Area Forecast Discussion
Issued by NWS La Crosse, WI

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583
FXUS63 KARX 172305
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
605 PM CDT Tue Jul 17 2018

.SHORT TERM...(This evening through Wednesday)
Issued at 313 PM CDT Tue Jul 17 2018

Main forecast concerns in the short-term are on the potential for
patchy fog tonight in some river valley locations.

Surface high pressure edges east of the region late tonight through
Wednesday as a trough approaches from the high plains.   The high
will provide mainly dry and quiet weather across the area at least
through Wednesday. Mostly clear skies and light winds will set
the stage for patchy valley fog, mainly in river valleys. Winds
increase just above the surface through around 1500 feet tonight
which should limit any fog in the main channel of the Mississippi
River. The main fog concerns will be along tributary river valleys
to the Mississippi River. Clouds will gradually increase across
the region from west to east late in the day on Wednesday as the
trough edges into eastern South Dakota/Nebraska.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 313 PM CDT Tue Jul 17 2018

Main forecast concerns are on the potential for heavy rainfall
Wednesday night through Thursday.

The trough is expected to push into the Upper Mississippi River
Valley on Thursday with surface low pressure deepening.  Moisture
will be pumped northward into the area ahead of this low with
precipitable water values increasing to around 1.8 inches with
warm cloud depth of 3.5 km. Most of the CAPE will be confined to
areas south of Interstate 90, potentially only portions of
northeast Iowa and far southwest Wisconsin. This will limit the
potential for any stronger storms as far as wind and hail go, with
the primary threat being heavier rains. The bulk of the rain
should spread from west to east across the area after 12Z Thursday
with rain then persisting through much of the day, with scattered
embedded thunderstorms. Any storms could produce locally heavy
rainfall in addition to the ongoing rains. Some differences noted
in the forecast models on the low track, especially with respect
to the 12Z NAM, which takes the low southeast into northern
Illinois. This is the only model doing this at this time so
treating it as an outlier. The ECMWF, GFS, and Canadian take the
low into eastern/northeast Wisconsin Thursday night into Friday
then stalls. The low then sits over this area into Saturday, and
then the big question is how far west will rain showers occur. The
showers could continue into Saturday for areas along and east of
the Mississippi River. High pressure should then finally build in
Saturday night into Sunday, but with the cutoff low in the area,
there is the potential for rain showers to linger, especially over
central Wisconsin. By the time the rain ends, totals of 1.5 to 2
inches are expected. The widespread cloud cover and rain over the
region on Thursday will result in cooler high temperatures in the
upper 60s to lower 70s.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 605 PM CDT Tue Jul 17 2018

Quiet VFR conditions expected through Wednesday afternoon. An
area of high pressure centered over Lake Superior will remain
nearly stationary and keep skies clear with light east to
southeast winds. Some concern for valley fog overnight with the
light winds. Forecast soundings from the 17.21Z RAP would suggest
it will be too dry tonight for fog to form as it does not show
saturation occurring at the surface. However the 17.18Z NAM
forecast soundings would be supportive of valley fog with a
visibility reduction at KLSE. For now, will maintain continuity
with the previous forecast and not include any fog at KLSE but
will monitor trends through the evening for possible inclusion
with later forecasts.

&&

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

$$

SHORT TERM...Wetenkamp
LONG TERM...Wetenkamp
AVIATION...04



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