Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 242342

Area Forecast Discussion
National Weather Service La Crosse WI
642 PM CDT Sun Sep 24 2017

.SHORT TERM...(This evening through Monday)
Issued at 310 PM CDT Sun Sep 24 2017

Not much change with the overall pattern this afternoon, with
upper level ridging over the eastern half of the CONUS and a
trough to the west. A slow-moving cold front extended from western
Minnesota southwest into western Kansas.

Very warm air remained over the forecast area this afternoon,
with 925 mb temps only 1-2C cooler than Saturday, leading to highs
in the upper 80s to lower 90s once again. There were even some
records tied or broken. Please see the climate section below for
more details on the records at Rochester and La Crosse.

Attention then turns to timing of precipitation and cloud cover
overnight and into Monday. The upper level ridge will begin to
slide off to the east, allowing the cold front and associated
precip/clouds to move into the area. The 24.12Z GFS and ECMWF both
bring precip into the far western portions of the forecast area
during the overnight hours. Both models also show over 1000 J/kg
of MUCAPE, so should see some thunder as the precip moves through.
That said, moisture transport looks to be fairly weak and the
strongest frontogenesis is confined to the 900 mb layer, so
currently thinking showers will decline in coverage as they move
through and thunder will become more embedded in nature. Also, do
not expect any severe weather with this system, as the strong deep
layer shear will remain behind the front.

.LONG TERM...(Monday night through Sunday)
Issued at 310 PM CDT Sun Sep 24 2017

From Monday night into Tuesday morning, a cold front will move
east through the region. It continues to look like that much of
the synoptic forcing will move northwest and north of the area as
the short wave trough moves northeast into Ontario. As a result,
all of forcing occurs below 800 mb. As a result, there is concern
that the rain chances may be too high. With limited instability,
lowered the chances of thunderstorms on Monday night. This was
especially the case over western Wisconsin.

On Tuesday afternoon, the front will move quickly east across
western Wisconsin. With 0-1 km mixed layer CAPES climbing up
250 J/kg, there may be a few isolated storms. With little shear,
no severe weather is anticipated.

With the models in good agreement that the rain will be well east
of the area on Tuesday night. The small rain chances were removed
for this time period.

Thursday through Sunday...

The GFS and ECMWF are in good agreement that an Alberta Clipper will
pass through the region late Thursday into Friday. There are some
timing differences, though. The GFS is more progressive with the
upper-level shortwave, while the ECMWF develops a closed upper
low over Michigan by Friday evening. The slower ECMWF would
prolong rain chances associated with this system through Friday.

The GFS and ECMWF remain in agreement on the evolution of the
synoptic pattern through the weekend, but the GFS continues to be
a slightly more progressive solution. The clipper system is
progged to be followed by brief ridging on Saturday and another
shortwave late Sunday. At the surface, Canadian high pressure will
build in behind the departing clipper, making for pleasant days
and cool nights through the weekend.


.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 642 PM CDT Sun Sep 24 2017

A cold front currently bisecting MN will begin its eastward push
over the next 24 hours. While dry VFR conditions are expected at
KRST/KLSE this evening and overnight, a low MVFR/IFR cloud deck
along and behind the front could get very close to KRST by Monday
morning. For now, will keep cloud bases above 3000 ft agl, but
something to watch closely. Otherwise, should see a general
increase in scattered showers and thunderstorms after 25.12Z, but
especially into the afternoon. Will introduce VCTS/CB at KRST at
25.18Z, but keep KLSE dry at this time. Given the scattered nature
of convection, will need to adjust timing and likely introduce
precipitation at KLSE in later forecasts. Light south-southeast
wind this evening and overnight will gradually turn to the
southwest through the period.


.CLIMATE...Sunday Record Highs
Issued at 310 PM CDT Sun Sep 24 2017

At 1:53 PM, Rochester International Airport climbed to 88F. This
tied the record high for this date set back in 1886 and 1937.

At 2:03 PM, the temperature at La Crosse Regional Airport climbed
to 93F. This is a new record for the date. The previous record was
92F in 1891. This was the third consecutive day in which a record
high has been set in La Crosse.  All 3 records were back in 1891.

It was only the 7th time in La Crosse that we have had 3
consecutive days with highs of 93F or higher after August 31st.
This is the latest in a calendar year that this has occurred. The
other years were 1922 (Sep. 5-7), 1937 (Sep 1-3), 1939 (Sep
13-15), 1955 (Sep 16-18), and 1978 (Sep 6-11 - 6 days).




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