Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 250755

Area Forecast Discussion
National Weather Service La Crosse WI
255 AM CDT Mon Sep 25 2017

.SHORT TERM...(Today through Wednesday)
Issued at 255 AM CDT Mon Sep 25 2017

Early this morning, the cold front extended from far northwest
Wisconsin into southcentral Minnesota and then southwest Iowa.
This placed the front just to the west of the local area. Local
and regional radars are showing scattered showers along and
behind the front.

Water vapor satellite continues to show the upper level ridge
centered over the eastern conus with the ridge axis extending west
back into the region. The 25.00Z models show almost no movement in
the position of the ridge or ridge axis today. As a result, the
short wave troughs coming out of the western long wave trough will
continue to move by just to the west of the area. This will also
limit the eastward progression of the surface front and by late
afternoon it should extend from north central Wisconsin into
northeast Iowa. With this slow eastward progression of the front,
not expecting the rain associated with it to push very fast to the
east either and slowed down the eastward expansion of the rain
chances. The most widespread rain will be behind the front where
the deeper moisture resides, so limited any rain chances of higher
than 50 percent to parts of northeast Iowa across southeast
Minnesota into far western Wisconsin north of KLSE. Just how much
thunder will occur today is somewhat questionable. While there
will be around 1000 J/Kg of ML CAPE available, this will all be
ahead of the front. There could be some embedded thunder behind
the front where the forcing will be better as the short wave
troughs rotate through but expect the majority of the activity to
be just showers.

The upper level ridge should finally start to show some eastward
movement late tonight allowing the long wave trough to also edge
to the east. Some differences start to show up between the models
as to whether the surface front also moves slowly east (25.00Z
NAM), remains nearly stationary (25.00Z GFS) or slightly
retrogrades (25.00Z ECMWF). Having the front retrograde some
actually makes sense as a rather strong short wave trough rotates
around the upper level low over western North Dakota. This should
cause the weak surface low over Kansas to move north into central
Minnesota tonight. Again, with the deeper moisture behind the
front along with the stronger forcing, the best concentration of
showers could end up being west of the area. The hi-res models
that go out far enough, seem to support this idea with little to
no rain over the area tonight. Tried to work this scenario into
the rain chances with the highest values this evening and then
slowly reducing them through the rest of the night.

The upper level low will be weakening/filling Tuesday as it lifts
northeast into Manitoba and Ontario. The short wave trough
rotating around it will track across South Dakota into northern
Minnesota pushing the surface low northeast to over Lake Superior
and the cold front into eastern Wisconsin and central Illinois.
This should allow the deeper moisture and forcing behind the
front to finally spread over the entire area. The best chances for
rain look to be in the morning and diminishing through the
afternoon as everything works east behind the front. Very minimal
thunder chances for Tuesday afternoon as the CAPE will be pushed
out of the area ahead of the front with a small chance of some
embedded activity from the forcing behind the front. The rain
chances will then quickly diminish Tuesday night with just some
lingering chances over the eastern sections. Wednesday then looks
to be dry as high pressure builds in from the west.

.LONG TERM...(Wednesday night through Sunday)
Issued at 255 AM CDT Mon Sep 25 2017

Some rain chances will return to areas along and east of the
Mississippi River for Thursday afternoon and evening. This will be
in association with a northern stream short wave trough that will
be dropping across the Great Lakes. The trend has been for this
wave to be a bit stronger and there are also some hints of some
weak CAPE being in place as well. For now, will leave any thunder
mention out, but this may need to be added if the strengthening
trend of the short wave trough continues. Ridging aloft and at the
surface will be over the Upper Midwest for Friday and Saturday
before a Pacific short wave trough crossing the Rockies starts to
form a long wave trough/upper level low over the Plains to close
out the weekend. Some differences between the models on how this
evolution will take place but will have a chance of rain with this
feature starting Sunday afternoon.


.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1028 PM CDT Sun Sep 24 2017

A cold front across eastern MN into central IA will continue to
slowly push eastward overnight into Monday. Guidance is now in
better agreement that a low MVFR/IFR cloud deck will ooze into the
southeast MN, impacting KRST around 25.12Z and continue for much
of the morning before cloud bases lift back above 3000 ft agl.
KLSE will remain VFR through Monday morning.

For both TAF sites, shower and thunderstorm chances increase in
the afternoon into the early evening. Thunderstorms will likely
be scattered necessitating at least VCTS/CB mention for now, but
timing may need adjusting with possible predominant or TEMPO
thunder added in later forecasts.

Ceilings look to drop back into the MVFR range at KRST Monday
evening with VFR conditions continuing at KLSE. Winds through the
period will be light, generally from the south-southeast and
eventually shifting to the southwest. At KRST, winds will turn to
the northwest late in the period behind the front.




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