Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 140352

Area Forecast Discussion
National Weather Service La Crosse WI
1052 PM CDT Sun Aug 13 2017

.SHORT TERM...(This evening through Tuesday night)
Issued at 244 PM CDT Sun Aug 13 2017

The focus in this period will be on the rain chances. The first
chance of rain will be from late tonight into Monday evening. The
upper level low currently over eastern North Dakota will progress
east tonight across northern Minnesota and rapidly become an open
wave. A short wave trough on the back side of the low is expected
to swing through the base of the long wave trough and come across
the region from late tonight through Monday afternoon with a band
of moderate pv advection in the 500-300 mb layer. In the low
levels, there is not much of a surface reflection of this system
other than an inverted trough. This inverted trough will work east
into the area tonight and then become oriented more east/west into
Monday. Some weak moisture transport will occur ahead of the short
wave trough and become focused on the area by late tonight into
Monday morning along with about 2 ubar/s of up glide on the 300K
isentropic surface. With the timing of the forcing, expecting that
most of this evening will be dry with scattered showers and a few
storms then spreading east across the area. The best chances for
rain should be late Monday morning and afternoon across north
central Wisconsin which will be closest to the remains of the
upper level low.

Behind this system, high pressure will briefly build in over the
Great Lakes for Monday night and Tuesday morning as the surface
trough transitions into a warm front across southern Iowa. The
warm front will then start to lift north back toward the area
Tuesday. By Tuesday night, there are some model differences on how
far north the front will make it. The 13.12Z NAM lifts the front
into the area by late Tuesday night while the 13.12Z GFS and ECMWF
keep the front to the south of the area. There is not much forcing
along the front as it starts to return to the area while ridging
aloft will be transitioning from west to east across the Upper
Midwest. With the lack of forcing, any activity north of the
returning front should be widely scattered at best and will just
have some very low rain chances for Tuesday into Tuesday evening.

For the overnight hours of Tuesday night, the ridging aloft will
move east of the area allowing the next system to begin
approaching the region. There looks to be several short wave
troughs embedded in the long wave trough with this next system
with the first of these moving into the western sections of
western Minnesota and Iowa. With the short wave trough remaining
to the west, the best pv advection will also remain to the west of
the area. The low level moisture transport will increase ahead of
the short wave trough and will be focused into western Minnesota
but the eastern and convergent side of this will be over the local
area. Along with the moisture transport, there should also be 1 to
2 ubar/s of up glide on the 300K isentropic surface. This should
allow for an increase in coverage to the showers and storms and
will go with a 30 to 60 percent chance from east to west across
the area.

.LONG TERM...(Wednesday through Sunday)
Issued at 244 PM CDT Sun Aug 13 2017

The best period of forcing from this system will occur Wednesday
as the short wave trough and a surface low move east across the
region. The moderate pv advection in the 500-300 mb layer will
come through with the short wave trough and the low level moisture
transport should be focused into or near the area Wednesday
morning, with this then diurnally weakening during the afternoon.
The best instability will be south of wherever the warm front
sets up with the GFS showing 1000-1500 J/Kg of ML CAPE south of
the front. Deep shear with this system appears to be lacking but
there could be around 30 knots in the 0-3 km layer. If the cape
can be realized, there could be a few strong to possibly severe
storms. A bigger threat may be locally heavy rains with the warm
cloud depths looking to be in the 3.5 to 4 km range along with
precipitable waters near 2 inches. Fortunately, the area has dried
out and likely can take some rain before flooding becomes a
concern. This system will not be any hurry to move east which will
necessitate keeping some rain chances across much of the area for
Wednesday night into Thursday.


.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1052 PM CDT Sun Aug 13 2017

A weakening batch of showers and a few thunderstorms continues to
work east out of southern Minnesota and northern Iowa, and should
impact RST and to a lesser extend LSE through mid morning - just a
little faster than earlier expected. Overall, precipitation should
exhibit a weakening trend as it outruns the better instability,
though a brief period of heavier rain is possible for RST along
with a thunderstorm or two. As the rain departs, we will be
watching the possibility that lower MVFR stratus will develop.
This is a trend hinted at strongly by most model guidance, though
confidence isn`t the highest given a lack of clouds at the moment,
though with stronger moisture advection still to take place before
sunrise. Any lower stratus should "mix out" into more of a VFR
cumulus cloud deck through the afternoon hours, as winds shift
from the southeast to southwest and eventually north behind a
passing cold front.

Looking just beyond the current TAF period, there have been pretty
good signals for a widespread MVFR or even IFR stratus deck to
redevelop prior to sunrise Tuesday. Will need to watch trends
closely with this one.




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