Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 190805

Area Forecast Discussion
National Weather Service La Crosse WI
305 AM CDT Sat Jun 19 2021

.SHORT TERM...(Today and tonight)
Issued at 305 AM CDT Sat Jun 19 2021

Today should be a rather pleasant day as an area of high pressure
builds in from the northwest. The high will bring in some cooler
and drier air with highs today expected to top out in the lower to
middle 80s for much of the area. With some decent mixing,
afternoon dew points should drop into the upper 40s to lower 50s.

The area of high pressure will move off to the east tonight ahead
of the next system. A short wave trough rotating through the long
wave trough from a system moving across Canada, is expected to
cause an MCS to form overnight in the lower Missouri River
Valley. This should then track northeast toward the area, possibly
starting to move into the area west of the Mississippi River
toward daybreak.

.LONG TERM...(Sunday through Friday)
Issued at 305 AM CDT Sat Jun 19 2021

The next significant chance for rain will be Sunday. An upper
level low will be over central Canada with the long wave trough
extending south into the Upper Midwest. At this time, the models
suggest there will be more than one embedded short wave trough
that will rotate across the region, which will create a rather
messy set up for convective development. The first short wave
trough should move across the region in the morning followed by
another wave or waves in the late afternoon and evening. The hi-
res meso-scale models for the most part all suggest an MCS should
develop Saturday night over the lower Missouri River Valley with
the weakening remnants of this moving across the southern parts of
the area Sunday morning. How the atmosphere then recovers in the
wake of this activity will have a big influence on the
afternoon/evening severe threat.

As the next short wave trough moves in, an area of low pressure
is expected to develop along an incoming cold front and move
across southeast Minnesota into western Wisconsin late in the
afternoon into the evening. This will help to spread lower to
middle 60s dew points back across much of the area with around 70
approaching northeast Iowa and far southwest Wisconsin along a
secondary warm front/moisture boundary. As a result, the best
MLCAPE pool of 2000+ J/kg will remain to the south but the RAP and
19.00Z HREF suggest a narrow ribbon of 1000-1500 J/kg right along
and ahead of the incoming cold front. Deep layer shear along the
front looks to be on the order of 40 to 45 knots suggesting
damaging winds and hail would be the main threats along this
boundary. Farther to the south, the low level moisture transport
looks to be focused on the secondary boundary with up to 25 knots
of shear in the 0-1km layer possible, indicating some tornado
potential. This is also the area the HREF shows will have the
highest SRH potential of 150-200 m2/s2 and the highest
probabilities of updraft helicity exceeding 75 m2/s2. Just how far
north the tornado threat will exist will depend on the location
of the boundary and the new day 2 outlook now has a slight across
much of the south to cover this scenario. Some locally heavy rains
will also be possible with this activity as precipitable water
amounts should be between 1.5 and 2 inches with warm cloud depths
of 3 to 3.5 km.

Looking out into next week, the pattern looks to remain somewhat
active. Some activity could develop Tuesday night in a return
flow/warm air advection pattern. The upper level flow looks to
transition from northwest to quasi-zonal toward the end of the
week with the potential for several short wave troughs to impact
the region. Confidence on timing is not very high leading to an
extended period of rain chances starting Wednesday night and
continuing into Friday.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1030 PM CDT Fri Jun 18 2021

Scattered showers are beginning to develop west of Rochester and
a few isolated thunderstorms may develop as well and push east
southeast overnight. Due to spotty coverage will mention vcsh for
now. Otherwise, look for VFR conditions through the TAF period.
Light winds from the southeast through southwest become northwest
behind the cold front. Surface winds may briefly gust behind the
cold front tonight and Saturday morning.




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