Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 151705

Area Forecast Discussion
National Weather Service La Crosse WI
1205 PM CDT Fri Sep 15 2017

.SHORT TERM...(Today through Sunday)
Issued at 300 AM CDT Fri Sep 15 2017

Smattering of light showers, running north-south across the area -
diminishing as a the night wears on. Fueled by a weak frontal
boundary and some moisture transport. Most of the shower/storms
continue well north, over northern MN, associated with an upper
level shortwave. Some hints in a few of the meso models that another
small area of showers could develop over MN late this morning into
the afternoon, likely along another weak boundary/ripple in the
upper level flow. Trends favor keeping chances west/north of the
local area...for now.

The late summery taste to the air will continue for today and
Saturday as an upper level ridge continues to pull warm air
northward. Highs from the mid 80s to around 90 are expected - well
above the low 70s normals for mid Sep. Humidity levels won`t be too
high, but some air conditioners might get their last workout in for
the year. A cold front is slated to sweep west to east across the
Upper Mississippi River valley Sunday, with cooler-more seasonable
air flowing in.

Finally, a decent shot for showers and storms for the local area,
although still can`t say its a slam dunk that everyone will get wet,
nor in amounts that will help the browning grass.

The models have been in good agreement with driving a 500 mb trough
from the PAC NW, across the northern plains Sat, and then northeast
into Canada Sunday. While the bulk of the upper level forcing will
hold west/north of the local area, the system`s associated cold
front is progged to move west-east across the region Sat night. Some
low level moisture transport leads the front in, and there is some
instability that lingers through the overnight hours to aid
shower/storm production. The stronger, deeper shear still displaced
west of the instability axis, but there could be enough overlap for a
stronger (severe?) threat for the late afternoon/evening just to the
west. Bears watching. The GFS and NAM both agree that a sct/bkn line
will be found post the front, with the GFS hinting at a pre frontal
band - mostly moisture transport related. Would expect a diminishing
trend in coverage/intensity as Sat night wears on (weakening

.LONG TERM...(Monday through Thursday)
Issued at 300 AM CDT Fri Sep 15 2017

The cool down Sunday looks to be brief as the GFS and EC are already
back to upper level ridge building for the start of next week,
digging a trough into the west coast and amplifying the ridge over
the east. It doesn`t look quite as warm as this week, but highs back
into the mid 70s to around 80 look reasonable at this time.

As for rain chances, the GFS remains the more pessimistic (or
optimistic, depending on your viewpoint) model, spinning bits of
shortwave energy into and across the ridge, resulting in periods of
shower/storm chances. The EC is more insistent on the ridge winning
out for much of the next week, favoring a drier forecast (although
still produces some spotty chances here and there). Nothing too
clear when it comes to rain chances at this time - so will stick
with consensus.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1205 PM CDT Fri Sep 15 2017

The primary aviation concerns will revolve around the low level
jet tonight. This will increase shortly after sunset across the
area ahead of a short wave trough moving north/northeast across
the Missouri River Valley. The main axis of the low level jet will
be to the west of the area, but the eastern side of this will
spread over the region to create a dramatic increase in wind
speeds to include low level wind shear for both airports tonight.
The other concern from the low level jet is the development of
any convection on the convergent side of the low level jet. The
15.15Z RAP also suggests there will be a weak mid-level short wave
trough ahead of the main short wave trough to provide some lift.
Expecting these two features to be enough to generate some
convection west of the Mississippi River and have opted to include
a VCTS at KRST. This activity should diminish around sunrise with
just mid-level clouds and VFR conditions Saturday morning for
both airports.




LONG TERM....Rieck
AVIATION...04 is the U.S. government's official web portal to all federal, state and local government web resources and services.