Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 130344

Area Forecast Discussion
National Weather Service La Crosse WI
1044 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 06Z TAFS through 06Z Sunday night)
Issued at 1044 PM CDT Sat Aug 12 2017

High clouds continue to filter into the area from the west, and
that trend will continue through sunrise, with clouds gradually
thickening and lowering into Sunday afternoon. An outside risk for
a brief shower or sprinkle will exist, mainly near RST, though
with no impact to VFR conditions. As of now, the risk for fog at
LSE continues to diminish, owing to earlier mixing of dewpoints
into the 40s and clouds a little faster to thicken up overnight.
Will still need to monitor trends the next few hours, but the risk
appears much lower than earlier thought despite light winds
through the lowest 6kft of the atmosphere. Speaking of winds,
surface speeds will remain light through sunrise before winds
shift south to southeast at 8-15 knots into Sunday and Sunday




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