Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 140459

Area Forecast Discussion
National Weather Service La Crosse WI
1100 PM CST Mon Nov 13 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 312 PM CST Mon Nov 13 2017

At 3 PM, southerly winds were advecting warm air into the
Upper Mississippi River Valley. The coldest temperatures (mid-30s)
were found in north-central Wisconsin (due to snow cover) and
southwest Wisconsin (due to dense fog earlier in the day).
Elsewhere, temperatures ranged from the upper 30s to mid-40s.

The 285K isentropic surface shows that the lift and moisture
transport will increasing as warm air advection develops across
the region. The HRRR, NAM, and RAP soundings show that the low
level moisture will increase into the 1 to 2 km range by late
evening and then continue into Tuesday. As this occurs, drizzle
will develop. This should occur along and west of the Mississippi
River between 8 PM and midnight. Temperatures will initially fall
into the mid-30s and then slowly rise. As a result, not seeing any
threat for any icing threat.

Meanwhile, in central and north-central Wisconsin, the drizzle
will be develop between midnight and 6 AM. Temperatures at this
time will be likely near 30 with dew points in the upper 20s. This
provide up to a 3 hour period in which freezing drizzle may occur
before the temperatures and dew points rise above freezing; thus,
ending the icing threat. The highest potential for any icing will
be in Clark, Jackson, and Taylor counties. Thought about issuing
a Winter Weather Advisory for this freezing drizzle threat, but
the threat is such a short period decided to go with a Special
Weather Statement instead and monitor how things evolve.

Besides the freezing drizzle threat, another potential threat will
be the development of dense fog. With the SREF not hitting this
hard; thus, confidence was not high enough to add it to the
forecast at this time. This will just be another thing that we
will have to monitor tonight.

The final concern for tonight is whether we are too cold with the
low temperatures along and west of the Mississippi River. Dew
points have been slowly increasing to our south this afternoon and
these higher dew points along with LIFR/IFR clouds will be moving
into this area during the evening. This will limit the cooling.
As a result, expect that our low temperatures will occur this
evening and then they will slowly rise through the remainder of
the night.

On Tuesday, the drizzle will continue as synoptic lift continues
to produce omega in the saturated low levels of the atmosphere.

.LONG TERM...(Tuesday night through Monday)
Issued at 312 PM CST Mon Nov 13 2017

From late Thursday night into Saturday, the 13.12z models
continue to struggle with evolution of a low pressure system that
will be affecting the Upper Mississippi River Valley. The GFS is
faster with its southern stream short wave; thus, more phasing
occurs. As it result it is the fastest at developing its surface
low, producing more QPF, and bringing cold air into the region.
Meanwhile, the Canadian and ECMWF do not have these two waves
phasing until it is well to our southeast. This result in the
colder air moving into the region slower and lighter QPF.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1100 PM CST Mon Nov 13 2017

Cigs/Vsby/WX: low level saturation continues to increase across the
region, with responding cigs from ifr to lifr. These have moved in,
and should hold through Tue night. Fog is accompanying the lower
cigs - generally MVFR. That said, as the saturation deepens it will
start to touch low level warm air advection/isentropic
upglide...resulting in areas of drizzle. Deeper cloud mass suggests
a change over to mostly rain or a ra/dz mix by Tue evening...but
will leave dz for simplicity for now. Mostly 1-3SM vsbys with the
pcpn. This too should hold across the region into Tue night.

Wind: will stay southerly through tomorrow, taking a turn to the
northwest with a frontal passage late Tue night.




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