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Area forecast discussion
National Weather Service La Crosse WI
525 am CDT Tue Sep 17 2019

Short term...(today through Thursday night)
issued at 225 am CDT Tue Sep 17 2019

Summer weather fans rejoice - the next 5-7 days look to feature more
in the way of late season warmth and humidity, though with the
occasional risk for some showers and storms. Early morning analysis
shows a relatively quiet weather regime in place across the local
area, with low level ridging branching westward across the region
from the northern Great Lakes, while a weak shortwave rippling north
across South Dakota as of 06z is helping drive scattered convection
well to our north and west while also delivering thicker cirrus over
this way. Airmass just off to the west is impressively moist,
featuring low 70s dew points even at this early hour, while things
are a bit more comfortable over this way for the moment (warning:
that won't last). Just a touch too much wind aloft this morning for
widespread dense fog, though will watch eastern areas through
sunrise for some pockets of reduced visibility, especially in the
river valleys where the boundary layer is sufficiently decoupled.

Pretty quiet weather looks to be in place today as high pressure
slowly gives up its influence, allowing higher dew points to work
east up to about the Mississippi River. However, not much in the way
of any forcing to work with that added moisture, as the stronger
moisture transport points well to the north into southwest Ontario
by afternoon. Things change a bit by late evening and overnight as
the tail of a stronger shortwave lifting across North Dakota helps
drive additional low level jet reinforcement/moisture transport
realignment ahead of a slow-moving frontal boundary into western
Minnesota/Iowa after 06z.

That feature then looks to slowly "ooze" eastward into our area
sometime Wednesday before becoming increasing diffuse with time into
Thursday. That setup should allow for unseasonably high moisture to
overtake the entire area through mid/late week, helping fuel
occasional chances for showers and storms. Not much at all in the
way of background forcing for storm organization, and a lack of deep
layer shear backs that up, but anomalous moisture (pwat values up
near 1.7+ inches) coupled with deep warm cloud depths of 4+ km and
slower storm motion vectors hint that localized heavy rainfall is
likely with any convection. As such, will continue to message a
heavy rain/localized flooding potential.

Long term...(friday through monday)
issued at 225 am CDT Tue Sep 17 2019

Highly amplified upper pattern continues into the weekend as broad
upper ridging dominates much of the eastern Continental U.S. While broad
troughing remains out west, sending periodic shortwaves in our
direction before the broader trough axis eventually lifts through
the region toward late Saturday or Sunday. Midweek boundary looks to
be completely "washed out" by Friday, with a lack of any forcing
leaving little to drive significant rain chances. Changes then occur
by the weekend as the western trough works east, with an uptick in
stronger forcing arriving sometime later Saturday or Sunday (timing
differences among guidance) fueling better convective chances as
another anomalously moisture airmass briefly works back north into
the area. As such, localized heavy rain remains a risk, though at
the moment, any convection looks rather progressive. Could also be
at least some severe weather potential given stronger shear ahead of
the upper trough axis, though plenty of details remain to be worked
out so far out in the forecast.

Looking beyond, there are increasing hints of another pattern shift
into early next week and beyond, as strong North Pacific jet energy
comes ashore and dominates the Continental U.S. With a pseudo-zonal flow
regime. That would likely allow for a more variable weather pattern
more typical for late September, but again, lots of details to be
worked out as we go forward with large run-to-run variability in
guidance likely over the coming days during the transition period.

&&

Aviation...(for the 12z tafs through 12z Wednesday morning)
issued at 525 am CDT Tue Sep 17 2019

Winds off the surface have remained strong enough to limit fog
formation along the MS river at klse this morning, with only some
high cirrus streaming across the area and a narrow scattered-broken MVFR
stratus field approaching the MS river. Other than possibly a
very short-lived period of MVFR ceilings at klse early this
morning, VFR conditions are expected today and tonight with
southerly winds gusting to around 20 kts at times today. A
weakening band of showers/storms may approach krst by 12z
Wednesday, but given the timing, will not include mention in the
taf yet.

&&

Arx watches/warnings/advisories...
WI...none.
Minnesota...none.
Iowa...none.
&&

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