Area Forecast Discussion
Issued by NWS La Crosse, WI

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688
FXUS63 KARX 222359
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
659 PM CDT Tue May 22 2018

.SHORT TERM...(This evening through Thursday)
Issued at 201 PM CDT Tue May 22 2018

Visible satellite this afternoon continues to show the clouds
remaining over areas from along the Mississippi River east into
Wisconsin. Still expecting most of these to scatter out through
the evening as a brief period of upper level ridging builds over
the area. With the recent rains adding moisture to the boundary
layer, the clearing skies and light winds, concerned about fog
formation, especially along and east of the Mississippi River
where skies will be clear the longest. Potential that some of this
may be dense but confidence on where and when is not high enough
for an advisory at this point.

Farther west, the potential for fog will be mitigated by
increasing clouds ahead of the short wave trough. The 22.12Z
models remain consistent in showing a wave topping the upper level
ridge tonight. The focus for the low level jet will be over the
Missouri River Valley which should help the wave generate a
complex of showers and storms over northwest Iowa into southeast
South Dakota this evening. This complex should migrate east with
the short wave trough, but the low level jet/moisture transport
will rapidly weaken as the complex moves east which should bring
in mainly showers with some embedded thunder to locations west of
the Mississippi River late tonight.

This activity should continue to drift east/northeast Wednesday
while also weakening but enough to continue mentioning at least a
chance for some showers/isolated storms across much of the area.
Another weak short wave trough should top the ridge again
Wednesday night with the low level jet/moisture transport focused
more into the Upper Mississippi River Valley. This low level
jet/moisture transport doesn`t look to be all that strong but
enough to regenerate another complex of showers and storms from
about the Mississippi River off to the east. The severe threat
with these storms looks rather minimal with only about 750 J/kg
of ML CAPE that quickly drops off into northern and central
Wisconsin. Similar to Wednesday, this complex should continue to
drift to the east/northeast during Thursday while also gradually
weakening and diminishing in areal coverage.

.LONG TERM...(Thursday night through Tuesday)
Issued at 201 PM CDT Tue May 22 2018

There looks to be on and off rain chances through much of this
period with a rather weak pattern that should allow for more hours
of dry weather than wet conditions at any given locations. The
exception to this looks to be Friday afternoon and evening. The
current upper level low over the Desert Southwest will ride up the
backside of the ridge and then slide east through the top of the
ridge to close out the work week. Still some timing and strength
differences between the models, but they all show at least some
weak pv advection in the 500-300 mb layer sliding across most of
Minnesota and Wisconsin. This should push a cold front across the
area Friday afternoon and evening with some convection firing
along it. The 22.12Z NAM suggests between 1000 and 2000 J/kg of ML
CAPE could be in place ahead of the front but the deep shear looks
to be lacking and only about 20 to 25 knots in the 0-3 km layer.
At this point, would expect much severe weather but heavy rain may
be more of a threat with warm cloud depths of 3 to 3.5 km and
precipitable water values forecast to be between 1.5 and 1.75
inches.

Temperatures will also be warming back up during this period with
little to no cool down behind the front Friday. Highs should be
back in the 80s by Thursday and stay there through the holiday
weekend. Highs in the middle to upper 80s are currently expected
across the entire area from Friday through Memorial Day.
Fortunately, it looks like the dew points will remain in the 60s
for this period which will help to keep the heat index values in
check.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 659 PM CDT Tue May 22 2018

A challenging forecast over the next 18 hours at both KRST and
KLSE.

A stubborn MVFR OVC deck has just now shifted east of KLSE and has
set up SKC/SCT skies with very light winds through nearly 10kft.
This, in concert with less than 10F dewpoint depressions, will set
the stage for fog development at KLSE. It looks like it has the
potential to be LIFR by later tonight and thus have kept that
pessimistic forecast on track. There could be a bit of cloud that
moves in from the west late that could pose some issues for the
final drop to LIFR.

Some MVFR fog may develop at KRST but with mixing today and
earlier clearing, but IFR is not probable it seems. However,
increasing instability moving in from the west overnight and
forcing to initiate and sustain TSRA/SHRA looks to affect KRST in
the Wed morning hours. The forecast may be updated to prevailing
TSRA based on overnight development of TSRA further east, and
provided the shift eastward does occur with instability and
forcing. Confidence was high enough to include VCTS and prevailing
RA.

&&

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

$$

SHORT TERM...04
LONG TERM...04
AVIATION...Baumgardt



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