Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 202333

Area Forecast Discussion
National Weather Service La Crosse WI
533 PM CST Mon Nov 20 2017

.SHORT TERM...(Tonight through Tuesday)
Issued at 215 PM CST Mon Nov 20 2017

Models have been in good agreement with taking an upper level
shortwave trough (currently churning across southern Canada)
southeast across the Upper Mississippi River Valley tonight through
Tue. Accompanying sfc cold front will slide across the local area
overnight, exiting southeast by 12z Tue. Not a lot of forcing with
the boundary, with most of the models` qpf tied to the shortwave and
deeper saturation in the north. That said, with strong low level
cold air advection post the front, low level lapse rates favor
cu/shower potential - if there would be enough saturation to play
with. Latest bufkit soundings suggest there isn`t. Will keep the
area "dry" for now - but monitor the passage of the front late
tonight and any percolation for Tue.

While the surge of cold likely won`t lead to shower activity, the
steepening lapse rates will promote mixing and enhanced wind gusts.
Couple this with a tightening sfc pressure gradient tonight through
Tue morning and it looks to be a blustery period for the local area.
Wind gusts upwards 35 mph possible in the open/unsheltered areas.
With very little daytime bump in the temps from the morning lows
Tue, wind chills in the teens will be common. Don`t see a need for a
wind advisory at this time.

.LONG TERM...(Wednesday through Monday)
Issued at 215 PM CST Mon Nov 20 2017

Start of the long Thanksgiving weekend will be rather nice...the
end? Not so much.

Shortwave upper level ridging and a southerly fetch will usher in
some seasonable to mild air for Thu and Fri. 850 mb temps will
rebound from the chilly -12C at 18z Tue to +14C by 18z Fri. NAEFS
850 mb temp anomalies are +1/+2.  Fri should see highs in the 50s -
but if the EC is correct, a few locations will flirt with 60. Going
to hold with consensus blend for temps for the moment. Too many
questions with cloud cover/frontal passage.

While still a few differences in strength, etc...the GFS and EC
coming into better agreement (at least within the last couple runs)
with a shortwave trough/sfc front combo slated to drive across the
region Friday afternoon/evening. Both dig the trough across northern
parts of the region, but extend the upper level forcing southward to
the local forecast area - working with the sfc front that slides
through by 00z Sat. Some aid in the lift from left exit region of
the upper level jet also looking likely. The deeper saturation is
still holding to the north, but models do suggest some low level
moisture transport ahead of the front - helping with pcpn potential
in the south. As it stands, trends in the GFS and EC suggest
increasing chances for showers (rain) for Friday afternoon, mostly
north of I-90. The bulk of the system`s qpf still well north.

Cold air drops southward across the Upper Mississippi River Valley
for Sat and Sun - post the departing shortwave trough. 850mb temps
tumble back to -10C by 18z Sat and highs in the 30s for most Sat/Sun
are expected. Looks a little blustery on Sat too - so wind chills
will add bit to the air.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 533 PM CST Mon Nov 20 2017

The winds will be the major concern during the entire period. An
area of low pressure over southwest Ontario will move east into
Quebec through the period. A pre-frontal trough will pass through
the area overnight swinging the winds around to the west. This
will be followed by the cold front late tonight when the winds
will shift to the northwest and become quite gusty. Good low level
lapse rates behind the front will allow gusts into the upper 20s
to lower 30s knot range to be common for much of the day after
sunrise Tuesday. The 20.18Z NAM and 20.22Z RAP are both showing a
band of higher moisture behind the front at 850 mb that will swing
across the area Tuesday morning. Have included this with a period
of lower VFR ceilings for both airports but will have to watch
this for a possible period of MVFR ceilings.




LONG TERM....Rieck
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