Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 161703
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1203 PM CDT Sat Sep 16 2017

.SHORT TERM...(Today through Sunday)
Issued at 212 AM CDT Sat Sep 16 2017

Somewhat complex setup for shower/storm chances today-tonight, with
a lot of small scale features to contend with - aside from the
front/trough combo for tonight.

Currently, some high based showers/storms over northeast WI reacting
to weak upper level shortwave tracking across the region. Meanwhile
a north-south running thin line of showers/storms has sparked just
to the west, which most of the meso models would hold together long
enough to reach the Mississippi river. Forcing here would seem to be
the edge of the instability/moisture axis.

Current watervapor satellite imagery places an upper level shortwave
across the northern plains, with the various models shifting it
northeast through the day, into southern Canada by Sunday morning.
The trough`s associated north-south running cold front moves in
tonight, gradually making its way into eastern WI Sunday morning.
Most of the forcing with the boundary is post the sfc reflection,
and thus the models all develop showers/storms mostly west of the
front rather then along/ahead of it. A consistent signal for the
past few days. The GFS and NAM point to upwards of 1000 J/kg or so
of MUCAPE to play with, even post the front. The deeper, stronger
shear is even farther west of the front, but enough overlap with the
instability where a few strong (severe?) storms will be possible
early on - mostly this evening.

Later tonight, another bit of upper level energy is progged to shift
northeast across eastern IA, then across the Great Lakes. This
feature will/should result in the convection becoming more focused
along southern parts of the sfc front overnight. Locally, that puts
northeast IA/and southwest WI in the more favorable shower/storm
areas.

Much cooler, more seasonable air returns Sunday-Monday.

.LONG TERM...(Monday through Friday)
Issued at 212 AM CDT Sat Sep 16 2017

Models all in pretty good agreement now with lifting a ripple in the
upper level flow from the southern plains to across the region
mon/mon night. The NAM takes more of a northern track, while the the
EC and GFS a bit more south. The Canadian is in between. The
differences would generally favor rain either north, or south of I-
90. Leaning more on the southern path, but without a clear favorite,
will let consensus drive the ship for now.

The swing from a dry to more unsettled weather pattern looks to
persist for the better part of the new work week. The upper level
ridge looks to re-establish itself over the eastern half of the u.s.
while a west coast trough continues to carve itself out, looking
more energetic and amplified compared to previous model runs. Both
the GFS and EC more or less lock this positioning into the weekend -
thanks mostly to hurricane Jose off the east coast holding any
movement east at bay. This puts the Upper Mississippi River valley
under the gun for ejecting bits of shortwave energy to churn across
the area, acting on what should be a nearly quasi-stationary north-
south running sfc front. This setup would result in repeated rounds
of showers/storms for the later half of the week. No guarantees that
the upper level pattern won`t shift a bit (again, Jose will have
some day here) - which could move this favored rain path west or
east. See some threat for stronger storms too.

Temperatures still looking to be 5 to 10 degrees above the mid Sep
normals.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1203 PM CDT Sat Sep 16 2017

Late this morning, the cold front extended from a weak area of low
pressure over extreme east central Minnesota into western Iowa. A
few showers and storms continue ahead of the front over northern
Iowa into southeast Minnesota that were moving northeast. Barring
any additional development, this activity looks to slide east of
KRST and west of KLSE so do not plan to have any rain in at the
start of either forecast. The hi-res models show that convection
will continue to burble ahead of the front during the afternoon,
but with this being scattered in nature, hard to pinpoint a time
when either site may be impacted and will update the forecasts as
needed. More widespread activity is expected this evening into the
early overnight as the front moves through with good isentropic up
glide expected on the 305K isentropic surface. Will continue to
include the categorical rain for both airports and add a period of
thunderstorms with MVFR conditions until the front moves through.
Behind the front, enough low level moisture should linger to keep
the MVFR ceilings in place until early Sunday morning when these
should lift to VFR as drier and cooler air starts to move in.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...Rieck
LONG TERM....Rieck
AVIATION...04



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