Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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351
FXUS63 KMKX 221208
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
708 AM CDT Thu Jun 22 2017

.UPDATE...

The band of showers/isolated thunderstorms are expected to
continue to weaken this morning as the 850 mb low level jet moves
off to the east. However more storms are developing well west
across northeast and north central Iowa in areas of greater
instability. Still expect things should quiet down until a line
of storms (possibly severe) tracks through southern WI this
evening ahead of an approaching cold front.

&&

.AVIATION(12Z TAFS)...

The band of showers/isolated thunderstorms are expected to
continue to weaken this morning as the 850 mb low level jet moves
off to the east. However more storms are developing well west
across northeast and north central Iowa in areas of greater
instability. Still expect things should quiet down until a line
of storms tracks through southern WI this evening ahead of an
approaching cold front. MVFR cigs and IFR vsbys in the stronger
storms this evening.

A band of MVFR ceilings expected behind the line of storms/cold
front late tonight.

&&

.PREV DISCUSSION... (Issued 247 AM CDT Thu Jun 22 2017)

SHORT TERM...
Today...Forecast Confidence is Medium...

A strong 125 knot upper level jet in the zonal flow across the
U.S. Canadian border will move east, as a trough begins to
develop north of Montana and North Dakota. This causes the jet to
become more southwest tonight. Upper divergence and 700 mb upward
motion increases over central Wisconsin this afternoon and across
the south tonight.

Forecast sounding show a capped environment later this morning,
and into the afternoon. Mesoscale models suggest a round of severe
storms later this afternoon over central Wisconsin, near the
advancing cold front with a weakening of the cap, that could reach
our northern areas late this afternoon.

Given the expected mean layer Zero to 1 km mixed layer CAPE around
2000 Joules/kg, and deep layer/low level shear, damaging winds
and large hail are the primary threat, but EHI values also
indicate that a potential of a tornado can not be ruled out,
especially toward early evening. The Storm Prediction Center has
the entire forecast area in a Slight Risk of severe. Heavy
rainfall is also a concern, given the precipitable water values
approaching 2 inches.

Tonight...Forecast Confidence is Medium.

The main focus for this period is the risk for severe thunderstorms
and heavy rainfall.

The low amplitude wave will progress east-southeast from the
northern Great Plains toward Wisconsin. An increase in westerly
mid to upper tropospheric flow will accompany the wave, with jet-
level divergence maximized this evening. Respectable height falls
will precede the wave`s arrival with synoptic scale forcing for
ascent maximized early Tonight. Deep layer moisture will be in
place ahead of the wave, with low-level forcing provided via
frontogenesis along a southward sinking cold front. All of this
suggests a good environment for continued thunderstorm
development.

This will provide around 35 knots of speed shear in the 0-6 km
layer, with around 25 knots occuring in the lowest kilometer. Some
directional shear will also be present in the low levels, as the
flow backs south-southwesterly. This shear profile would
potentially support both multicell and supercell storm modes, with
a risk for damaging winds and hail. The combination of low level
directional and speed shear also presents a tornado risk.

Moderate instability expected into the early evening, with
dewpoints well into the 60s, and Zero to 1 km CAPE values still
around 2000 joules/kg at 10 pm south of the front. This
environment would be supportive of both large hail and damaging
wind gusts, in addition to the tornado threat, especially
southeast into the mid to late evening.

After Midnight CAPE values decrease ahead of the front far
southeast, and the severe threat decreases. meanwhile the rest of
the forecast area will be behind the cold front.

LONG TERM...
Friday and Saturday...Forecast confidence is High.

Surface low pressure will pull away from the region with northwest
flow lingering behind it. An upper trough will dominate with a
shortwave moving through during the afternoon. The upper jet will
shift southward directly over southern WI, pushing the favorable
divergent flow area well south of the region. Low level lapse
rates will be steep and with mixing we`ll be close to tapping
25-30 knot winds at 850mb. The GFS continues to be the only one
spitting out some showers. Soundings are showing a lot of dry air
so will keep a dry forecast going. Temperatures will be a little
cooler with the influence of the trough.

A strong shortwave trough will dig through on Saturday. Models are
in better agreement showing some afternoon warm air advection
precip. Most models show very little to no cape so will hold off
on thunder mention. Temperatures will be even cooler with the
reinforcing trough.

Sunday through Wednesday...Forecast confidence is Medium.

The upper trough lingers through Sunday and shortwave activity
continues Sunday and Monday. There is a lot of model variability
with instability. Cooler temperatures will continue.

A mid level ridge starts to push into the region on Tuesday and
temperatures start to rebound. Surface high pressure slides
southwest of the state. Tuesday looks dry then there are timing
differences of the next system moving through. Low pressure will
move through sometime after Tuesday bringing the best chance for
rain and thunderstorms.

AVIATION(09Z TAFS)...

Strongest storms will remain north of Fond du Lac and Sheboygan
early this morning, however scattered showers/thunderstorms are
developing farther south, so there is a chance of showers this
morning. Otherwise, most of Thursday should be quiet until a line
of storms tracks through southern WI Thu evening.

Low level wind shear is going on tonight and then gusty southwest
winds are expected during the daytime hours on Thursday.

There is a chance of MVFR or lower ceilings behind the line of
storms/cold front late Thursday night.

MARINE...

Gusty southwest winds around 25 knots are expected Today, which
may should south to southeast as a Lake Breeze tries to push along
the shore areas. Winds should not be as gusty over the cool waters
due to a low level stable layer. Will issue a Small Craft advisory
for the gusty shore winds. Thunderstorms are expected this
evening, and some may be severe. There is a chance of
thunderstorms during the afternoon, especially north areas toward
sheboygan.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...Small Craft Advisory until 7 PM CDT this evening for LMZ644>646.

     Small Craft Advisory until 4 PM CDT this afternoon for LMZ643.

&&

$$

Update...Hentz
Today/Tonight and Aviation/Marine...Hentz
Friday THROUGH Wednesday...Marquardt



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