083
FXUS63 KMKX 181050
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
550 AM CDT Tue Aug 18 2026

.KEY MESSAGES...

- Chance of showers and storms today; coverage remains
  uncertain. A few stronger storms with hail and gusty winds are
  possible in the mid to late afternoon into the evening.

- Dry and pleasant conditions Wednesday through Thursday. Rain
  chances return Friday, but overall nice weather is expected
  for the weekend.

&&

.UPDATE...
Issued 550 AM CDT Tue Aug 18 2026

As expected seeing a decaying area of showers moving into the
area this morning. Areas along and southwest of I-90 will have
the better chances of seeing measurable rain from this round,
but will remain very light in nature given lack of instability
and synoptic forcing. Much of the area further east will remain
mostly dry, maybe see an isolated sprinkle or two through the
morning. Otherwise, trending drier late morning into the early
afternoon before additional shower and storm chances return late
afternoon and evening ahead of the cold front.

While chances increase this afternoon and evening for additional
development, some uncertainty remains in coverage, timing and
strength of this activity. Main driver of this will be the
upper-level shortwave and low-level moisture return.

00z global models have been bouncing around with the mid-level
vorticity max associated with a shortwave trough rounding the
base of the main trough so difficult to hone in where it will
set up this morning and into the afternoon.

Otherwise, need to keep an eye on the low-level moisture and
subsequently the instability. 00z-06z model PWATs have been
trending further south into IA/IL and barely topping off above
1.5 inches for southern WI. Thinking this can be attributed to
some models such as the 00z RRFS hinting at a dry layer between
900-700mb in the model soundings. Also another limiting factor
could be the ongoing convection over NE/IA as it has not been
handled well by this morning`s CAMs and maintaining it strength.
But this could limit the northward progression of the higher
dewpoints.

On the flip side, the 00z-06z CAMs such as the HRRR and ARW
favor seeing dewpoints climb back into mid and upper 60s and
allow for SBCAPE to build above 1000 J/kg. And given the
current surface obs this seems to be on trend. Thus the 00z HREF
min SBCAPE parameter still brings in up to 1000 J/kg of SBCAPE.
Pair this with the cold front sliding southward and some modest
deep layer shear (~30 kt) and should be enough for
thunderstorms to develop along the front later today. Also
looking to be enough to support a few stronger storms with this
activity, especially if development trends toward the early
evening window allowing for SBCAPE to build toward 2000J/kg
along with any localized mesoscale from nearby convection. If
any stronger storms develop main concern will be gusty winds
and potentially some hail.

Overall, while there are still some concerns with the upper-
level thermo, do think there is enough model evidence and
current trends to lean more into the HRRR solutions for
afternoon/evening round of storms. Thus, have bumped up PoPs
from the previous forecast.

Wagner

&&

.SHORT TERM...
Issued 1201 AM CDT Tue Aug 18 2026

Rest of Tonight through Wednesday:

The storms in MN this evening are focused along the nose of the
low level jet (LLJ). The jet will shift into WI overnight, veer
to the west, and have a weakening trend; thus the storms will
too. However, these diminishing showers could get a boost
Tuesday morning over south central WI as low-level, moist, warm
advection focuses over us. The RAP is showing a slight increase
in CAPE (up to 500 j/kg) and modest bulk shear up to 40 kt
during the morning hours (between 8 and noon). This would be
enough forcing to justify general thunder. Coverage of these
showers/storms is still uncertain due to them moving into an
already-dry environment.

A mid level shortwave trough and coincident right entrance
region of an upper jet that is currently over southeast Montana
will track into southern WI around midday Tuesday. This stronger
synoptic forcing will give us a better environment for
thunderstorms during the mid afternoon to early evening hours,
but there is a lack of focus for these storms. There will have
to be be very small-scale feature such as an outflow boundary
or MCV that plays a role in where these storms pop.

Depending on how much/little cloud cover lingers over southern
WI during the day, we could build up to 1500 j/kg CAPE with
continued bulk shear around 40 kt. This would be enough for a
few near-severe gusts or small hail. Since precipitable water
values are going to be increasing to around 1.5 inch by late
afternoon, heavy rain with any storms is likely.

The upper trough and slow-moving surface front will finally exit
southern WI by Wednesday morning. Expect sunshine with diurnal
cumulus clouds and highs in the upper 70s. Light winds under
high pressure will allow for a weak lake breeze as well. With
the dry air mass in place, Wed nt lows will drop into the mid
50s for inland areas.

Cronce

&&

.LONG TERM...
Issued 1201 AM CDT Tue Aug 18 2026

Thursday through Monday:

Northwest flow at 500 mb is generally expected across the Western
Great Lakes region during this period, with a digging shortwave
trough/low shifting through Lake Huron and southeast Ontario
Saturday night into Sunday. Another digging shortwave trough may
move southeast into the region early next week.

Ensembles suggest that high pressure at the surface Thursday will
push off to the southeast Thursday night into Friday, with low
pressure shifting east southeast through a similar path to the
digging shortwave trough later Friday/Friday night, then shifting
southeast on Saturday. This should bring a cold front southeast
through the area, with some low level warm air advection ahead of
it.

Forecast soundings are showing some tall skinny mean layer CAPE
developing ahead of the front Friday afternoon, though deep layer
bulk shear is modest at best. Ensemble members are generally
showing measurable precipitation later Friday into Friday night,
so will continue to mention chances for showers and thunderstorms
in the forecast. Some of this could linger into Saturday if the
frontal passage is a little slower, or the shortwave trough helps
bring some scattered showers behind it.

Ensembles suggest temperatures near seasonal normal values with
dew points into the 60s Friday into Friday night ahead of the
cold front. High pressure should then bring quiet weather for
Saturday night into Sunday night or Monday, depending on when the
next digging shortwave trough approaches the region from the
northwest.

Wood

&&

.AVIATION...
Issued 550 AM CDT Tue Aug 18 2026

A decaying line of showers are beginning to make their way into
southwestern WI this morning. Overall, may bring a light
shower/sprinkle to the southwest MSN and JVL along with some
mid-level clouds. Much of the morning should remain mostly dry
with sporadic showers and southwesterly winds into the
afternoon. Then looking at additional shower and thunderstorm
chances to develop mid-afternoon into the evening. Still some
questions in coverage, strength, and timing, but the potential
remains as the cold front slides southward through the day. Will
trend toward a later development toward early evening as the
previous TAF issuance. However, could see a few stronger storms
with this activity not only bringing lower flight conditions,
but also gusty winds and maybe even hail. This activity will
gradually diminish through the evening as the front slides
further south. Expect lighter northerly winds into Wednesday
behind the front and VFR conditions.

Wagner

&&

.MARINE...
Issued 1201 AM CDT Tue Aug 18 2026

A slow-moving low pressure trough of around 29.9 inches will slide
across Lake Michigan during the day Tuesday. Winds will be
predominantly westerly. A few storms are possible within the
vicinity of the trough/front through Tuesday night. An isolated
stronger storm with gusty winds and hail cannot be ruled out. High
pressure of 30.0 inches will move through the western Great Lakes
Wednesday through Thursday, with light and variable winds
prevailing across the waters. The next thunderstorm chances arrive
Friday afternoon and evening, when the next cold front is
expected to move across Lake Michigan.

Cronce

&&

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

$$

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