Flash Flood and Hail in Wisconsin, April 18, 2025

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
312 AM CDT Fri Apr 18 2025

.KEY MESSAGES...

- Morning round of strong to severe thunderstorms with large
  hail potential between 5am - 10am CDT.

- Lull in thunderstorm activity late this morning/early
  afternoon. Then additional thunderstorm development later this
  afternoon/evening ahead/along the cold front. Will be another
  window for large hail and damaging winds across southeastern
  WI.

- Quieter and cooler for Saturday.

- High chance (70-90%) of rain Sunday afternoon into early Monday

- Cooler Sunday and Monday, warming back to near or above
  normal temperatures by mid-week

&&

.SHORT TERM...
Issued 307 AM CDT Fri Apr 18 2025

Today through Saturday night:

Will continue to see scattered off an on showers with a few
rumbles of thunder early this morning across southern WI. The
next window for showers and storms will spread in around day
break and continue into the the mid morning hours. Monitoring
the elevated convection that developed upstream around the
NE/SD/IA border and progressing eastward, which is being driven
by the 850mb WAA/frontogenesis along the northern fringes of the
weak surface low feature. Following the trends of the latest
HRRR runs, expecting this activity to move into southern WI
later this morning. As the activity treks eastward toward our
neck of the woods, thinking they will remain elevated as they
tap into the MUCAPE around 1000 J/kg and effective shear of.
greater than 40 kt with the main hazard being large hail. Thus
cannot rule out a few warnings being needed between 5-10AM CDT
this morning across southern WI.

Then expecting a bit of lull after the morning round of
showers/ elevated storms. Environment looks to take a bit to
recover as the weak surface low disturbance lifts across
northwest MO/eastern IA into WI by the early afternoon.
Accompanying it looks to be a secondary warm front associated
with the higher surface moisture where dewpoints look to climb
into the 50s and even lower 60s. This feature is progged to lift
into at least the southern portion of the area before the cold
front pushes through. The main question will be timing and
alignment of the warm sector in WI. This will factor into how
the environment recovers for late afternoon/evening convective
development. May see a brief window for more isolated cells as
things develop, but the front-parallel shear favors rapid
upscale growth of any convection into a linear mode ahead/along
the cold front. However, given the limited synoptic
forcing/upper-level dynamics support still lends way to
uncertainty on the exact timing and placement of storm
development. Nevertheless, there remains enough of a consensus
for increased PoP chances and another window for storms to
develop late this afternoon/evening. Continue to see
southeastern WI favored for any strong to severe development
that is more surface based. The main concern will be large hail
and damaging winds through the evening, but once the cold front
pushes through thunderstorm chances dwindle later this evening.

While our main focus is with the activity ahead and along the
cold front toward the evening, it is worth noting that if there
is a long enough lull an isolated storm or two could develop
well out ahead (potentially on a remnant outflow/boundary) in
the warm sector. The potential remains low as only the recent
06z HRRR picks up on this scenario at the moment, but if it was
to materialize then said storm could bring not only large hail
and damaging wind threat, but could not rule out a brief
tornado. Again the chances of this remain on the low side, but
it is not out of the realm of possibility.

Quieter and cooler weather is expected behind the cold front
for Saturday as high pressure works its way across the Upper
Midwest.

Wagner

&&

.LONG TERM...
Issued 307 AM CDT Fri Apr 18 2025

Sunday through Thursday:

Deterministic models are in pretty good agreement that a
strong, negatively tilted shortwave will lift through Wisconsin
later Sunday into Monday. The resultant surface low is expected
to lift northeast through southern Wisconsin Sunday night into
Monday. At the moment, there is very good agreement among models
with this surface low track and timing. Strong warm air
advection and theta-e advection ahead of this low is expected to
kick off widespread showers by Sunday afternoon, with showers
continuing through at least the first half of Sunday night as
the surface low lifts through. GFS and ECMWF ensembles have a
very high (90%+) chance for measurable precip over a 24 hour
period during this time, so confidence is increasing in a round
of wet weather late this weekend. Given the anticipated low
track, much of the milder and unstable airmass should remain
south of Wisconsin, though wouldn't be surprised if thunder
chances sneak into the southeast forecast area.

Temps on Sunday will likely warm towards 50 in the morning,
before cooling back down once the rain moves in. Breezy easterly
winds ahead of the low will aid in keeping it cooler the
remainder of the day as well.

