Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 221745 AAB
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
1245 PM CDT Thu Jun 22 2017

.UPDATE...


&&

.AVIATION(18Z TAFS)...

Scattered showers and a few thunderstorms may affect Madison early
this afternoon, as well as northern and western portions of the
area. Otherwise, gusty south to southwest winds should develop or
continue, especially in areas that get some sunshine. This will be
most likely in the southern and southeastern portions of the area.
Weaker winds will be found in cloudy areas to the north. Ceilings
should remain above 3000 feet this afternoon.

Thunderstorms are expected to slide southeast across the area
mainly tonight, from northwest to southeast. They will spread
southeast during the evening, then end later tonight. Look for
visibilities down to 2 miles, and ceilings down to around 2000
feet. These may be lower in any stronger storms. Strong to severe
storms are possible, with gusty winds, hail and heavy rainfall.

May see ceilings around 2000 feet linger after the storms exit the
area later tonight into Friday morning, before mixing out. West
winds should become gusty by the afternoon hours. Some diurnal
cumulus clouds are possible as well.

Wood

&&

.PREV DISCUSSION... (Issued 1157 AM CDT Thu Jun 22 2017)

UPDATE...

Mesoscale models have had some trouble with the showers and storms
this morning across the region. The latest HRRR model run has
started to get a better handle on the current activity. It looks
like the current showers over the area should continue to slowly
weaken over the next hour or two.

The showers and storms to the west appear to be tracking to the
east northeast. Development continuing to the southwest of there
seems to be tied to the influence of the low level jet
nose/convergence. This has been showing a general weakening trend,
though they still may affect the western counties at times into
the afternoon.

There are breaks in the clouds developing in southern portions of
the area, and should continue into this afternoon. This should
allow at least southern/southeastern portions of the area to get
warm and unstable this afternoon. There is still time for the rest
of the area to get some breaks in the clouds this afternoon,
before convection develops late this afternoon with low level
frontogenesis response from the cold front just northwest of the
area. Just how much of the area sees sunshine and gets unstable
will be the big question.

Mesoscale models are still generally developing convection along
the front and taking it southeast across the area tonight. As long
as some areas get some sunshine this afternoon, there should be
enough mean layer CAPE and deep layer shear to bring strong to
severe storms.

The cloud cover will be an issue if it lingers longer, so there
is some uncertainty over how strong/severe storms will become.
Will continue to mention strong to severe storms in our messaging,
but may play down the impacts a bit, until a better handle on the
potential instability is gained.

Wood

MARINE...

Small Craft Advisory continues until 21Z this afternoon north of
Port Washington, and until 00Z Friday south of there. Clouds have
limited sunshine and low level mixing this morning, which has led
to a delay with the onset of the gusty south to southwest winds.

There are some breaks in the clouds developing, and should
continue into the afternoon. This should allow for these gusty
winds to occur. Already seeing gusts in the low 20 knot range at
McKinley Marina and Kenosha Harbor. Thus, will continue the Small
Craft Advisory as is. Waves should build to 2 to 4 feet during the
afternoon.

There are chances for thunderstorms this afternoon. Thunderstorms
are then expected to develop to the northwest of the area later
this afternoon, then slide southeastward across the area tonight.
The timing for the nearshore waters of Lake Michigan should be
generally between 03Z and 09Z Friday. Strong to severe storms are
possible, with gusty winds and hail.

May see gusty west northwest winds reach Small Craft Advisory
levels at times on Friday, with cooler and drier air moving into
the region. Good mixing and a tight pressure gradient will help
with mixing these gusts to the surface. Highest waves will be over
the open waters.

Wood

PREV DISCUSSION... (Issued 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...Wood
Today/Tonight and Aviation/Marine...Hentz
Friday THROUGH Wednesday...Marquardt


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