NWS-Milwaukee/Sullivan Forecast Discussion

						FXUS63 KMKX 012355
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
655 PM CDT Tue Sep 1 2026

.KEY MESSAGES...

- Unseasonably warm and muggy conditions continue through the
  end of the work week. Upper 90s to near 100 degree heat
  indices will be possible daily in areas experiencing ample
  sunshine.

- Occasional storm chances continue into the beginning of the
  weekend, with plenty of dry periods in between. Some storms
  could be on the stronger side.

&&

.UPDATE...
Issued 700 PM CDT Tue Sep 1 2026

Judging by the latest radar / satellite / surface obs trends,
our next corridor of potential shower and storm activity (for
this evening into tonight) appears to be setting up over
portions of central and east-central WI, mainly north of WI-
Hwy-60.

As with the last several rounds of storms, there is plenty of
instability overhead, but we're very warm at 700mb (over 10
Celsius) and strongly capped, and the forcing for ascent to
trigger the storms is not particularly strong. Better confidence
in storm presence and coverage is later tonight, as the LLJ
strengthens and reinforces the lift along the east-west surface
frontal zone bisecting the state.

Sheppard

&&

.SHORT TERM...
Issued 230 PM CDT Tue Sep 1 2026

Tonight through Thursday night:

The sfc stationary front as sagged south and is roughly along a
line from Boscobel to the Dells to Clintonville. With the
stationary front just to our west, hot and humid air remains
across all of southern Wisconsin. Rain and storms have continued
to fire along this frontal boundary in northern Iowa and travel
east into the CWA. Rain and storms are predominately along and
south of Interstate 94. Storms are firing along the weak sfc
convergence into a very moist airmass. This is right along the
boundary of higher moisture in the mid levels (850-700mb). This
WAA in the mid levels remains a force to be reckoned with. So
the combo of the weak sfc convergence and WAA in the mid levels
rain/storms continue to develop. The inversion/capping across
southern Wisconsin is strong and this is likely to prevent any
storms from becoming sfc based. Storms will need to become sfc
based to get any stronger to severe storms. While chances are
low its non-zero. So if an isolated stronger storm does become
sfc based then gusty winds and hail will be the primary
concerns. Confidence on any stronger storms developing is very
low especially considering the strong capping. So what this
means is with the high mid level moisture and PWATs is that
these will be efficient rain producers with a few rumbles of
thunder. These storms are fairly transient as the storm motion
(both 4-6 km and 9-11km wind fields) are due westerly. This is
why storms that are developing in northern Iowa are traveling
all the way through southern Wisconsin. Well that and the
saturated and unstable environment (hot and humid). Rain chances
should start to come to an end when we begin to lose the diurnal
instability.

Heading into tonight with a stationary front draped across
Wisconsin, there will be additional chances for rain and
thunderstorms once the LLJ develops. The LLJ will provide more
forcing and organized convection during the overnight and early
morning hours. The LLJ is expected to set up across central and
far southwestern Wisconsin. This should keep shower and storm
development along and west of the stationary front. This means
that the far northwest corner of the forecast area is likely to
get clipped by rain and storms (since the stationary front is
going through the Dells). If the frontal boundary retreats north
a bit or sags further south then more of the CWA will be in
play. The only other things to keep an eye on overnight is if
any outflow boundary picks up steam and barrels through southern
Wisconsin. Given the warm and humid conditions any forcing in
the low levels will likely develop storms either thats a front,
outflow boundary or lake breeze. CAMS have come into more
agreement that the far northwest corner is all the really sees
rain tonight into Wednesday morning.

