Special Weather Statement, NWS Milwaukee, WI

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
354 PM CDT THU AUG 18 2016

.TONIGHT AND FRIDAY...Forecast Confidence Is Medium.

Kept small chances for showers and storms going along lake breeze
boundary until around sunset, with uncapped airmass. Anything that
develops will be isolated in nature, but could produce brief heavy
rainfall and cloud to ground lightning. Could not rule out a brief
funnel cloud either, as there is ample 0-1 km CAPE with some
surface vorticity available with the lake breeze boundary. There
has been a funnel cloud report with the current cell in Manitowoc
County.

Next issue is convection in far southern Minnesota moving
southeast toward southwest Wisconsin. HRRR experimental and
operational versions have this convection moving through the
western counties from mid to late evening. Corfidi vectors support
this propagation, though the CAPE becomes more elevated this
evening as the boundary layer cools off. Will update the forecast
to bring in some POPs to the western counties this evening. Not
anticipating anything severe if this activity gets here, as deep
layer shear is weak and CAPE gets more elevated in nature.

Kept POPs mainly in the north later tonight into Friday morning,
as some focused warm air advection and low level jet nose push
through the area. Some of the mesoscale models take the showers
and storms north of the area, with the latest HRRR experimental
trying to bring another round through the area overnight. Some
uncertainty here with what may happen.

The rest of Friday may end up dry across the area, as very warm
and humid airmass pushes in. Area may remain capped in this
scenario, though did hold onto some POPs in case some elevated
convection develops. Still looks like the cap may win out, per GFS
forecast soundings. Highs in the upper 80s should give heat index
values in the lower to mid 90s Friday.

.FRIDAY NIGHT THROUGH SUNDAY MORNING...Forecast Confidence Is
Medium.

A robust low pressure system will move through Wisconsin on
Saturday bringing the first taste of autumn to the region. By 00z
Sat, the base of a deep trof at the jet level will be over the
western Dakotas just the jet oriented from SW SD into NW MN. That
feature will gradually translate eastward, with the base of the
trof reaching Wisconsin by 12z on Sunday. The trof/jet over the
are will result in quit a bit of upper-level divergence across the
area throughout the period, especially prior to 00z Sun. A very
similar progression is evident at 500mb, with a few leading
shortwaves traversing the parent wave. Southern WI will see a few
rounds of DCVA, beginning around 12z Sat, and ending by Sun
morning.

In the lower-levels, models are in good agreement placing a
trof/closed low roughly over the WI/MN/IA borders. However, the
GEM, and especially the NAM, continue to generate an
unrealistically strong closed low. The NAM is also quite a bit
slower then the other models in moving the low across the area.
Other guidance shows the low deepening and moving NE across the
CWA on Saturday, reaching the UP of Michigan by 00z Sat. The low
will drag the front through the CWA between 18z Sat and 00z Sun.

Due to the DCVA and location of the low on Saturday morning, am
going with definite PoPs through much of the day Sat, and likely
PoPs remain until 00z Sun. The unrealistically deep low in the NAM
brings quite a bit of wrap-around precip into the are Sat evening.
This solution is clearly an outlier amongst the guidance, so am
not buying all the wrap around precip the NAM is showing. Saturday
will be a very muggy day, but given the overcast skies,
temperatures should be relatively cool, especially once the front
pushes through in the afternoon. The cooler temps should keep
instability fairly low. The best shear will be in northern
Wisconsin, where 0-6 km bulk shear will be in the 50kt
neighborhood. The shear quickly drops off into Illinois, with
shear values closer to 20kts. Lack of instability and shear should
really limit severe chances, as highlighted in the SPC day 3.

.SUNDAY AFTERNOON...Forecast Confidence Is Medium.

Cyclonic flow will remain in place on Sunday, and quite a bit or
moisture will remain in place in the lower levels. This should
keep skies mostly cloudy, and bring a slight chance of light
showers as well. Given the post frontal air and cloudy skies, high
temperatures will be quite a bit cooler than we've seen lately.
925 temps around 16C suggest highs will only make it into the low
70s at best.

.MONDAY AND TUESDAY...Forecast Confidence Is Medium.

A few days of very nice weather are expected as we ridging/high
pressure move into the area. That high will move across southern
IL on Monday, meaning that flow in WI will back from the NW to SW.
This SW flow will begin to usher warmer air into the area.

.TUESDAY AND WEDNESDAY...Forecast Confidence Is Medium.

Another low pressure system will impact the region in this
period. At this point, the low looks to pass well to our north,
dragging the cold front through WI. Best chances for precip look
to be Wednesday afternoon and evening, though it looks like the
guidance is stalling the front just to the south of WI, so precip
may linger in southern WI, especially south.

&&

.AVIATION/00Z TAFS/...

Southeast winds at eastern sites will become south this evening,
with south winds at Madison. Winds will shift southwest later
tonight into Friday at all sites, increasing a bit on Friday.

There may be an area of showers and thunderstorms that reaches
Madison from middle to late evening. Some uncertainty if this will
occur, but may put a vicinity thunder mention in Madison TAF
during this time. Brief gusty winds and brief heavy rainfall would
be the main hazards.

Another round of showers and storms may occur later tonight into
Friday morning, though this activity may remain north of TAF
sites. Given the uncertainty here, will leave mention out of TAFs
for now. May see light fog at TAF sites between 08Z and 13Z
Friday.

The rest of Friday may end up dry across the area, with the very
warm and humid airmass in place. Scattered to broken diurnal
cumulus clouds should develop by afternoon.

&&

.MARINE...

Winds and waves should remain below Small Craft Advisory levels
tonight into this weekend. South to southwest wind gusts may
reach to around 20 knots at times Friday into Saturday night, as
the pressure gradient tightens somewhat ahead of an approaching
cold front and low pressure.

&&

.MKX Watches/Warnings/Advisories...
WI...None.
LM...None.
&&

TONIGHT/FRIDAY AND AVIATION/MARINE...Wood
FRIDAY NIGHT THROUGH THURSDAY...BSH

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