NWS-Milwaukee/Sullivan Forecast Discussion

						FXUS63 KMKX 160020
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
720 PM CDT Fri May 15 2026

.KEY MESSAGES...

- Confidence continues to increase in strong to severe
  thundestorms pushing across southern WI overnight tonight
  (after 10 PM CDT). Hail and damaging winds are the main threat
  for southwestern Wisconsin as the line moves in, with damaging
  winds becoming the dominant threat across south-central to
  southeastern Wisconsin, especially along the WI/IL border.

- Very warm and increasingly humid conditions through the
  weekend.

- Additional thunderstorm chances Saturday night through
  Tuesday. Storms may be strong to severe at times with the best
  potential later on Monday.

&&

.UPDATE...
Issued 720 PM CDT Fri May 15 2026

Scattered supercells have developed in northwest Iowa this
evening along a developing frontal feature. Additional, more
isolated, development has occurred in southern Iowa and eastern
Nebraska. These storms are expected to remain discrete through
the evening hours, potentially through the early overnight
hours as they are driven eastward under 50 kt of bulk shear. As
they impact southwestern Wisconsin (expected between 10 PM and
12 AM CDT), large hail and damaging winds remain possible under
~2000-2500 J/kg of MUCAPE. Around midnight tonight, an 850 mb
LLJ around 35-40 kt is expected to develop, with the nose into
south-central Wisconsin. This will serve to bring in MUCAPE
around 1500-2000 J/kg, and should also allow storms to gain a
more linear structure and transition to a wind threat. There
will be an inversion/moist adiabatic layer in the low levels
(~850 mb to the surface), but a strong enough rear inflow jet
will be able to overcome this concern and result in damaging
winds at the surface. Still expecting the worst conditions from
southwestern Wisconsin through the WI/IL border. Areas near
Lake Michigan may stay more stable and be largely gapped by
severe potential.

MH

&&

.SHORT TERM...
Issued 219 PM CDT Fri May 15 2026

Tonight through Saturday night:

Today is much warmer than previous days with temps in the 70s
this afternoon. The milder temps associated with the low-level
WAA will continue tonight along with milder dewpoints advecting
into southern WI as a weak area of low pressure along the cold
front draped across the regions lifts from NE into IA. This will
be the focus for the increasing potential for storm development
overnight as the upper- level trough moves across the Upper
Great Lakes Region situating the upper-level jet's right
entrance region over MN/WI. This upper-level forcing aligns with
a mid-level vorticity max as well as the low- level WAA and
band of frontogenesis as the front pushes eastward. Overall,
synoptic forcing and dynamics align over the region to support
increase thunderstorm chances.

The more favorable environment for severe storm development
remains back to the west across IA into southern MN this
afternoon/evening where higher instability (SBCAPE +2000 J/kg)
and daytime heating pairs with +40 kt of effective shear along
the cold front. However, hi-res models continue to project this
environment to shift more into southwestern WI later this
evening and tonight as a wedge of higher dewpoints push into the
western portions of the CWA. While the upstream activity across
IA gets going this evening, CAMs have it transitioning into a
line of storms as it pushes toward the Mississippi River Valley.
Although we lose a bit of instability (~1000 J/kg of MUCAPE)
with daytime heating and questions remain on how storms would
react to increasing capping inversion, there is ample deep layer
shear (+40 kt) on the nose of an increasing LLJ tonight that
continue to hint that stronger to severe thunderstorms remain
possible as the line pushes in overnight. Given the steep mid-
level lapse and linger MUCAPE values, large hail will be
possible, but given the more linear convection mode and steep
low-level lapse rates, gusty to damaging winds remain the main
concern tonight. 12z HREF members all hint at a potential to
see a bowing segment develop within the line as well and given
the RAP and NAM showing 0- 3km shear of +30kt from west to east,
could not rule out an embedded tornado or two with in the line
associated with any bowing features. However, will be dependent
on the LCLs which are progged to be around 1000m and if the
lower-level saturate more than models suggest at this time.

Nevertheless, there continues to be a a growing concern for
overnight strong to severe line of thundestorms bringing
damaging winds and some hail to southern WI. Timing will be as
early as 03- 05z (10pm-12am) for areas west of I-39, but more
likely between 05- 07z (12-2am) and pushing east through 08z-10z
(3-5am). Although the line seems like it may lose some momentum
as it approaches Lake Michigan, it still may have enough energy
to bring the severe threat that far east. Given the overnight
timing, be sure to have a way to receive warnings that may wake
you up about any potential severe threats.

