Area Forecast Discussion
Issued by NWS La Crosse, WI

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399
FXUS63 KARX 251727
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1227 PM CDT Thu May 25 2017

.SHORT TERM...(Today through Friday night)
Issued at 320 AM CDT Thu May 25 2017

Upper ridging has been building eastward into the Mississippi
Valley overnight with deep subsidence through the atmosphere. Some
shallow moisture has lingered with patches of cloud cover. Light
winds within the surface ridge axis has resulted in some patchy
fog where skies have cleared so far this morning. So far, more
clearing has occurred west of the Mississippi. Where skies do
remain clear early this morning, will have to monitor to see how
widespread and dense the fog becomes, given favorable conditions
for fog formation.

After any fog dissipates this morning, a very pleasant day is
expected with light winds as the ridge axis slides east. Temps
will warm back up into the upper 60s to mid 70s.

Behind the upper shortwave ridge, the upper flow will become west
to southwest by tonight. As shortwave energy approaches, 850-700
mb warm advection will increase along with increasing deep layer
moisture (precipitable water values rising to around an inch). NAM
soundings show much of the moistening near or above 700 mb. A
band of high-based showers/sprinkles are possible overnight into
Friday morning before the weak shortwaves move off to the east. A
weak surface boundary will approach from the west later on Friday
with a narrow instability with perhaps around 1000 J/kg by
afternoon into eastern Minnesota/far western Wisconsin. A few
storms could fire near the boundary during the afternoon, possibly
supported by a weak shortwave tracking across Minnesota. However,
given the weak upper forcing, any showers/storms would be
dissipating by early evening.

.LONG TERM...(Saturday through Wednesday)
Issued at 320 AM CDT Thu May 25 2017

Southwest flow aloft will continue on Saturday with 25.00Z models
suggesting a few embedded upper shortwaves crossing the
Mississippi Valley, while a surface low and primary frontal
boundary of interest sets up south of the area. With some
uncertainty with timing/placement of shortwaves and the impact of
convection to the south on the evolution of the regional
environment, confidence is rather low. The 25.00Z ECMWF continues
to indicate a mainly dry day with most of the precip well south,
while the GFS is a little farther north bringing a mid-level
deformation zone into the area with some showers/isolated thunder.
For now will continue to carry low precip chances for Saturday
afternoon, but it could end up being a dry day.

By Sunday an upper low will begin to dig into the Upper Midwest.
This feature will dominate the regional weather pattern through
the early to middle part of next week. Some differences have
remained among the models with the amplitude of the upper trough
and the strength/timing of various shortwaves rotating around the
upper low. The ECMWF being slightly slower bringing the upper
southward would mean a drier day yet on Sunday compared with the
GFS. Otherwise, a trend towards cooler temps is expected early
next week with periodic showers under the upper trough and
potentially a few diurnally driven thunderstorms under the cold
pool aloft. By the middle of next week, the upper low will begin
to move east with dry weather returning.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1227 PM CDT Thu May 25 2017

Main taf concerns is the possibility of MVFR ceiling conditions
late in the taf period. Weak surface ridge will build into
Wisconsin this afternoon and allow for VFR conditions to prevail
at both RST/LSE taf sites. A weak surface front is expected to
move into western Wisconsin by 18z Friday. The 25.12z GFS/NAM show
weak lift and moisture convergence in association with the
surface front. The combination of weak lift/moisture
convergence...may produce isolated showers across the area after
00z Friday. At this time will leave shower mention out of both
tafs. VFR conditions will continue to prevail...as ceiling
heights are expected to be above 10000 feet through 15z Friday.
After 15z... ceilings will lower into the MVFR conditions. Will
introduce MVFR conditions after 14z Friday at both RST/LSE taf
sites.

&&

.HYDROLOGY...
Issued at 320 AM CDT Thu May 25 2017

Runoff from recent heavy rainfall will continue to cause minor
flooding along portions of the Mississippi River into next week
and also the Trempealeau River at Dodge through today. Be sure to
monitor river levels closely if you have any plans or interests
along these rivers. You can access this information on our website
at weather.gov/arx.

&&

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

$$

SHORT TERM...JM
LONG TERM...JM
AVIATION...DTJ
HYDROLOGY...JM



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