Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 181722

Area Forecast Discussion
National Weather Service La Crosse WI
1222 PM CDT Fri May 18 2018

.SHORT TERM...(Today through Saturday)
Issued at 235 AM CDT Fri May 18 2018

Overall weather layout favors dry conditions into Sat for the local
area, but a couple features of note to keep an eye on.

First is the frontal boundary/upper level trough combo currently
over the northern plains - with ongoing convection in response to
this forcing. Models press the front into central MN late this
afternoon/evening, with parent shortwave lifting northeast into
southern Canada. The boundary will also vacate the favorable low
level jet/moisture transport that is currently feeding it. Some
instability builds over MN later today - skinny/long CAPE per bufkit
soundings with generally weak shear. Might be enough to trigger
isolated/sct showers/storms late afternoon/night. Pretty dry as
you near the Mississippi River eastward though, evidenced in RH
fields and bufkit soundings. With this in mind, will keep any pcpn
threat off to the west/northwest of the local area.

The other feature of note is a shortwave lifting northeast from the
mid mississippi river valley. While most models take this feature
across the Oh river valley, some spin enough energy northward for
small rain chances across parts of southern WI later tonight. Dry
air in place will hamper the threat somewhat, and trends again favor
chances east of the local area.

All in all, dry forecast still looks good into Sat - with a couple
feature to monitor.

.LONG TERM...(Sunday through Thursday)
Issued at 235 AM CDT Fri May 18 2018

What originally looked like a solid bet for widespread rains Fri
night/Sat continues to get pushed back a day or two...and at least
in one model (the GFS), any wide spread threat could hold off until
Mon night/Tue.

Overall, the models favoring a weak upper level trough over the
rockies/desert southwest with a meandering, cutoff low gradually
lifting toward the region Sat night/Sun. Sfc front likely hanging
around west-east across mid mississippi valley for the upper level
forcing to work on. Not much moisture transport to speak of, with
the nocturnal low level jet favored to point into northern MO/central
IL Sat/Sun nights. The GFS and NAM`s CAPE pool also well south with
best deeper shear also south. Still some thunder threat, but any
stronger/severe threat should hold south.

Flow relatively weak, so trying to pinpoint timing on any of the
various shortwaves can be problematic. Going to hold with the
consensus solution for pcpn chances for now. With that in mind,
generally higher shower/storm chances south of I-90 with proximity
of sfc boundary and ripple coming out of the southwest.

High pressure starts to build in for Tue, with the GFS holding onto
its influence for the local area through Friday. The EC is not as
bullish, pealing off a bit of upper level energy out of a weak
Rockies trough Fri - resulting in some pcpn chances for the Upper
Mississippi River Valley. Relatively weak flow persists in the EC, so
confidence low in this outcome.

Both models do favor a shot for pcpn on Sat with the GFS adding in
some influence of a northern stream 500mb shortwave. The EC does not
have this feature, and thusly not as robust of a system, nor with as
much cooler air. Differences keeps confidence low in the details,
although expectation is still for a decent shower threat Sat.
Depending on timing, instability meager, and very little/if any
shear. Should have some thunder threat, but potential for strong
looks low for the moment.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1222 PM CDT Fri May 18 2018

The upper level low over Illinois will become an open wave and
move toward lower Michigan through tonight. The ongoing showers
and storms with this system are expected to lift north/northeast
and will stay off to the east of both airports. However, the
increase in moisture as this system moves north might push some
lower clouds over the area along with allowing some fog to form.
With the light winds and mostly clear skies confidence is higher
that fog will form than the lower clouds, so have included a MVFR
visibility reduction for both airports while putting some MVFR
ceilings into a tempo group for late tonight and early Saturday
morning. These should both burn off by the middle of the morning
for VFR conditions into the afternoon. A weak cold front will
start to move across Saturday morning turning the winds to the
northwest and possibly allowing a few showers and storms to
develop during the afternoon.




LONG TERM....Rieck
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