Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 191740
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1240 PM CDT Sat Aug 19 2017

.SHORT TERM...(Today through Monday)
Issued at 226 AM CDT Sat Aug 19 2017

Today still looks to be a pleasant day across the region as
ridging aloft and at the surface builds in over the area. After
some early morning fog, the remainder of the day looks to be
mostly sunny with temperatures topping out in the upper 70s to
lower 80s. The ridging will quickly move past the area tonight and
now some of the models are suggesting some rain could form to the
west of the area and move into very late tonight. The 19.00Z GFS
is the most aggressive with this idea as it spins up an MCV out
convection that forms this evening over the Missouri River Valley.
It then brings this feature across northern Iowa overnight and
combines it with some isentropic up glide on the 305K surface and
the convergent side of the low level moisture transport to push
some showers into the western sections of the forecast area. The
19.00Z NAM and ECMWF do not develop nearly as much activity as the
GFS and what they do have, stays farther to the west. The few hi-
res models that go out that far would support keeping the activity
farther west as well. So, to honor the GFS solution, will just
have a very small rain chance for late tonight across the far
western sections.

There is a bit more of a model consensus of the possibility of
some showers getting into the area Sunday morning from the
activity that forms over the Missouri River Valley. With the low
level moisture transport weakening during the morning, just how
much of this activity can make it into the area is questionable
and will confine the rain chances to west of the Mississippi
River. By Sunday afternoon, the upper level flow will have
transitioned into a zonal pattern with the expectation that a
short wave trough will be coming out of the Dakotas into
Minnesota. This wave should only produce some very weak pv
advection in the 500-300 mb layer across central Minnesota into
northern Wisconsin. The GFS again is the more aggressive as it has
a stronger moisture transport signal ahead of the wave and
develops more activity over the area than either the NAM or ECMWF.
For now, will favor the weaker model solutions and just have a 20
to 30 percent chance across the area.

The model differences continue right into Sunday night as well.
All the models agree that the Sunday afternoon short wave trough
will push a weak cold front into the area that will set up
west/east across the area. Another short wave trough in the zonal
flow should move across the area, but models showing differences
in placement and whether it will be south of the area or move
right over the baroclinic zone of the front. The GFS again has the
most activity over the area as it again is stronger with the
moisture transport into the front than either the NAM or ECMWF.
Will have rain chances across the entire area for Sunday night but
as of now, the model consensus would keep these in the 40 to 50
percent range.

The stalled front should still be over or very near the area for
Monday. As of now, the models would suggest the local area would
be in between short wave troughs as the next short wave trough
will still be out over the Dakotas. There will still be some low
level support for activity along the front. Weak moisture
transport into the front and 1 to 3 ubar/s of up glide on the 305K
surface should be enough to keep some scattered activity going.
Unfortunately, this will also produce plenty of clouds and skies
look to be mostly cloudy to cloudy at the time of the eclipse.

.LONG TERM...(Monday night through Friday)
Issued at 226 AM CDT Sat Aug 19 2017

The highest chances for rain will come Monday night as the short
wave trough over the Dakotas moves across the region. This wave is
expected to produce weak to moderate pv advection in the 500-300
mb layer. An increase in the low level warm advection ahead of the
wave is expected to kick the isentropic up glide up into the 2 to
5 ubar/s range on the 305K surface but the low level moisture
transport looks to be more parallel to the front than into it. The
greatest threat from this system will likely be locally heavy
rains as warm cloud depths will be in the 3.5 to 4 km range both
Sunday and Monday nights with precipitable water values of 1.5 to
1.75 inches. As of now, the threat for severe weather looks rather
low as the deep layer shear stays behind the front and never
overlaps the CAPE pool ahead of the front.

There will be some lingering rain chances into Tuesday until the
whole system finally moves past the area. The rest of the week
then looks to be dry as ridging aloft and at the surface sets up
over the Upper Midwest.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1240 PM CDT Sat Aug 19 2017

Predominantly VFR conditions are expected the next 24 hours across
the area, with the lone exception of possible MVFR ceilings at RST
into mid afternoon with the development of some thicker cumulus
clouds. Increasing flow aloft through the night should preclude a
repeat of widespread river valley fog for LSE tonight, though do
anticipate seeing some fog in the Kickapoo and Wisconsin River
Valleys for an aviation interests there. Returning moisture aloft
should provide some increasing mid level clouds late tonight into
Sunday, with perhaps a brief shower for either LSE or RST Sunday
morning, though expected isolated nature (at best) of any
convection precludes any mention at this time.

&&

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

$$

SHORT TERM...04
LONG TERM...04
AVIATION...Lawrence


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