Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 180014

Area Forecast Discussion
National Weather Service La Crosse WI
714 PM CDT Mon Oct 17 2016

...Severe Weather Potential Quickly Diminishing...

.UPDATE...This evening
Issued at 714 PM CDT Mon Oct 17 2016

The threat for severe storms across north central Wisconsin is
really starting to diminish this evening. The main threats are
quickly turning to brief heavy rain, lightning and perhaps some
wind gusts to 30 mph. 0-1 km shear is rather strong into Clark and
Taylor Counties but now that we are really starting to lose
surface bases CAPE and CIN is building thinking the tornado
potential has passed.


.SHORT TERM...(This evening through Thursday)
Issued at 215 PM CDT Mon Oct 17 2016

The main concern in this period is with the rain chances tonight.
Water vapor satellite this afternoon shows a short wave trough
coming across Wyoming into Nebraska. This wave will continue to
move east/northeast tonight and should reach northeast Wisconsin
into the eastern part of the U.P. of Michigan by early Tuesday
morning. The wave does come through with a bit of a positive tilt
which will keep the best pv advection on the poleward side with
just some weak pv advection in the 500-300 mb layer coming across
the northern sections of the forecast area. A strong upper level
jet will come through with the short wave trough, but both the
17.12Z NAM and several runs of the RAP today indicate that most of
the area will be under the right exit region of this jet and the
subsidence associated with it.

At the surface, a low was over the Missouri River Valley near
KSUX. A cold front extended northeast from the low into northern
Wisconsin. This surface low will continue to deepen through the
day and move rapidly northeast along the cold front reaching Lake
Superior by 06Z. Visible satellite is showing an extensive low
cloud shield out ahead of the surface low in the warm sector and
even if this does start to break up at this point, there will not
be enough hours of heating to significantly add to the CAPE.
Despite this, with the unusually warm and moist airmass already in
place, ML CAPES values are in the 1000-1500 J/kg range ahead of
the cold front. Looking at the latest RAP soundings for KEAU and
KRCX, the cap in place gets weakened but may not get totally
eliminated. If this is the case, then the threat for getting
surface based storms appears to be slim, pretty much ruling out a
tornado threat. The MU CAPE does extend farther back to the
northwest into the cool sector which could still be enough to
support a large hail and damaging wind threat. The timing of any
severe threat still looks to be from late this afternoon into
early this evening before the surface low moves into the area and
pushes the CAPE out of the area. The latest sets of hi- res meso
scale models all seem to have a good handle on how things should
develop through the afternoon into the evening with the best
chances for storms generally remaining just to the northwest of
the forecast area into Clark and Taylor Counties. Plan to show a
60 to 80 percent rain chance for Clark and Taylor and then quickly
drop this down to less than a 15 percent south of Interstate 90.
This should then lift out to the northeast through the night with
the entire area expected to be rain free by late tonight.

The rest of the period looks to be dry, although another positive
tilt short wave trough looks to come across the region Wednesday
night. This wave should go south of the local area which with the
positive tilt, could bring some lift to the southern sections of
the forecast area. The front that goes through the region tonight
will be over the Ohio River Valley and there is concern whether
there will be enough moisture behind this front for the short wave
trough to work with. The 17.12Z NAM and GFS keep the area dry
while the ECMWF and GEM do show some light rain. For now, will
maintain forecast continuity and stay with a dry forecast.

.LONG TERM...(Thursday night through Monday)
Issued at 215 PM CDT Mon Oct 17 2016

Not many changes in this period as the GFS and ECMWF are still in
agreement with a longwave trough moving through the area and bringing
seasonable temperatures. Much of the period also looks dry, though
the ECMWF has consistently hinted that a shortwave may move through
Friday night into Saturday morning and bring a bit of precipitation.
While the GFS does show a similar feature, it has it farther north
and not nearly as amplified, therefore keeping all of the
precipitation north. Beyond Saturday, northwest flow will dominate
and temperatures will remain near normal.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 656 PM CDT Mon Oct 17 2016

IFR conditions are possible late tonight in low stratus and BR.
Otherwise, a cold front will slide through the region tonight into
Tuesday. The stratus will scatter out by mid morning Tuesday and
cloud bases will rise to VFR by late morning into the afternoon.
Look for skies to become mostly clear during the afternoon hours.




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