Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 101800

Area Forecast Discussion
National Weather Service La Crosse WI
100 PM CDT Tue Oct 10 2017

.SHORT TERM...(Today and tonight)
Issued at 332 AM CDT Tue Oct 10 2017

Early this morning, clouds have been slow to clear across the
northern forecast area. The 3 am temperature at Medford was still 40
degrees, although the clearing line was just west across Chippewa
County. Think temperatures across Taylor County will still be able
to touch the mid 30s prior to sunrise, but not for long, thus
limiting frost potential. That said, will keep the Frost Advisory
going through 8 am.

Also watching a weakening band of light rain showers now mainly
south of I-90. These showers will continue to weaken and dissipate
as they drift to the south through sunrise.

For the rest of today and tonight, a 500 hPa closed low currently
over KS will lift northeastward, nearing the IA/IL border by 06Z
Wednesday. The surface low follows a similar path, bringing warm
advective wrap-around showers into the southern forecast area this
afternoon and evening. Main question will be the northern extent of
rain given dry air to the north. Still some differences between the
global/regional models, but latest CAMs would suggest the majority
of showers will remain along/south of I-90. Look for rainfall
amounts ranging from a tenth along the I-90 corridor to 0.75 inches
across far southwest WI.

Cloud cover and rain will keep today cool with highs only in the
50s. Winds will be from the northeast, gusting at times across the
south to 20 mph.

.LONG TERM...(Wednesday through Monday)
Issued at 332 AM CDT Tue Oct 10 2017

The upper low quickly swings east and weakens heading across the
Great Lakes on Wednesday with southwest flow aloft developing in its
wake as longwave troughing overspreads the Pacific Northwest.
Southerly low-level flow will become established through late week
with high temps remaining near to above average for mid to late
week, generally in the 60s, possibly some low 70s by Friday.
Model soundings/RH fields indicate potential for quite a bit of
stratus Wed/Thu along with possibly some light showers on Thursday
with increasing 925-850 mb warm advection. With soundings
indicating most of the moisture confined to below 800 mb on
Thursday and only modest lift, anything that does fall would be
light and may be more drizzly in nature.

For Friday through the weekend, the main focus is on rain chances.
Although models generally agree on overall synoptic evolution,
details remain less clear. An upper shortwave trough kicking out
across southern Canada from the western trough late in the week will
help to strengthen a low-level baroclinic zone over the region, with
rain chances increasing later Friday and Friday night as the
frontal zone sinks south across the area. Then, as a stronger
shortwave trough within the broader longwave ejects eastward later
in the weekend, a surface low is expected to lift northeastward
along the baroclinic zone. Questions remain on the placement of
the baroclinic zone, and especially the timing of the ejecting
trough from the west, impacting where/when the best rain chances
will be. The faster 10.00Z GFS would have much of the rain out of
the area by Sunday, while the 10.00Z ECMWF is quite a bit
stronger and slower with the surface low, keeping rain chances
through Sunday. Plenty of moisture will be advected northward
ahead of the cyclone, with potential for widespread rainfall, but
overall model variability still limits confidence on exact
track/timing. Given the differences, stuck with consensus PoPs,
with the highest chances Saturday afternoon and night. Dry weather
will return behind the system with high pressure. Temps overall
should remain near average for the weekend.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1259 PM CDT Tue Oct 10 2017

Low pressure passing through IA/northern IL will continue to spread
pockets of rain northward into the area tonight, mainly south of I-
90. It looks like KRST may be missed by the rain but KLSE may be in
and out of -shra. Otherwise, mainly VFR conditions expected into
this evening, but will have to watch for some MVFR cloud cover at
KLSE after midnight and at KRST toward sunrise.




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