Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 270502 AAC
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
1202 AM CDT Thu Jul 27 2017

.UPDATE...


&&

.AVIATION(06Z TAFS)...

Showers and a few storms will affect portions of the area into
the overnight hours. Locally heavy rainfall may occur. These
showers and storms should weaken and/or move southeast of the area
by 12Z Thursday. Low ceilings and fog are expected to continue to
develop overnight, with visibilities down to around 1 mile
possible. Lower values will be found in low lying areas. Ceilings
may lower to around 1000 feet or so. Light winds are expected
overnight.

VFR conditions should return by middle to late morning, as
northeast winds increase and bring drier air into the area. Gusty
northeast winds are expected near Lake Michigan Thursday afternoon
and evening, with gusts up to 20 knots possible. Ceilings should
gradually mix out by later in the day and into Thursday night.

Wood

&&

.PREV DISCUSSION... (Issued 853 PM CDT Wed Jul 26 2017)

UPDATE...

Scattered showers with a few thunderstorms will continue to slide
east southeastward across southern portions of the area this
evening. Main issue will be locally heavy rainfall, with
relatively slow movement, and high precipitable water and related
heavy rainfall parameters. Could see cloud to ground lightning
and gusty winds to 30 knots with any stronger storms as well.

The area of storms in northern Illinois should remain just south
of the area into the overnight hours, as Corfidi vectors keep its
motion due east. If far southern portions of the area get clipped,
they could see the locally heavy rainfall and gusty winds as
well.

The wind shift boundary will push southeast through the area
overnight, exiting around 12Z Thursday. May see some showers and
a few storms develop until this feature moves through. There is
little synoptic forcing for upward vertical motion, which is
hindering its development. Clouds and some light fog are expected
later tonight into Thursday morning, before mixing out later in
the morning.

Wood

MARINE...

Gusty northeast winds and building waves are expected for Thursday
afternoon into Thursday night. Frequent gusts to 25 knots are
forecast, along with waves of 3 to 6 feet. Winds and waves will
remain elevated Friday morning, then increase again Friday
afternoon into Friday night. Thus, a Small Craft Advisory will
likely be needed for Thursday afternoon into Thursday night,
perhaps extending into Friday night.

Wood

BEACHES...

Gusty northeast winds and building waves are expected along the
beaches of Lake Michigan Thursday afternoon and night. This should
bring high swim risk conditions during this time. A Beach Hazards
Statement may be needed for Thursday afternoon into Thursday
night. Similar conditions may linger into Friday night.

Wood

PREV DISCUSSION... (Issued 712 PM CDT Wed Jul 26 2017)

UPDATE...

AVIATION(00Z TAFS)...

Scattered showers with a few thunderstorms will continue to slide
eastward across the area into the early evening hours. These may
or may not hold together. Could see cloud to ground lightning,
brief heavy rainfall and gusty winds to 30 knots with any stronger
storms, as well as greatly reduced ceilings and visibilities.

The area of storms in northwest Illinois may make its way into
far southern portions of the area this evening. They are moving
relatively slowly, so there would be a risk for locally heavy
rainfall in that area. Cloud to ground lightning and gusty winds
to 30 knots are possible with these storms as well, with greatly
reduced ceilings and visibilities.

The wind shift boundary will push southeast through the area later
this evening and overnight, exiting around 12Z Thursday. Scattered
showers and storms are still possible until this feature moves
through.

Winds will shift to the northeast on Thursday behind the front,
becoming gusty near the lake in the afternoon and evening. There
should be a period of ceilings down to 1500 to 2000 feet later
tonight into Thursday morning, along with light fog with
visibilities down to 1 mile at worst. VFR conditions will return
by later in the morning.

Wood

PREV DISCUSSION... (Issued 331 PM CDT Wed Jul 26 2017)

SHORT TERM...

Rest of this afternoon and tonight - Confidence...Medium
Very moist airmass with precipitable water values running just s
smidge under 2 inches. MLCAPE values are reasonably high
especially given the degree of cloud cover. Mid level lapse rates
are very poor with 0-6km shear that`s pretty much aob 30 knots. So
with the upstream wave and slow moving surface trough we are
dealing with mainly a locally heavy rain threat. SPC has pulled
the Slight Risk and this looks really good. But some cells may
still put out some impressive rainfall rates but everything is
moving along pretty good with the influence of the upper jet
stream to our north. So meso models are all showing fairly
disorganized convection well into the evening hours with a
diminishing trend later in the night. So will trend POPS in this
direction given the consensus and consistency.

Thursday - Confidence...Medium
Any lingering shower potential looks to be early in the morning.
There is some 850 convergence and the hint of some low level
inverted troughing. Northeast winds will prevail and low level rh
progs are indicating cloud cover may be stubborn to clear out with
a better dry push for the afternoon. The big dropoff in 925 temps
holds off until Thursday night despite the shift in winds to the
northeast but areas closer to the lake will be cooler.

LONG TERM...

Thursday night through Friday... Forecast confidence is high.

The upper trough will swing through the Great Lakes region Thu
nt through Friday. That vorticity advection may be enough to
generate a few showers Thu night, otherwise expect dry weather.

Cyclonic flow aloft and persistent, dry north to northeast winds
will allow for dry weather with diurnal cumulus clouds.

Saturday through Tuesday... Forecast confidence is high.

Surface high pressure and upper ridging will give us light winds,
mostly sunny skies, and a weak lake breeze each day. Highs will
be below seasonal average, right around 80.

Monday through Wednesday... Forecast confidence is medium.

There are a couple slight chances of storms sprinkled into the
forecast for Monday and Tuesday afternoons. This is due to the
GFS showing more weak shortwaves riding over the ridge in the
southern WI area and producing scattered showers and storms. The
ECMWF holds onto that high over us all the way through Wednesday
night.

The GFS and Canadian models bring a front through southern WI Wed
afternoon/night while the ECWMF holds it off until Thu morning.
That would be the more organized focus for storms in our forecast.

AVIATION(18Z TAFS)...Very moist airmass with weak trough advancing
into the area from NW WI. Scattered to numerous shra/tsra across
srn WI with more activity upstream. Dynamics not impressive but a
moist and somewhat unstable environment will keep the threat for
storms going well into the evening hours. SPC dropped all mention
of the Slight risk so severe threat looks minimal. Overall VFR
conditions though some MVFR cigs/vsbys possible esp later tonight
into Thursday morning.

MARINE...NE winds ramp up on Thursday which will lead to some
higher wave generation. These NE winds continue into Friday so a
Small Craft may be needed at some point during this NE wind
regime as waves gradually build up.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...None.
&&

$$

Update...Wood
Tonight/Thursday and Aviation/Marine...Collar
Thursday Night through Wednesday...Cronce


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