Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 312148 AAA
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
448 PM CDT WED AUG 31 2016

Updated for high swim risk on Thursday

.TONIGHT AND THURSDAY...Forecast confidence is High

Gusty winds right behind the cold front that is pushing south out of
southern WI will begin to lower by daybreak. A low cloud band has
developed in the lakeshore counties with moist onshore flow, but is
expected to break up as drier air spreads in behind front.

Cold advection dips 925 mb temperatures between 16C and 17C which
yields highs in the mid 70s inland and low 70s lakeside with cool
onshore flow. A weak short wave trough currently seen on water vapor
imagery over northern Minnesota is forecast to drop across the state
this afternoon, but moisture is lacking so no precipitation
expected. There will be a second surge of colder air behind this
wave spreading northeast to southwest overnight as the broad
surface high builds down over the western Great Lakes.

This surge brings delta T`s to around 10C tonight and brings the
potential of lake effect clouds/light precipitation. However strong
subsidence inversion behind the afternoon short wave trough on
forecast soundings limits cloud depth and pcpn chances so no PoPs
for now. Overnight lows in the lower to mid 50s inland to around 60
lakeside with a fall-like warmer onshore flow.

.THURSDAY NIGHT THROUGH FRIDAY...Forecast confidence is High

Positively-tilted upper-level trough will slowly move through the
central Great Lakes Thursday night, along with some lingering weak
500 mb positive vorticity advection. North to northeast winds off
Lake Michigan will help keep temperatures warmer compared to inland,
with lows in the upper 50s along the lake shore Friday morning.

Mid-level ridge axis looks to move in during the day Friday.
Onshore flow looks to continue through the day Friday, with sfc
high pressure situated over the northern Great Lakes. 925 mb
temperatures will drop into the 14C to 16C range, leading to
highs in the lower to mid 70s. Onshore flow looks to be pretty
weak, with magnitudes below 10 kts along the nearshore waters, so
low possibility of lake clouds developing.

.SATURDAY THROUGH SUNDAY...Forecast confidence is Medium

High pressure translates to the east Saturday, causing winds to
veer to a more southerly direction, which helps support better
warm air advection as evident in the increase of the 925 mb temps,
into the mid to upper teens Celsius. Continued dry conditions
expected through Saturday and Sunday before a surge of 850 mb warm
air advection with an associated low-level jet moves into the
region. The axis of best convergence with the LLJ looks to stay
to the west, but some of the enhancement may scratch our western
counties. Best mid-level support looks to remain west as well.
Latest 31.12z guidance shows any QPF holding off until Monday.

.LABOR DAY THROUGH WEDNESDAY...Forecast confidence is Medium

The 31.12z GFS is the quickest model in bringing in some QPF
compared to the other guidance, by 12z Monday. The ECMWF and
Canadian models are in better agreement in keeping any QPF out
Monday. The GFS may be influenced by stronger WAA forcing
compared to the other models, so went with a slower and lighter
solution for Monday precipitation, keeping more slight chance PoPs
than chance PoPs.

The better chances of precipitation look to be on Tuesday and
Wednesday as an upper-level trough builds in, strengthening the
250 mb jet, with a jet streak magnitude of 110+ kts. Southern WI
looks to be in the right-entrance region of the jet, enhancing
upper level divergent flow Tuesday morning. An associated
shortwave with good isentropic lift will help to support showers
and thunderstorms into Wednesday. The GFS/ECMWF/DGEX are all
showing some decent QPF amounts later in the day Wednesday.
Sufficient instability looks to build in behind the WAA, so kept
definite thunder in the forecast.

.AVIATION/00Z TAFS/...

Cold front is exiting to the south of the state with gusty north
winds up to 25 knots right behind the front expected to ease by
daybreak. Post-frontal MVFR/IFR stratus deck near the lake expected
to lift by 12Z as drier air overspreads the region. There may be
some lake-effect strato-cu over the eastern forecast area late this
afternoon and tonight but bases should remain above 3k ft so VFR
conditions expected through the forecast, with any fog tonight
limited to the river valleys.

.MARINE...

A cold front is exiting to the south of Wisconsin and gusty north to
northeast winds are expected to ease to a sustained 10 to 15 knots
with a few gusts to 20 knots by daybreak. This keeps wave heights
below small craft levels as well so no headline but small boaters
should exercise caution. Winds and waves approach small craft
advisory levels again on Thursday as a second surge of cooler air
moves down the lake ahead of broad high pressure building over the
western Great Lakes.

.BEACHES...

A moderate swim risk is expected tonight into Thursday morning
with a high swim risk expected Thursday afternoon through
Thursday night as north to northeast winds increase and gust to 25
knots. Waves on thursday will build to 3 to 5 feet Thursday
afternoon into Thursday night.

&&

.MKX Watches/Warnings/Advisories...
WI...BEACH HAZARDS STATEMENT from Thursday afternoon through Friday
     morning for WIZ052-060-066-071-072.

LM...SMALL CRAFT ADVISORY from noon Thursday to 7 AM CDT Friday for
     LMZ643>646.

&&

$$

TONIGHT/Thursday AND AVIATION/MARINE...DDV
Thursday NIGHT THROUGH Wednesday...JTS



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