Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 160447

Area Forecast Discussion
National Weather Service La Crosse WI
1147 PM CDT Thu Jun 15 2017

Issued at 859 PM CDT Thu Jun 15 2017

Thunderstorms will continue to drift south and eventually away
from Chickasaw, Fayette, Clayton, and Grant Counties through 11
PM. Most of the MLCAPE has now pushed south of the forecast area
and with the thicker anvil and boundary pushing south across
northeast Iowa into far southwest Wisconsin, not expecting much
more activity outside of a stray shower or isolated thunderstorm.
It appears the severe weather threat has just about ended. Will be
holding onto the severe thunderstorm watch just a little bit
longer until the last batch of storms in Bremer and Buchanan
County in northeast Iowa weaken or shift well to the south. The
severe thunderstorm watch will likely be cancelled well before 1
AM for the La Crosse Forecast Area.


.SHORT TERM...(This evening through Friday)
Issued at 225 PM CDT Thu Jun 15 2017

At 3 PM, a weak boundary was located across central Iowa. Dew
points north of this boundary were in the lower and mid 50s and
around 60F south of it. The HRRR and RAP suggest this boundary
will slip northeast into northeast Iowa this afternoon and early
evening. As this occurs, the mean layer CAPES are forecast to
climb into the 500 to 1500 J/kg range. In addition, a 500 to
700 mb speed max will increase the effective wind shear up to
60 knots. While the hodograph is favorable for splitting
supercells, any left movers will move quickly into a drier
environment, so their impact will be limited. Due to this, the SPC
1630Z Day 1 Convective Outlook moved the marginal risk up to the
Interstate 90 corridor and the slight risk into northeast Iowa and
a small portion of southwest Wisconsin. This risk is a very
conditional and highly dependent upon our dew points getting as
high as what the CAM models indicate (around 60F). If convection
can develop, the main severe weather threats would be large hail
and damaging wind (primarily based on the strong winds aloft and
inverted V in the sub-cloud layer).

From late Friday morning into Friday afternoon, another short
wave trough will move east through the region. The 0-1 km mean
layer CAPES climb into the 500 to 1000 J/kg range. While the deep
layer shear is not that impressive, the 0-3 km shear does get into
the 25 to 35 knot range and there is an inverted V in the sub-
cloud layer, so maybe an isolated wind threat.

.LONG TERM...(Friday night through Thursday)
Issued at 225 PM CDT Thu Jun 15 2017

On Friday night, it continues to look like a mesoscale convective
complex will develop in western Iowa on the northern edge of an
unstable air mass (2-4K J/kg) in southwest Iowa. This system will
then track either southeast through southeast Iowa and northern
Illinois or dive south into the Ozarks. In either scenario, this
would push the front across Iowa much further south. This has
been a trend in the GFS, ECMWF, and GEM. If this does indeed
occur, the threat of severe weather for Saturday afternoon and
night will likely to shift further south. Even if this does
occurs, the combination of a coupled jet and strong short wave
trough will still produce showers and scattered storms across the
Upper Mississippi River Valley for Saturday afternoon and night.

On Sunday, wrap around moisture and cold air aloft will result in
the development scattered showers and isolated thunderstorms.

Strong cold air advection in the wake of this system will bring
much colder air into the region. Temperatures on Sunday and Monday
will range from the upper 60s to mid 70s and then slowly moderate
as we move into mid week.


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1147 PM CDT Thu Jun 15 2017

Scattered showers and thunderstorms are possible at the TAFs late
Friday afternoon into Friday evening. However, latest indications
are that this activity could occur south of the TAF sites.
Otherwise, plan on VFR conditions with sct to bkn ceilings at
times. The lowest ceilings will be Friday afternoon when they may
lower to around 5 kft. MVFR conditions are possible near any
showers or storms that develop.




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