Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 211912

Area Forecast Discussion
National Weather Service La Crosse WI
212 PM CDT Mon May 21 2018

.SHORT TERM...(This evening through Tuesday)
Issued at 1246 PM CDT Mon May 21 2018

Water vapor satellite showing an upper level low over the Dakotas
today and a short wave trough that came out of that system was
responsible for the showers over the area. As this wave continues
to move away the area, the showers will move northeast and should
pretty much be out of the area by late afternoon. There could be a
few lingering showers or drizzle left behind but not expecting
this to be much of an impact. Very little change in the atmosphere
will occur in the wake of the showers; skies will remain mostly
cloudy with light winds. With the showers adding moisture to the
boundary layer, the concern is that fog will form overnight
despite the clouds. Not expecting this to become dense overnight
unless there is some localized clearing. With some weak ridging
moving in for Tuesday, the fog should burn off during the morning
with some sunshine making an appearance during the afternoon.

.LONG TERM...(Tuesday night through Monday)
Issued at 1246 PM CDT Mon May 21 2018

The ridging is not expected to remain in place very long as most
models are showing a short wave trough coming out of the southwest
flow from the Rockies and breaking through the top of the ridge
late Tuesday night into Wednesday. While there is decent consensus
on the track of this short wave trough, there is significant
differences on how strong it will be. The low level moisture
transport with this wave looks to be strongest over northwest
Iowa/southwest Minnesota Tuesday evening and then weakening
rapidly as the wave moves to the east with only some very weak
moisture transport focused on the area by late Tuesday night. The
isentropic upglide is very weak and only about 1 ubar/s on 305K
surface. This pattern should allow showers and some storms to form
to the west and then move toward the area Tuesday night but weaken
as they approach. Enough forcing form the short wave trough to
carry a chance of rain across the entire area through Wednesday.

The activity may increase in coverage Wednesday night as another
weak short wave trough tops the ridge and moves across the area.
This wave looks to help focus the moisture transport into the area
Wednesday evening before it then weakens and moves east by
Thursday morning. Much light Tuesday night, not a lot of
instability will be present and expecting showers with some
embedded thunder to occur.

The occasional rain chances continue from Thursday night into
Saturday as the upper level ridge looks to get flatten by a
system moving along the Canadian border. While there is general
agreement between the models on this system, still plenty of
differences on strength, timing and placement and will just show a
chance for some rain with this system. The upper level ridge
should then build back in over the area with dry weather for
Sunday and Monday.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1212 PM CDT Mon May 21 2018

Local and regional radars suggest the showers will be past or very
close to being past KRST by 18Z and plan to start with a dry
forecast. The showers should end by the middle of the afternoon at
KLSE and will carry them through about 20Z. Ceiling heights are
not expected to move much today with a very weak pattern in place
over the region. With the rain priming the low levels of the
atmosphere and light winds overnight, concerned about fog forming
and producing MVFR/IFR conditions at both airports. Any fog the
forms overnight should dissipate by mid morning Tuesday allowing
the visibility to come back up, but ceilings will be much slower
to improve with these coming up from IFR to MVFR. These could also
improve to VFR by late Tuesday morning, but confidence is not high
enough to include at this time.




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