Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 122019

Area Forecast Discussion
National Weather Service La Crosse WI
319 PM CDT Sat Aug 12 2017

.SHORT TERM...(This evening through Sunday)
Issued at 319 PM CDT Sat Aug 12 2017

At 3 PM, a 1020 mb surface high was centered over south-central
Minnesota and north-central Iowa. A scattered 5-7K deck of
diurnally driven cumulus clouds have developed across the region.
The 12.12z models are in good agreement that these clouds will
rapidly dissipate this evening with the loss of daytime heating.

With the clouds not arriving until late, dry dew points, and light
winds, the low temperatures for tonight were lowered. This was
especially the case for central Wisconsin where the temperatures
fell to around 40 degrees this morning. Black River Falls was 11
degrees below their MAV guidance. Since these were nearly the
identical conditions as this morning, it made sense to go that
cold tonight too.

Soundings show that the light winds will extend up 875 mb
initially during the early overnight and then decrease to 925 mb
toward morning. Only the NAM is suggesting any valley fog late
tonight. With this being the case, only expect shallow valley fog
in the Mississippi tributaries, lower Kickapoo, and Wisconsin
river valleys.

On Sunday, a short wave will approach the area from the Northern
Plains. A warm front will lift north across the region. Much of
the lift associated with this front will go into saturating the
layer between 650 and 450 mb. Below this layer, the air mass
remains very dry. As a result, there are questions on whether any
of the rain will reach the ground, so kept the forecast dry in
southwest, central, and west-central Wisconsin.

Meanwhile further west, there is deeper moisture and greater 925
and 850 mb moisture transport. It continues to look like the best
chance for showers and isolated storms will remain along and ahead
of the cold front. As a result, just kept a 20 to 40 percent
chance of rain in parts of southeast Minnesota, northeast Iowa,
and north-central Wisconsin.

.LONG TERM...(Sunday night through Saturday)
Issued at 319 PM CDT Sat Aug 12 2017

On Sunday night and Monday, a short wave trough will move through
the region. With the best moisture transport remaining near the
cold front, much of the area will likely remain dry until the cold
front moves through the region. With limited instability, kept the
thunderstorms isolated to scattered. Rainfall amounts will likely
only be up to a quarter inch at most.

From Tuesday afternoon into Wednesday night, another short wave
trough will move through the region. Like the past couple of days,
the GFS continues to have much greater instability and shear than
the ECMWF. As a result, confidence remains very low whether there
will be any severe weather with this system on Wednesday.


.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.




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