Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 201629

Area Forecast Discussion
National Weather Service La Crosse WI
1120 AM CDT Tue Sep 20 2016

Issued at 1121 AM CDT Tue Sep 20 2016

Decided a bit ago to fire out a Flash Flood Watch for the entire
CWA from tonight through Thursday evening. Just way too many
signals there (stalled boundary, tropical moisture, antecedent
very wet conditions) to ignore the potential for additional
flooding over the next few days. Still not exactly sure where each
individual round of convection will track, with strong hints from
near term guidance that we may toss out an outflow boundary into
southern MN/northern IA late tonight and continue to fire storms
with modest low level jet mass convergence/moisture transport into
and over that boundary. However, of course we will have to work
out the details in near-real time as convection unfolds, but there
is potential there for some 1-3" amounts in spots tonight as this
first round serves as a primer for possible additional heavy rain
later tomorrow into Thursday.

In addition, have been watching persistent/pesky convection firing
across southern MN the past few hours, tied to persistent moisture
transport into that area within an increasingly steep lapse rate
environment. Wouldn`t be totally shocked to see some showers work
toward areas along and just east of I-35 the next few hours, so
will have to watch that closely. Otherwise, most areas should
simply remain dry (in line with our going forecast) with some mild
late summer conditions prevailing.


.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 12Z TAFS through 12Z Wednesday morning)
Issued at 634 AM CDT Tue Sep 20 2016

Dense fog at KLSE this morning expected to dissipate by 20.15Z
with VFR conditions thereafter. VFR will be the rule through the
day at KRST. Primary aviation concern comes late in the period
with increasing chances for showers and thunderstorms overnight.
Timing/coverage questions remain, but will introduce VCTS at KRST
by 21.04Z and at KLSE by 21.07Z. Can expect at least MVFR
conditions with any thunderstorms, but IFR a real possibility as
well. Gusty/erratic winds possible with any thunderstorms that
move across TAF airfields. Otherwise, winds will generally be
light from the southeast through the period.


.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...Flash Flood Watch from late tonight through Thursday evening
     for WIZ017-029-032>034-041>044-053>055-061.

MN...Flash Flood Watch from late tonight through Thursday evening
     for MNZ079-086>088-094>096.

IA...Flash Flood Watch from late tonight through Thursday evening
     for IAZ008>011-018-019-029-030.



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