Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 172020

Area Forecast Discussion
National Weather Service La Crosse WI
320 PM CDT Sat Jun 17 2017

.SHORT TERM...(This evening through Sunday)
Issued at 320 PM CDT Sat Jun 17 2017

At 3 PM, there are 3 cold fronts across Mid and Upper Mississippi
River Valleys. The first front is located across southern Iowa and
it is separating the dew points between the 60s and 70s. The
second cold front extends from near Green Bay to Cedar Rapids
Iowa. Behind this front, the dew points have dropped into the 50s.
The final cold front extends from the western tip of Lake
Superior to Sioux Falls South Dakota. It is behind this cold
front where temperatures fall into the 60s and 70s. With the best
instability well to our south, the 0-1 km mean layer CAPES are
less than 1000 J/kg. In addition, the 0-6 km shear is rather weak
at this time. It does increase this evening and becomes favorable
for supercells, but by this time the instability is generally
less than 250 J/kg. As a result, the chances for strong to severe
storms look very small. As a result, SPC has continue has
continued to shift this threat further and further south.

On Sunday, 850 mb temperatures will cool to around 9C. This will
result in 950 to 800 mb lapse rates of greater than 9C/km.
This along with cyclonic flow aloft, expect to see some diurnally
driven showers and storms to develop by late morning and continue
into the evening. With very weak shear and winds aloft of 20 to
30 knots, not anticipating any severe storms.

.LONG TERM...(Sunday night through Saturday)
Issued at 320 PM CDT Sat Jun 17 2017

On Monday, the combination of a short wave trough moving through
the region, steep 900 to 850 lapse rates, and cyclonic flow aloft
will once again cause scattered instability showers and storms to
form. Like on Sunday both the 0-3 km and 0-6 km shear remain
weak, so not anticipating any severe storms.

On Wednesday afternoon and night, the models are in good agreement
that a short wave trough will move southeast through the region.
While this is the case, there continues to be plenty of
uncertainty on the amount of instability that will be located
across the region. The ECMWF keeps our area on the cool side of
the system; thus, the instability across the region remains weak.
Meanwhile, the GFS has the area entering the warm sector of this
system with 0-1 km mixed layer CAPES into the 1000 to 2000 J/kg
range south of Interstate 94. The amount of instability is very
important, because both models have sufficient deep shear for
supercell development.

Beyond this time period, the models continued to show little
consistency, so made little change to the model consensus.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1244 PM CDT Sat Jun 17 2017

VFR conditions will be the rule at KRST/KLSE through at least
18.10Z, after which forecast guidance suggests a broken MVFR
cloud deck will spill southward across the area. Will introduce
lower ceilings at KRST at 18.10Z, but keep KLSE just above 3000 ft
agl. Scattered showers/thunderstorms are possible later this
afternoon/ evening, but confidence in thunder at TAF airfields is
low, so will keep VCSH or predominate showers for now. Any
convective activity is expected to end by midnight tonight. Light
westerly winds will persist through much of the period, shifting
to a more northwesterly direction at KRST by Sunday morning.




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