Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 220440

Area Forecast Discussion
National Weather Service La Crosse WI
1140 PM CDT Sat Oct 21 2017

.SHORT TERM...(This evening through Sunday)
Issued at 230 PM CDT Sat Oct 21 2017

Water vapor satellite this afternoon shows the mean long wave trough
has progressed through the Rockies. The trough and embedded short
wave troughs will continue to work east through the night and then
start to shear out Sunday as a closed upper level low looks to take
shape across the lower Mississippi River Valley. This trough and
embedded short wave troughs will provide the necessary lift for
showers and storms along the cold front. Out ahead of the front,
there is a strong cap in place above 800 mb and the 21.17Z RAP and
21.12Z NAM forecast soundings continue to show this cap will remain
in place until the front arrives this evening. This will provide a
very narrow window where there will be enough forcing and CAPE to
support thunderstorms right along the front. Forecast soundings from
the RAP show between 500 and 750 J/Kg of CAPE along the front in a
tall, skinny profile with deep layer shear of 30 to 40 knots, most
of which resides in the lowest 3 km. The main concern continues to
be if this CAPE profile will be enough to balance out the shear and
allow some stronger to potentially severe storms to form. If it
does, strong to damaging winds continue to look to be the main
threat and in that regards, no real change to current thinking or
messaging. The storms should also be efficient rain producers with
precipitable waters expected to be in the 1.5 to 1.75 inch range and
warm cloud depths around 3.5 km. Expecting the front with the
showers and storms to start moving into the western sections of the
area right around 22.00Z and then move steadily east across the area
through the night. The threat for strong to severe storms should
diminish by late evening as the CAPE becomes minimal. Lingering
showers behind the front Sunday morning, mainly over Wisconsin, will
come to an end by early afternoon. The clouds will take a bit longer
to clear but look for skies to become mostly sunny starting in the
morning across the west and during the afternoon for the eastern

.LONG TERM...(Sunday night through Saturday)
Issued at 230 PM CDT Sat Oct 21 2017

Much cooler weather still expected for this coming week. Overall,
little change made to the previous forecast, although forecast
confidence erodes heading through the middle and latter stages of
the week.

There will be one more mild/seasonable day Monday ahead of an
upper trough that will dig southeastward towards the Great Lakes
and drive a strong cold front through the area on Monday.
Following the frontal passage, temps will fall below average on
Tuesday with highs in the 40s to near 50. As the cold air aloft
and steep low-level lapse rates interact with weak ripples within
the upper trough, some showers are possible especially Monday
night into Tuesday, with otherwise mostly cloudy, breezy and
chilly conditions. Some differences exist Tuesday night into
Wednesday with the 21.12Z GFS a little farther southwest with a
reinforcing shortwave trough in northwest flow compared with the
ECMWF, bringing light precip and increased clouds to the area.
This could ultimately impact low temps and possible frost concerns
Wednesday morning.

Short-lived shortwave ridging is expected on Thursday with return
southerly flow helping temps moderate to near or slightly above
seasonal norms ahead of low pressure developing to the west.

Beyond Thursday, confidence in forecast details remains lower,
especially regarding any precip chances. Deterministic and
ensemble guidance both support another upper trough moving into
the central US late this week, a little farther west than the
early week trough. A reinforcing surge of cooler, below average
temps is expected for Friday/Saturday (850 mb temps falling to -6
to -9C by Saturday), with another chance for a widespread
frost/freeze by late this week. However, run-to-run consistency
among models has been poor during this time with the evolution of
the upper trough, so stuck with model consensus pops late this
week for now. It`s possible it could be cold enough for some
snowflakes by late this week with any precip.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1140 PM CDT Sat Oct 21 2017

A cold front continues to gradually work through the region, and
should clear LSE by 06Z. Earlier band of thunderstorms along that
front has pushed east, but a larger axis of rain will continue to
impact LSE for a few more hours, while RST should now be clear of
the rain (can`t rule out a brief shower through 07Z). Current MVFR
ceilings at LSE will improve to VFR by 07Z, further rising through
the coming day for most areas as drier air works into the region.

With that said, rain does look to hang on much longer east of LSE
through at least midday, and it is possible that some showers will
linger at that site longer than currently indicated. Should that
occur, however, ceilings will still remain VFR for LSE proper.




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