Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 200836

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
336 AM CDT Tue Oct 20 2020

(Issued 335 AM CDT Tue Oct 20 2020)

Today and Tonight:

A broad low pressure system will push across the Midwest today
with an associated strong upper level shortwave. This will feature
plenty of PVA and some frontogenesis in the upper levels and
plenty of low to mid level WAA and moisture. However the track of
this part of the system will likely track north of our CWA for the
most part with only the far northern parts of the CWA likely to
see impact from this part of the system, especially with lack of
midlevel moisture in the southern half of Wisconsin.

The primarily chance for precipitation across the CWA will
actually come later this afternoon and into the evening as a
decently strong LLJ will push northeast and nose into the southern
half of Wisconsin. This will bring strong WAA, plenty of low
level moisture and perhaps some frontogenetical forcing in the
low levels. Models indicate essentially full saturation up to
about 700 mb which will likely allow us to see a period of
widespread drizzle this evening as the system pushes out tonight.
Drizzle may linger overnight but with most of the moisture and
associated low level WAA expected to push out by around 6z we
should see most of the area dry out for the the rest of the
overnight period.



(Issued 335 AM CDT Tue Oct 20 2020)

Wednesday through Thursday morning:

Wednesday looks to remain mostly dry as high pressure moves east
across Wisconsin. Mid-level clouds will gradually thicken during
the afternoon as some weak warm air advection starts to move in
from the west as southerly 850mb return flow becomes established
over the central Plains. However, it will take a while for the
low-levels to moisten enough for precipitation to reach the
ground, thus have backed off of chances for precipitation during
the day on Wednesday. Have now delayed the arrival until the
stronger theta-e advection associated with a strengthening warm
front developing upstream of the area lifts north Wednesday night
through Thursday morning. Moderate to heavy rain will be possible
as the front lifts north, with the potential for a few embedded
thunderstorms to accompany the strong isentropic ascent and
increasing PWATs.

Thursday afternoon through Friday morning:

There is some model disagreement in regards to how fast this warm
front will lift north across southern and central Wisconsin, but
the overall consensus is for the front to lift into northeast
Wisconsin as the main surface low develops over northwest
Wisconsin Thursday afternoon/evening. This will favor better
chances for precipitation to lift north of the area, with the warm
sector overspreading southern and central Wisconsin. It will
certainly feel increasingly humid during the afternoon as a
result with temperatures warming into the 60s and 70s.

As the main vort max lifts northeast over the Upper Great Lakes,
the surface low developing along the open wave lifts northeast
into Ontario late Thursday night into Friday morning. The
trailing cold front will then track west to east across southern
and central Wisconsin, bringing another round of showers and the
potential for some embedded thunderstorms once again. There is
some uncertainty in the speed/timing of the front, but do expect a
breezy, mild night Thursday night ahead of the front and a cool
down behind the front. Do have non-diurnal temperatures in the
forecast for hourly temperatures as a result Thursday night into
Friday morning.

Friday afternoon through Saturday night:

Expect non-diurnal temperatures behind the cold front as it exits
east over Lake Michigan, with day time highs expected early on in
the day. Precipitation will come to an end from west to east, as
the eastern edge of high pressure anchored back in the northern
Rockies moves in behind the front. Friday night will be a cold
night with lows back in the 20s and 30s. Saturday will remain dry,
with temperatures only rebounding into the 40s.

Sunday into early next week:

Medium range guidance tends to stay on the cooler side for late
weekend into early next week, but ensemble spreads increase
drastically as numerous shortwaves embedded within longwave
troughing dig across the western and central CONUS. Given the
uncertainty in how the forecast will trend, have opted to leave
the model blend as is for now.



(Issued 335 AM CDT Tue Oct 20 2020)

Much of the region will start off with VFR condition with parts of
far southern Wisconsin potentially seeing borderline MVFR CIGS.
Later today we will see broad MVFR CIGS push in with a low
pressure system pushing across the Midwest. Across central
Wisconsin we will see shower chances for much of the afternoon
with the best chances in the evening. Across southern Wisconsin we
will see increased chances for drizzle in the evening with fairly
low CIGS expected with IFR CIGS expected for a period with
potential for a period of LIFR CIGS. In addition, we could see
brief periods of reduced VSBYS with showers and patchy fog but
for the most part VSBYS should remain VFR. This system will push
out overnight with IFR/MVFR CIGS expected to push out by daybreak
though clouds are expected to stick around through Wednesday.



(Issued 335 AM CDT Tue Oct 20 2020)

High pressure will move across the northern Great Lakes and
Ontario today. A trough of low pressure will then approach from
the west and move across Lake Michigan tonight. East to
southeasterly winds today, then to breezy southwest to westerly
winds for tonight. Prior to the switch over to southwest winds a
period of 4 to 5 foot waves which will then be followed by breezy
gusts around 20 to 25 kts. Thus we have issued a Small Craft
Advisory from 3z until 15z for the nearshore from Port Washington
to Sheboygan. The breezy westerly winds will continue Wednesday
followed by light and variable winds Wednesday night as high
pressure moves across the lake.

Breezy southerly winds approaching gales will then develop later
Thursday into Thursday night as low pressure tracks from the
central Great Plains to eastern Lake Superior. The winds will then
shift to brisk west to northwesterly on Friday with the passage
of the cold front.



LM...Small Craft Advisory...LMZ643...10 PM Tuesday to 10 AM Wednesday.



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