Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 151934
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
234 PM CDT Tue Aug 15 2017

.SHORT TERM...(This evening through Thursday night)
Issued at 233 PM CDT Tue Aug 15 2017

Water vapor satellite this afternoon shows a long wave trough over
the Rockies with downstream ridging over the Midwest. This ridging
will get pushed east of the area tonight and Wednesday morning as
the long wave trough moves out of the Rockies. The 15.12Z models
continue to suggest there will be several short wave troughs
embedded in the long wave trough and they have also trended slower
bringing the whole system into and across the region. The first
short wave trough to impact the area looks to come across the area
late tonight into early Wednesday morning. This wave will only
produce some weak pv advection but may also come in as the leading
edge of the low level moisture transport and warm air advection
starts spreading over the area. This may lead to some showers and
a few storms over the western portions of the forecast area from
overnight into early Wednesday morning. Once this wave moves past
the area, all the forcing then shifts back to the main long wave
trough which will still be well west of the area. With this slower
arrival of the main forcing, dropped the rain chances back into
the 40 to 50 percent range across the west for late Wednesday
morning into the early afternoon and also slowed the arrival of
the rain chances across the east until the afternoon.

The main short wave trough will then swing across the area
Wednesday night followed by a secondary short wave trough for
Thursday when the long wave trough will also move across the
region. As these waves move across, they should produce periods of
weak to moderate pv advection in the 500 to 300 mb layer. The low
level moisture transport will continue to slowly increase and
looks to be at a maximum Wednesday afternoon but will be focused
to the north of the area ahead of the main short wave trough. This
will then start to diminish Wednesday night as it moves off to the
east with the short wave trough. The isentropic up glide looks to
be fairly continuous Wednesday and Wednesday night and should
average 1 to 4 ubar/s on the 305K surface. As the forcing starts
to increase, will spread 60 to 80 percent rain chances from west
to east across the area Wednesday afternoon and evening and then
start to diminish these from west to east during the overnight
Wednesday into Thursday.

With the slower arrival of the system, this will give the
opportunity for more CAPE to build across the area Wednesday, if
there can be adequate breaks in the cloud cover. The GFS now
suggest a gradient of 500 to 1500 J/Kg of ML CAPE from about
Interstate 94 into northeast Iowa. The other severe parameters
really have not changed much with very weak deep layer shear and
25 to maybe 30 knots in the 0-3 km layer. This could be enough for
a few strong storms, but not expecting much severe weather. Still
looking for the possibility of some locally heavy rains as well.
Warm cloud depths look to be in the 3.5 to 4 km range with
precipitable waters approaching 2 inches. Flooding concerns remain
low thanks to the recent stretch of dry conditions and most
counties having 6 hour flash flood guidance in the 3.5 to 4 inch
range.

.LONG TERM...(Friday through Tuesday)
Issued at 233 PM CDT Tue Aug 15 2017

Northwest flow will set up over the Upper Midwest behind the mid
week system. The 15.12Z ECMWF and GFS have come into better
agreement that a short wave trough will move across the region in
this pattern for Friday afternoon and night. Still some
differences in the strength of this wave but enough there to go
with up to a 50 percent chance of rain for parts of the area
Friday night. Behind this system, a upper level low looks to form
across central Canada with nearly zonal flow over the Upper
Midwest. As of now, there is the suggestion of a weak short wave
trough moving across the region with a cold front and some rain
chances for early next week. Presently, this timing would
interfere locally with the ability to view the eclipse.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1225 PM CDT Tue Aug 15 2017

MVFR strato-cu will continue to lift/mix out early this afternoon
with diurnal warming and a bit of drier air advecting in from the
E/NE. Good VFR expected by mid afternoon, thru the evening into the
early overnight hours as weak high pressure holds over the western
great lakes. The leading moisture return ahead of low pressure
coming across the central/northern plains spreads across the area
centered on about 12z Wed. This over a weak sfc-850mb warm frontal
zone. Mdt signal among models for a band of SHRA/isolated TSRA to
lift NE across the taf sites during the late overnight hours or
early morning hours of Wed. Included a period of SHRA at KRST with
only a VCSH mention at KLSE (further east) for now. Left out VCTS/
TSRA/CB mention for now due to limit instability and lower
confidence in coverage of any TSRA with the precip band.

Much of the moisture return progged to be near/above 805mb, so left
cigs late tonight/early Wed more in the 3k-4k ft range. More clouds
later tonight looking to limit late night/early morning BR/FG
development and kept this to a minimum in the tafs this cycle.


&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...04
LONG TERM...04
AVIATION.....RRS



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