Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 211930

Area Forecast Discussion
National Weather Service La Crosse WI
230 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 18Z TAFS through 18Z Sunday afternoon)
Issued at 1210 PM CDT Sat Oct 21 2017

The first round of showers and storms has moved north of both
airports with regional radars showing nothing downstream at this
time. The cold front was over the eastern Dakotas and Nebraska
late this morning and it continues to look like there will not be
anymore convection until the front arrives. This is still expected
to move across both airports this evening with a period of
showers and a couple hour window for some storms. MVFR ceilings
expected to persist ahead of the front, although there could be
some periods of VFR conditions. Primarily MVFR conditions expected
as the front, showers and storms move through but the visibility
could briefly drop to IFR in the stronger storms. Once the front
moves through the winds will swing around to the west with the
MVFR ceilings remaining until late tonight when these will lift
to VFR. Expect the clouds to scatter out through Sunday morning as
drier air spread in behind the front.




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