Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 132113

Area Forecast Discussion
National Weather Service La Crosse WI
313 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 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1154 AM CST Mon Nov 13 2017

As the 925 mb and 850 mb moisture transport increases late this
afternoon and tonight, moisture from the Central Plains will surge
northward into the region. Ceilings will quickly become IFR and
LIFR tonight and then continue into Tuesday morning. In addition,
there will be fog advecting into the area. As drizzle develops,
visibilities will drop even further. This should occur at KRST
around 14.00z and KLSE around 14.03z. With temperatures above
freezing, there is no concern with any freezing drizzle at the TAF
sites. However, north of Interstate 94, there will be a concern
of freezing drizzle between midnight and 6 AM.




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