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Area forecast discussion 
National Weather Service La Crosse WI
256 PM CDT Sat Jul 4 2015

Short term...(this evening through Monday night)
issued at 138 PM CDT Sat Jul 4 2015

The focus of the forecast is on the rain chances for Sunday night
through Monday night.

Water vapor satellite shows an upper level low over west central
Canada with a positive tilt short wave trough to the southwest
into British Columbia. The consensus of the 04.12 models is to
continue moving this system to the east/southeast with the upper
level low remaining well to the north across Canada while the
short wave through drops over the northern sections of the country
in the Lee of The Rockies. The wave should maintain is positive
tilt as it comes across...although the 04.12z European model (ecmwf) has started to
show that the wave could take a more neutral tilt Monday with an
upper level low forming over southwest Ontario. If this did would seem the only impact on the forecast would be for
the rain chances to possibly linger longer into Monday night. Will
have to see if this trend continues with later model runs and
adjust accordingly if it does.

Because the wave is coming in with a positive tilt...the best pv
advection will remain on the poleward side of the wave and across
Canada. This leaves just some weak pv advection to come across the
upper Midwest for Sunday night through Monday. The lift should be
enhanced some late Sunday night into Monday as the area comes
under the Equator side of the upper level jet...although the true
right entrance region may remain to the west of the area. In the
low levels...the cold front will approach late Sunday night and
then move across the area Monday. The front looks to have deep
frontogenesis along it in the 1000-500 mb layer...but it only
looks to be weak in strength. Decent isentropic up Glide will
occur in the warm air advection ahead of the front with between 3
and 6 ubar/S possible on the 305k surface. Add all this up...and
there looks to be ample forcing for rain to occur as the system
moves through late Sunday and Monday. Will show a axis of 80
percent rain chances along the front over the entire area during
the day Monday.

Severe weather does not look to be much of a concern with this
system. Considerable differences between the models on how much
cape will develop across the area Monday ahead of the front. The
04.12z NAM builds up to 3000 j/kg of ml cape while the 04.12z GFS
just gets 1000 j/kg of ml cape into the southern sections of the
forecast area. Given that the NAM has a bias of being too high
with the dew points...which it has climbing into the middle to
upper develops too much cape. Will side with the GFS on
the lower cape values and looking at the forecast soundings...the
cape profile is tall and skinny suggesting strong up drafts will
have a hard time surviving. The saturated layer is very deep which
would lead more to heavy rain instead of hail. If a stronger storm
can get going...some potential for gusty winds if the 0-3km shear
to get into the 30 to 35 knot range as suggested Monday afternoon.

Long term...(tuesday through saturday)
issued at 138 PM CDT Sat Jul 4 2015

Once this system clears the region...the upper level flow looks to
go nearly zonal. Expecting that there will be some weak short wave
troughs embedded in this flow that will bring some on and off rain
chances. Some suggestion by the GFS and 04.12z European model (ecmwf) of one of
these waves coming across Wednesday night and Thursday and again
Friday night and Saturday. Because of this weak pattern...will
have some low rain chances from Wednesday night through the
remainder of the period.


Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1250 PM CDT Sat Jul 4 2015

Smoke from Canadian/alaskan wildfires continues to stream into
the region producing visibilities in the 5 to 6 sm range.
Otherwise...high pressure will remain in control providing quiet
weather through the taf period. The visibility restrictions in
smoke will persist into tomorrow. For the most part visibilities
should remain VFR but cannot rule out an occasional drop to MVFR.


Hydrology...(sunday night through Monday night)

Models continue to show a good return of moisture into the area
ahead of the cold front with precipitable waters climbing into the
2 to 2.4 inch range. With warm cloud depths of 3.5 to 4.5
km...the storms should be efficient rain producers. However...not
sure how widespread these more efficient storms will be with the
best forcing from the wave remaining to the north of the area. The
heaviest quantitative precipitation forecast axis looks to have shifted into northeast Minnesota
and northern Wisconsin closer to the short wave trough. This
results in total quantitative precipitation forecast over the local area of an inch or less. Not
expecting this will be enough to produce any flooding problems and
do not anticipate the need for any watches or esf if these trends

Arx watches/warnings/advisories...


Short term...04
long term...04

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