Otherwise, may see some of the lingering showers behind the
line linger through daybreak Saturday before things dry out with
westerly winds behind the cold front. This front then looks to
stall south of the WI/IL border during the day Saturday. While
post-cold front, temps on Saturday will warm back up in the
upper 70s and lower 80s. Will need to keep an eye on the
placement of this front as it looks to be the focus for
additional shower and thunderstorm development later Saturday
evening/night as a weak shortwave trough sweeps through the
area. While the chances for development along this boundary
looks to stay mostly south of the Cheddar Curtain, there could
be a few scattered showers and thunderstorms Saturday evening,
but coverage remains limited at this time.

Wagner

&&

.LONG TERM...
Issued 219 PM CDT Fri May 15 2026

Sunday through Friday:

A strong shortwave trough will dig sewd from the Pacific NW to
the central Rockies for Sun-Sun nt, while upper ridging proceeds
over the ern USA and the central to ern Great Lakes. Shortwave
troughs within the swly flow aloft will be traversing through
the central Great Plains into the Upper MS River Valley and wrn
Great Lakes during this time. At the surface, low pressure over
wrn KS will develop into ern NE by 00Z Mon with the warm front
extending ewd into central WI. Thus low to mid level warm, moist
advection and a shortwave trough passage will bring 60-80
percent chances for showers and isold to sct thunderstorms for
Sun AM over south central WI with lesser chances to the east and
toward the IL border. Temps will warm drastically with the warm
frontal passage Sun afternoon and remain very mild Sun nt
within the warm sector.

The aforementioned wave of low pressure over ern NE will then
track to nw WI and Upper MI by 12Z Mon, while a decaying MCS
from the Great Plains may be arriving after 09Z Mon over south
central WI. Another wave of low pressure will develop from the
central Great Plains to the NE and IA border by 00Z Tue, while
the main shortwave trough tracks from the central Rockies to ern
SD and NE. Both features will then continue to lift newd to the
nrn Great Lakes and Ontario into Tue. The actual trailing cold
front will likely not pass until Tue, but there is some
confidence in rounds of showers and storms for Mon-Mon nt given
srn WI is downstream of areas of new convective initiation and
possible MCSs. The severe storm probabilities look very low for
Sun-Sun nt but increasing into Mon- Mon nt. The SVR probs will
then decline on Tue as the cold front will pass during the
morning into the early afternoon. High pressure and a cooler air
mass will then follow for Wed-Thu with lgt sly winds and warm
advection returning late Thu into Fri as the high moves ewd and
a shortwave trough approaches from the west. Thus high temps in
the 60s for Wed-Thu will return to the 70s well inland from Lake
MI for Fri.

Gehring

&&

.AVIATION...
Issued 720 PM CDT Fri May 15 2026

VFR conditions through this evening into tonight before a line
of strong thunderstorms progresses northwest to southeast
between 10 PM CDT and 3 AM CDT. Have included PROB30 groups for
stronger storm potential, with high confidence in at least
thunderstorms late tonight and a lower chance of gusty winds.
Even without stronger storm impacts, expecting LLWS as these
storms move in with a strong low-level jet overnight. Will
amend as needed as storms approach.

After thunderstorms exit, expecting lighter rain to linger for a
few hours and conditions to improve back to VFR. VFR conditions
and west-northwesterly winds are then expected throughout
Saturday. Winds become light and variable Saturday night.

MH

&&

.MARINE...
Issued 219 PM CDT Fri May 15 2026

Low pressure tracking across Ontario is bringing increasing
southerly winds across Lake Michigan this afternoon into the
evening. Still looking at a few gale force gusts to occur over
the northern third of Lake Michigan into this evening and the
Gale Warning remains in effect. Then a line of showers and
thunderstorms will push across the region overnight into early
Saturday morning along the low pressure's cold front. Looking at
the the front to bring more southwesterly winds into Saturday
as the front stalls south of Lake Michigan. Then another low
pressure is progged to develop over the Plains Sunday and lift
into the Upper Midwest into the start of next week. This will
bring increasing southerly winds and additional shower and
thunderstorm potential through Monday night/Tuesday. Then high
pressure looks to build in for midweek behind the early week low
pressure system.

In addition to today's gales, expect periods of small craft
conditions to develop across southern WI nearshore waters with
the stronger southerly winds this evening through the start of
next week.

Wagner

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...Gale Warning...LMZ261-LMZ362-LMZ364-LMZ563 until 1 AM Saturday.

     Small Craft Advisory...LMZ643-LMZ644-LMZ645-LMZ646 until 4 AM
     Saturday.

&&

$$

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