Area Forecast Discussion
Issued by NWS La Crosse, WI

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258
FXUS63 KARX 170815
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
315 AM CDT Sun Sep 17 2017

.SHORT TERM...(Today and tonight)
Issued at 313 AM CDT Sun Sep 17 2017

At 2 AM, a cold front extended from Ontonagan Michigan (western
Upper Michigan) southeast to Prairie du Chien Wisconsin.  Much of
the precipitation associated with this front is either located along
or behind this front. With 0-1 km mean layer CAPE up to 700 J/kg,
there is enough instability for isolated to scattered storms.  The
latest MRMS QPE are showing hourly rainfall rates up to a half inch
an hour.

As the 850 mb moisture transport shifts off to the east early this
morning, these showers and storms will move out of the area.  The
meso models are in good agreement that the rain will move east into
eastern Wisconsin by 17.13z.  This matches up well with the western
edge of the rain on radar.

The combination of subsidence and drier air moving into the region
will cause skies to clear from the west this morning and the skies
will remain sunny this afternoon.  With soils still very dry at La
Crosse bumped up their temperatures up a couple of degrees above MOS
guidance.


.LONG TERM...(Monday through Saturday)
Issued at 313 AM CDT Sun Sep 17 2017

Monday continues to trend in the wetter direction as shortwave
trough energy currently over CA/NV ejects toward the area. Trough
looks to deepen some en route, possibly with convective
enhancement in CO later today/evening. By Monday afternoon, juicy
1.5"+ precipitable water air mass is positioned over eastern IA
/this is the air mass that was over us yesterday/ with moisture
transport converging into the southern forecast area. Possibly
even some enhancement from long, right-entrance region of fairly
straight-line upper-level jet to the northwest. At this time,
have increased west/south of La Crosse Monday afternoon and
evening. If signals continue, rain chances will need to be raised,
possibly further north depending on shortwave trough strength.
Some low instability /<500 J/Kg/ warranted TSRA south of I-90.

Moist air mass remains over IA Tuesday as longwave trough deepens
over the western U.S. before lowering heights in the Dakotas,
ranking up the southerly flow across the region. This will try to
activate the warm front with TSRA as it lifts northward Tuesday
night. But, the best low-level moisture transport is off to the
west /Dakotas/, where severe weather looks likely. This area is
on the southeast edge of that warm frontal forcing for elevated
storms with possibly 1000-1500J/Kg of elevated CAPE available.
The big question is can it initiate by saturating. Will have to
monitor this evolution in the coming days for precipitation
chance increases. I suppose we couldnt rule out a pulse hail
storm, but wind shear is poor.

The shortwave trough then ejects through the Dakotas/MN on
Wednesday driving a cold front through the area. Because the
shortwave trough ejects to the northwest of the area, forcing
looks meager locally, but CAPE could be appreciable in 1.5-1.8"
precipitable water air mass. The best wind shear appears to be
further northwest as well, but too early to rule out any severe
storms.

Bottom line: rain chances are out there for our dry area!

Thursday through Sunday...Generally a period of ridge building with
active weather to our west...possibly Maddox synoptic type flooding
events with a stationary boundary over the Dakotas/MN. All the
talk will likely be about hurricane Maria`s impacts in the
Caribbean which do not look good.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1150 PM CDT Sat Sep 16 2017

Batch of showers and thunderstorms continues to work across the
region, but has struggled thus far to make too much northward
progress. Still think RST will see some showers and likely a few
rumbles of thunder through 08Z, while LSE does the same through
about 09Z or so, before activity quickly shifts east of the area.
As envisioned, lower stratus of MVFR to even briefly IFR levels
continues to overspread the area, but it too will quickly exit
the terminal sites before sunrise as much drier air sweeps in from
the west. That drier air should set up increasing sunshine for
much of Sunday, with northwest winds averaging 10 knots up through
sunset.


&&

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

$$

SHORT TERM...Boyne
LONG TERM...Baumgardt
AVIATION...Lawrence



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