as the storms
build into the very unstable air south of this Interstate.
Meanwhile the GFS hints that the heaviest rain may end up north
of the forecast area. Due to this, there is a bit more uncertainty
for this time period.


.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...Tonight Through Thursday
Issued at 359 AM CDT Tue Sep 20 2016

With the 20.00z models shifting the stationary/warm front further
north, the uncertainty on where the heavy rain increased some. It
now appears that the axis of heavy rain for tonight will be north
of Interstate 90 tonight. For Wednesday, the NAM suggests that a
mesoscale convective complex could build south into the very
unstable air mass south of Interstate 94. Meanwhile the GFS has
its highest rainfall axis north of our area. Due to this
uncertainty, it was the consensus to hold off issuing a Flash
Flood Watch at this time.


WI...Dense Fog Advisory until 9 AM CDT this morning for WIZ029-

MN...Dense Fog Advisory until 9 AM CDT this morning for MNZ088-096.

IA...Dense Fog Advisory until 9 AM CDT this morning for IAZ011-030.



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