Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 141740

Area Forecast Discussion
National Weather Service La Crosse WI
1240 PM CDT Fri Jul 14 2017

Issued at 1240 PM CDT Fri Jul 14 2017

A little taste of early "fall-like" conditions out there early
this afternoon, with stratus proving to be quite stubborn as it
remains stuck beneath a robust inversion in a not very well mixed
environment. Per satellite trends, clouds aren`t going anywhere
for at least another few hours (though they are thinning on the
north/south ends), and we will really have to watch trends into
the evening just to make sure stratus isn`t still hanging around,
as the overnight light wind environment would be supportive of
additional expansion. Given trends, have lowered highs by a few
degrees, but nothing too crazy as it won`t take but an hour or
two of sunshine for readings to quickly pop up well into the 70s.
It is mid July after all! Enjoy it as heat and humidity return


.SHORT TERM...(Today through Sunday)
Issued at 242 AM CDT Fri Jul 14 2017

The focus in this period will be on the thunderstorms chances for
Saturday and Saturday night. Water vapor satellite early this
morning shows a large area of ridging over the Rockies that
extends well into Canada. A cut off upper level low off the
Canadian Pacific coast will send some short wave troughs into and
over the upper level ridging. Right now, the models are in
reasonably good agreement that a wave will top the ridge and drop
southeast across the Great Lakes and southwest Ontario Saturday
afternoon and evening. This projected path takes the wave quite a
ways northeast of the local area with almost no pv advection in
the 500-300 mb layer over the region. This essentially will leave
the low level forcing along a cold front to be the trigger for
any convective activity. The front should move across the region
during the favorable peak heating of the day Saturday afternoon
and into the early evening. A band of weak frontogenesis in the
1000-700 mb layer should accompany the front along with about 3
ubar/s of up glide on the 310K isentropic surface. Interestingly,
both the 14.00Z NAM and GFS show the low level moisture transport
starting post-frontal and coming into the the back side of the
front. This suggest the low level jet will not be involved to feed
any storms on the leading edge of the activity. Forecast
soundings suggest a good low level cap will be in place until
almost the time the front moves through, leaving the concern of
just how much time will there be for convective activity to
develop before the front moves through and the atmosphere begins
to stabilize? Given the relative weak forcing and short window of
opportunity for convection to form, a 20 to 30 percent chance of
storms seems appropriate with this system.

If convection can form, some threat for severe storms with it.
Ample instability will exist ahead of the front with up to 2500
J/Kg of ML CAPE. The shear looks to be pretty much all in the 0-3
km layer and should be around 30 knots or so. This would suggest
that damaging winds and hail would be the primary threats.

.LONG TERM...(Sunday night through Thursday)
Issued at 242 AM CDT Fri Jul 14 2017

The upper level ridge looks like it will get flattened during this
period and possibly also shift more over the central part of the
country. This would set up a near zonal flow across the Upper
Midwest or possibly a bit of a northwest flow. Either way, there
does not look to be a strong short wave troughs currently progged
to impact the area. However, there will be a front that should
drop into the region Monday night and Tuesday and then become
stationary either over the area for very close to it for the
remainder of the period. This will keep some 20 to 40 percent rain
chances in the forecast for each period from Monday night right
through to Thursday.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1240 PM CDT Fri Jul 14 2017

MVFR clouds are proving to be stubborn early this afternoon. Not
terribly surprising given a lack of mixing and a stronger
inversion aloft, and given trends, expect at least another few
hours of MVFR conditions before ceilings gradually try to lift and
then eventually dissipate into the evening as high pressure works
into the area. So...what could go wrong? If clouds are not able to
completely mix out by sunset, chances are high that lower stratus
will redevelop overnight, though we will simply have to watch
trends to see where they take us. Winds through sunrise will
remain light, gradually shifting back to the southwest at 8-15
knots by midday Saturday.




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