Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 251140

Area Forecast Discussion
National Weather Service La Crosse WI
640 AM 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 12Z TAFS through 12Z Friday morning)
Issued at 640 AM CDT Thu May 25 2017

IFR/MVFR stratus and valley fog with visibility reductions to 1SM
or less this morning at KLSE will lift and clear out by 25.13Z to
25.14Z. Thereafter, expect partly to mostly sunny skies at both
TAF airfields through the afternoon with increasing mid-level
cloud later this evening and overnight. There is a 20 to 30
percent chance for rain showers late this evening and overnight,
but timing/coverage uncertainty precludes mention in 12Z TAFs.
With high pressure overhead, winds will be light from the south-
southeast through the period.


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




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