Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 201656

Area Forecast Discussion
National Weather Service La Crosse WI
1056 AM CST Mon Nov 20 2017

.SHORT TERM...(Today and tonight)
Issued at 345 AM CST Mon Nov 20 2017

At 3 AM, a broken mid and high deck of clouds were located over
northeast Iowa and southwest Wisconsin. These clouds were
associated with the right entrance region of a 110 knots 250 mb
jet max. The 20.00z models are in agreement that these clouds will
thin and move to the east of the area this morning.

For this afternoon, warm air advection ahead of an approaching
short wave trough will cause 925 mb temperatures in both the GFS
and ECMWF to warm into the 6 to 9C range. Meanwhile the NAM and
RAP have these temperatures initially and then they cool them into
the 3 to 6C as the omega increases. However, this seems far too
aggressive considering the strength of the winds aloft. As a
result, went more toward the warmer GFS and ECMWF solutions with
high temperatures ranging from the mid 40s to mid 50s. Some high
clouds will move into the area during the mid to late afternoon;
however, these clouds will likely arrive after peak heating and
mechanical mixing will likely mix warm air in from aloft.

After midnight, a Canadian cold front will move southeast through
the forecast area. This will cause the 925 mb temperatures to cool
into -3 to 3C range by sunrise Tuesday. Low temperatures will
cool into the mid 20s to mid 30s.

.LONG TERM...(Tuesday through Sunday)
Issued at 345 AM CST Mon Nov 20 2017

On Tuesday morning, the 20.00z models show a rapid increase in
the 925 to 850 mb moisture. As a result, we will see a rapid
increase in the MVFR clouds across the region. In addition, the
950 to 850 mb lapse rates will increase to around 9C/km. There is
even some small CAPE in the NAM soundings. While the deterministic
models continue to show that this moisture will be likely too
shallow for the development of any snow showers, there were a few
CAM models that suggested that there may be some brief snow
showers mainly north of Interstate 94. Confidence was far too low
to include these in the forecast.

With a tight surface pressure gradient and steep lapse rates
during this time period, the sustained northwest winds will be in
the 15 to 25 mph range and the gusts ranging from 25 to 35 mph.
This will result in wind chills of 10 to 20 degrees.

On Tuesday afternoon, skies will clear rapidly from the northwest.
Despite the clearing skies, 925 mb temperatures of -6 to -9C will
greatly limit any warming. High temperatures will range from the
mid 20s to mid 30s.

On Wednesday afternoon and evening, another short wave trough will
move southeast through the region. This system will bring another
reinforcing shot of cold air to the region. With the air mass very
dry across the region, most of the lift associated with this
system will just go into cloud production.

On Friday, the deterministic models are in agreement that a
strong low pressure area will move east through southern Ontario.
If this does indeed occur, the 925 mb temperatures would warm
likely warm into the 9 to 15C range. This would result in surface
temperatures warming into the 50s. However, with the ECMWF
ensemble temperatures still showing anywhere from 5 to 7 standard
deviations confidence was not quite high enough to go quite that
warm yet. As a result, just went a couple of degrees above the
model blends for now.

On Friday afternoon and evening, the models continue to hint that
there will be deep enough saturation for some precipitation to
occur. Soundings at this time continue to suggest that if this did
occur, it would be in the form of rain.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1056 AM CST Mon Nov 20 2017

Generally VFR conditions expected this forecast with dry airmass in
place. Short wave dropping through region overnight will keep winds
up and produce wind shift during period.

About only potential issue is amount of cold air advection stratus
that will accompany wave and associated front. Some hints of this
upstream at moment and model guidance a bit mixed at how it resolves
this. Closer examination of Bufkit data suggests there could be a
brief period of MVFR to VFR ceilings at some point near or after
frontal passage Tuesday morning so introduced this to small extent.
Will have to be watched in future forecasts, but impact should be




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