Severe Thunderstorm Warning 174, NWS Milwaukee, WI

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
736 PM CDT Sat Sep 21 2024

.KEY MESSAGES...

- Showers and thunderstorms are likely through the overnight.

- Most of southern WI will see some beneficial rainfall from the
  cold front passage with amounts generally ranging from 0.1-0.7
  inches with locally higher amounts with any stronger
  thunderstorms.

- Cooler/near normal temps are expected behind the cold front
  Sunday through the start of the week with daily highs in the
  upper 60s and lower 70s.

&&

.UPDATE...
Issued 736 PM CDT Sat Sep 21 2024

Scattered showers and thunderstorms continue across the region
this evening. Any meaningful severe weather threat has likely
ended.

Occasional showers and thunderstorms will persist overnight and
into tomorrow morning, as a broad area of warm/moist advection
remains in place ahead of a slow moving cold front. Tough to say
just how widespread coverage will be, and it may ebb and flow a
bit as various weak shortwaves ripple through the area. More
organized shortwave energy will lift across northern Illinois
and extreme southeastern Wisconsin tomorrow morning, so suspect
will see some uptick in coverage over the eastern third to half
of the area early to mid morning.

The front will finally clear the area tomorrow afternoon, though
clearing will lag by several hours.

Boxell

&&

.SHORT TERM...
Issued 336 PM CDT Sat Sep 21 2024

This Afternoon:

Showers and thunderstorms are developing in a north-south
oriented line that is right along the 1000 j/kg CAPE gradient,
within an area of steady 925-850mb warm air advection. There
was an outflow boundary that developed from the morning
convection over the Twin Cities that is tracking straight down
the instability gradient, and this feature is causing the
intersecting storms to be stronger as the boundary propagates
south-southeast. The area of stronger storms is currently over
Dane County and is producing marginally severe hail and wind.

The next feature of interest is the surface warm front which is
moving into the Mississippi River Valley. There are showers and
a few thunderstorms along this front. The two areas of warm air
advection will slowly track across southern WI through the
evening and therefore, the chance of storms will persist all
evening. Severe storms are not expected, but if there are any
areas of extra forcing such as an outflow boundary of surface
convergence with a lake breeze, then isolated severe is
possible. We will have up to 1500 j/kg of elevated CAPE and
around 30 kt of bulk shear to support a few stronger storms.

Tonight through Sunday night:

The cold front is also going to move through in a couple waves
overnight. There is a surface wind shift to the northwest that
is already in southeast MN and north central IA. This surface
warm front may be enough to kick off a few thunderstorms as it
gets into western WI late this afternoon, but it is also
possible that our environment could be worked over by previous
storms and not amount to much. There is a secondary cold front
back in central MN which has more upper level support with the
mid level vorticity advection. Since the upper wave will be
weakening, its movement toward southern WI will slow down
overnight into Sunday. This means our chance for showers and
storms ahead of an along this secondary cold front will persist
all the way into Sunday afternoon, especially over southeast WI.

Cronce

&&

.LONG TERM...
Issued 336 PM CDT Sat Sep 21 2024

Monday through Saturday:

One more mid level shortwave will track along the WI/IL border
on Monday. This will keep clouds over southern WI. Then an
upper low should develop and stall over the Upper Midwest
Tuesday. This will keep us cloudy and keep our temps down in the
70s. Lots of uncertainty with how that low will move later in
the week, with a trend away from southern WI and retrograding
toward the south central U.S..

Cronce

&&

.AVIATION...
Issued 736 PM CDT Sat Sep 21 2024

Rather complex aviation forecast on hand for the next 24 hours,
with occasional showers and thunderstorms into the daytime hours
on Sunday, ahead of a cold front. Have utilized PROB30 groups
to indicate the more favored windows for thunderstorms, but
overall forecast confidence is low. Lower ceilings and
visibilities are expected as well, especially early Sunday as
the front starts to move through the region.

Winds this evening and tonight will be south to southeast,
though will be variable in and around thunderstorms. Look for a
turn to westerly and then northwesterly winds as the front moves
through on Sunday.

Boxell

&&

.MARINE...
Issued 336 PM CDT Sat Sep 21 2024

Expect southerly winds increase through the evening as low
pressure lifts from MN to Ontario, which will in turn drag a
cold front across WI and Lake Michigan overnight through Sunday.
Showers and thunderstorms will develop ahead of and along the
cold front. Southwesterly winds will become northerly behind
this front Sunday night into Monday. Given the strong cold air
moving over the warmer lake waters, expecting to see gusty
northerly winds during this timeframe. Gusts up to 30 knots and
a few gale force gusts cannot be ruled out, especially for the
southern portions of Lake Michigan. Will see winds weaken a bit
for Monday as high pressure builds in across the Great Lakes
region, but the northerly component will persist through the
start of the week.

Cronce

&&

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

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