Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 121136

Area Forecast Discussion
National Weather Service La Crosse WI
636 AM CDT Thu Oct 12 2017

.SHORT TERM...(Today and tonight)
Issued at 339 AM CDT Thu Oct 12 2017

Stratus continues across the entire forecast area early this
morning. There have been a few sites that reported drizzle,
including here at the office, but it appears rather patchy. Also
seeing some reductions in visibility due to mist, but nothing yet
below 2SM. Will continue to see patchy drizzle/fog through sunrise,
then gradually dissipating by mid-morning.

For the rest of today, the main challenge will be timing the
clearing from west to east. As the surface ridge axis moves into the
southern Great Lakes region by mid-day, flow aloft remains from the
south. Extensive upstream cloud cover as far south as southern MO
into northern TN ensures that most of the day will remain cloudy.
It`s not until late this afternoon/evening, that a surface trough
moves from the Dakotas into western MN, providing the kick necessary
to start pushing the stratus eastward. Problem being is that there
is additional low-mid level cloud cover with the approaching trough,
so any clearing may be short-lived or even non existent, especially
for areas along and east of the MS River.

All said, today will be dry after this morning`s patchy drizzle, but
overcast skies through most if not the entire afternoon will keep
temperatures in check with highs only in the upper 50s to lower 60s.
Winds will be from the south, generally between 8 and 15 mph.

The surface trough/cold front moves into the forecast area tonight.
NAM is showing some light QPF mainly north of I-90, but other models
are dry, so will maintain a dry forecast for now. Cloud cover will
keep temperatures elevated with lows in the upper 40s to lower 50s.

.LONG TERM...(Friday through Wednesday)
Issued at 339 AM CDT Thu Oct 12 2017

Impactful weather concerns through the period include rain/thunder
potential Friday into the weekend and possibly some frost by
Sunday night.

The cold front will slide southward on Friday. As mid-level
frontogenesis intensifies, expect showers to increase through the
day and persist Friday night, with 00Z models focusing the higher
chances over the southern half of the area. Instability is
expected to stay mainly south of the region Fri/Fri night, so will
not add any thunder at this point.

Attention then turns to an upper trough that will translate east
out of the Rockies on Saturday helping to induce surface low
pressure in the plains that will track northeastward along the
low-level baroclinic zone. The 12.00Z NAM/Canadian are a bit
slower with the surface low compared with the GFS/ECMWF, which
would impact timing of highest precip chances. However, in
general, expect increasing rain chances spreading northward
Saturday into Saturday evening with strong low-level moisture
transport/warm advection and convergence on the nose of an
increasing 850 mb jet. With anomalous PWATs rising near or above
1.5 inches and MUCAPE up to 500-1000 J/kg working northward,
locally heavy rain and isolated storms are possible, with the
favored area for storms/heavier rains northeast Iowa into
southwest Wisconsin. Despite the weak instability, strong forcing
and deep layer shear could result in a few strong storms. The
system will be progressive, so most of the precip will exit to the
east Saturday night.

The low will move away on Sunday with the faster GFS/ECMWF
consensus suggesting a dry day. However, it will be cool and
breezy, with temps staying in the 50s, and possibly only around
50 in spots. Clearing skies with high pressure building over the
area later Sunday and Sunday night could set the stage for frost
concerns by early Monday morning over parts of the area. Dry
weather under high pressure is expected through the first half of
next week, with highs warming back above average at least into the
60s through mid-week.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 636 AM CDT Thu Oct 12 2017

Primary challenge this period will be cloud coverage and ceilings.
MVFR/IFR ceilings will continue at KLSE/KRST respectively through
this morning with some increase in cloud bases this afternoon,
but will remain below 3000 ft agl. There could be some breaks in
cloud cover with VFR ceilings late this afternoon into the early
evening, but confidence is low given saturated model RH fields,
so will maintain MVFR conditions at this time. A surface trough/
cold front will then move into the region after 13.03Z,
reinforcing low-mid level saturation and keeping at least MVFR
ceilings late this evening into the overnight. Light east-
southeast wind this morning will gradually shift to the south
through the period.




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