Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 212344

Area Forecast Discussion
National Weather Service La Crosse WI
644 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 like 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 00Z TAFS through 00Z Tuesday evening)
Issued at 644 PM CDT Mon May 21 2018

Extensive cloud cover with ceilings around 1 kft or less is the
main aviation concern this forecast period.

As of 2330Z this evening, IFR ceilings are ongoing at KRST, while
KLSE is hovering around the IFR/MVFR threshold. Expect ceilings
to gradually lower this evening into the overnight hours. Ceilings
should eventually reach LIFR criteria at KRST. KLSE may also see
a period of LIFR ceilings, but confidence is not high enough to go
that low at this time. Ample low-level moisture along with weak
winds may allow some fog to develop overnight. But due to the
extensive cloud cover, do not expect visibilities to drop below 1
mile at this time. A gradual lifting of ceilings and scattering
out of cloud cover is expected mid to late Tuesday morning, with
much of Tuesday afternoon at VFR conditions.




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