Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 121650
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1150 AM CDT Sat Aug 12 2017

.SHORT TERM...(Today through Tuesday)
Issued at 210 AM CDT Sat Aug 12 2017

Weekend shaping up to be quite a nice one as high pressure remains
the main weather influence for the region. Ample sunshine today, but
some high/mid level clouds expected to encroach from the west later
tonight, and could temper/filter some of that sunshine for Sunday.
If it stays mostly clear tonight, setup favors widespread valley
fog.

The GFS and NAM have been in good agreement with taking a shortwave
trough currently churning over MT east/southeast today, gradually
sliding it across the local area Sun afternoon/night. Not much
instability or wind shear to play with. A little low level moisture
transport to help fuel rain/storm chances. The deeper saturation
holds off until late Sunday/Monday - and the bulk of the
shower/storm chances could hold off until then.

.LONG TERM...(Wednesday through Friday)
Issued at 210 AM CDT Fri Aug 11 2017

Models remain in good agreement with a mid week system, taking an
upper level shortwave trough from the pac nw eastward, sliding it
across portions of the Upper Mississippi River Valley late Wed
through Thu (potentially into Friday). Previous runs of the GFS and
EC both suggested the shortwave would track across the U.S/Canadian
border, with the bulk of the upper level energy holding there. It`s
associated sfc front, coupled with low level moisture return and
some instability would have the most influence on rain chances
locally. Latest GFS run is farther south with the main piece of
upper level energy, resulting in greater qpf and higher threat for
showers/storms in the south. Both gradually shift the system into
the eastern great lakes. The previous EC run was slower, lingering
pcpn into Fri, but more on pace with the GFS now. However, the GFS
would bring another shortwave down across the region Friday. The EC
shows something similar, but weaker and faster. The GFS paints a
good shot for another round of showers/storms...the EC favors dry.
Feel good about the rain threat for late Wed into Thu, shakier with
the details for Friday. Don`t see much of a severe potential at this
time - some instability but weak wind shear.

Both the GFS and EC do hold the promise of high pressure for next
weekend.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1149 AM CDT Sat Aug 12 2017

The main concern is whether valley fog will form overnight and
produce MVFR/IFR conditions for KLSE. The center of the surface
high pressure will move from over Minnesota and Iowa southeast to
over Illinois by 13.12Z. With the high drifting overhead, there
will be an extended period of clear skies and light winds.
However, concerned that saturation may not occur despite this
favorable environment. With the mixing this afternoon, expecting
the temp/dew point spread to be around 30 degrees which is right
on the edge of almost being too dry for valley fog to form. The
12.12Z NAM forecast soundings do show surface saturation briefly
occurring late tonight while the 12.15Z RAP does not. With the
high temp/dew point spread this afternoon and lack of saturation
in the RAP sounding, confidence not high that valley fog will
impact KLSE tonight and plan to stay with the BCFG that is already
in the forecast. A short wave trough will be slowly approaching
from the northwest Sunday morning that will spread mid level
clouds over the area with VFR ceilings developing from late
morning through the rest of the day. Any showers with this system
look to remain west of the area through Sunday morning and no
plans to include any rain mention.

&&

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

$$

SHORT TERM...Rieck
LONG TERM....Rieck
AVIATION...04



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