From here through Thursday night, the chances for rain will
continue to linger across southern Wisconsin. During the daytime
hours it will be a question of where the frontal boundary ends
up (which guidance suggests it sags south and bisects the
forecast area). This will provide the sfc convergence needed for
rain to percolate along the boundary. Storms will have to become
surface based to really get some good strong to severe storm
potential as shear will be good and so to will CAPE. The strong
inversion/capping will be the one obstacle to overcome. During
the overnight hours it will be the LLJ that becomes the focus
point for rain/storms. Rinse and repeat this process until
Friday. Confidence on location for this will be dependent on
where the LLJ and sfc front reside which varies quiet a bit. So
may be an instance where we need to see the whites of its eyes
before POPs change in any meaningful way (ie increasing for
those that get rain and decrease for those that dont versus the
widespread 10-30% chances).

Patterson

&&

.LONG TERM...
Issued 230 PM CDT Tue Sep 1 2026

Friday through Tuesday:

The warm and unsettled weather is expected to continue across
southern Wisconsin through the extended. With an upper level high
centered over the southern CONUS, south to southwest winds straight
out of the Gulf will remain through the extended. Therefore,
temperatures in the 80s and dewpoints in the upper 60s to 70s are
likely to continue. With that the high heat index is likely to
persist as well. With the sustained southwesterly flow, there will
be enough warm air and moisture for storms at any given time through
the extended. For Friday, there will be the combination of the LLJ
during the overnight hours with a lingering sfc boundary for day
time development. Similar to what we are seeing earlier in the week.

Once we get into the weekend the diurnal instability remains,
but there is a noticeable lack of sfc forcing. The LLJ shifts
north and west while the sfc frontal boundary sags to the south.
This will lead to more disorganized rain and storms chances.
Shear looks to weaken as well during this time which in turn
limits the potential for an pesky isolated stronger storms.
Realistically it should close the door on it, but if there is
enough diurnally driven instability then we don't want to count
out any single cell pop up thunderstorm just yet. A
thunderstorm that gets tall enough or water logged enough can
always cause some downburst type winds. But that should be very
few and far between, more an exception that a rule in the
environment we should have.

Looking further ahead into early next week there is a single aloft
that the ridge will weaken a bit and get undercut as a low pressure
system swings through Canada. If these trends continue its possible
that as the sfc low moves through the Hudson Bay area that a cold
front could move across the Great Lakes Region. All this is to say
is there is a signal for more organized rain/storms. The GFS by far
is much further south in comparison to the EURO and CAN run
(deterministic). With the uncertainty in the track and strength of
the low, we could end up with more of the same from the weekend
(unstable, hot and moist with no forcing). Something to keep an eye
on, but nothing of large concern at this point.

&&

.AVIATION...
Issued 700 PM CDT Tue Sep 1 2026

The instability needed to fuel a thunderstorm will remain
overhead through the entire TAF period, but the 00z TAFs remain
completely free of precip (except a PROB30 at KSBM) due to low
confidence in the existence / timing / placement of the
triggering mechanisms needed to "start up" the storms.

In the near term, our best confidence in storm development is
over central and east-central WI, mainly north of WI Highway 60,
including KDLL, KFLD, and KSBM. One or more waves of showers /
storms may move through this evening or late tonight. Very low
confidence in timing and placement.

Predominantly VFR cloud ceilings expected through the TAF
period. Patchy fog cannot be ruled out tonight into early
Wednesday morning, though the scattered cloud cover aloft ought
to keep fog potential slightly lower than last night.

Sheppard

&&

.MARINE...
Issued 230 PM CDT Tue Sep 1 2026

Fog and thunderstorms will be possible across Lake Michigan
through the next couple of days. A warm and humid air mass, over
the lake will keep the threat for fog overnight high, especially
across the northern third of the lake. While daytime heating
keeps the chances for pop up thunderstorms possible across the
southern two thirds. Widespread severe weather isn't expected,
though an isolated stronger cell with hail and gusty winds can
not be ruled out.

Light to moderate south winds today will shift to east-northeast
by mid to late this week as a high pressure around 30.1 inches
builds into the Hudson Bay. With a low pressure system around
29.6 inches moves into the Northern Plains, Wisconsin will be
trapped between these two system which will result in some
moderate winds at times. Winds are expected to remain below
small craft criteria.

Patterson

&&

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

$$

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