A little precip may linger into Monday morning as the shortwave
clears the forecast area, with mostly cloudy skies likely
hanging on through the afternoon hours. There is some
uncertainty with temperatures on Monday, as models vary a bit
with the timing of the cold front early in the day. If the front
clears the area by early to mid morning, temps will likely
struggle to get out of the 40s. If the front is slower, 50s may
be possible into at least early afternoon.

High pressure is expected to briefly slide into the area Monday
night, but will be exiting during the day Tuesday as a weak
shortwave pushes in from the west. Tuesday should be mainly dry,
with a chance for showers returning anywhere from late
afternoon to late evening, depending on the timing of the
shortwave. It looks milder on Tuesday, with temps back to near
or above normal.

It looks mainly dry Wednesday and Thursday, but the pattern is
open to shortwaves moving through during this period, so low end
precip chances will continue through mid-week. Near to above
normal temps are likely to continue for Wed and Thu.

DDV

&&

.AVIATION...
Issued 307 AM CDT Fri Apr 18 2025

Gusty southerly winds linger early this morning. Seeing a bit
of a break in the clouds as well. However, monitoring the
upstream storms across northern IA racing eastward toward
southern WI. Expecting this to bring a round of showers and
storms along with lower ceilings and visibility along with
potentially some hail across southern WI between 10z-14z. This
activity looks to exit the area mid to late morning. With the
lull expecting VFR conditions to return through the start of
the afternoon, then expecting an additional round of showers
and thunderstorms to redevelop later this afternoon and evening
ahead/along of a cold front. There will be a potential for
strong to severe storms across southeastern WI with this
activity, thus another window for lower ceilings and visibility
with it as well. Otherwise the cold front looks to push through
later this evening and shower chances diminish, but lower
flight conditions may linger behind the front into early
Saturday morning. However, looking for lighter northerly winds
into Saturday as high pressure build into the region.

Wagner

&&

.MARINE...
Issued 307 AM CDT Fri Apr 18 2025

Breezy southerly winds persist as low pressure continues
northeastward from IA into WI and then crossing central portions
of the Lake this afternoon. Expect to see small craft
conditions across southern WI nearshore zones today. A cold
front will extend southwest from the low crossing the southern
half of lake later this afternoon and evening. Modest south
winds over Lake Michigan today with the strongest towards the
southern third will veer northwest through this evening
(beginning with the northern lake, then the southern half as the
front passes). There are chances for thunderstorms along with
some hail and strong winds across southern half of the Lake
later today into tonight with the cold front. Modest northwest
winds continue behind the cold front overnight followed by
lighter north winds Saturday, as high pressure builds into the
Great Lakes region.

Wagner

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...Small Craft Advisory...LMZ644-LMZ645-LMZ646 until 7 PM Friday.

     Small Craft Advisory...LMZ643 until 4 PM Friday.

&&

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Hazardous Weather Outlook
National Weather Service Milwaukee/Sullivan WI
223 AM CDT Fri Apr 18 2025

WIZ046-047-051-052-056>060-062>072-190730-
Marquette-Green Lake-Fond du Lac-Sheboygan-Sauk-Columbia-Dodge-
Washington-Ozaukee-Iowa-Dane-Jefferson-Waukesha-Milwaukee-
Lafayette-Green-Rock-Walworth-Racine-Kenosha-
223 AM CDT Fri Apr 18 2025

This Hazardous Weather Outlook is for portions of east central
Wisconsin, south central Wisconsin and southeast Wisconsin.

.DAY ONE...Today and Tonight

Thunderstorms are expected early this morning and possibly again
later this afternoon into the evening. Large hail and damaging
winds are the main concerns if a few storms reach severe levels.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

Isolated thunder will be possible with widespread rain showers
Sunday evening into Sunday night.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed today.

LMZ643>646-190730-
Sheboygan to Port Washington WI-
Port Washington to North Point Light WI-
North Point Light to Wind Point WI-
Wind Point WI to Winthrop Harbor IL-
223 AM CDT Fri Apr 18 2025

This Hazardous Weather Outlook is for the nearshore waters from
Sheboygan Wisconsin to Winthrop Harbor Illinois.

.DAY ONE...Today and Tonight

A Small Craft Advisory remains in effect through the day today due
to breezy southerly winds and high waves.

There is a chance for thunderstorms today, a few of which may be
strong to severe.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

Small Craft Advisory conditions will be possible again on Monday.

LMZ080-261-362-364-366-563-565-567-669-671-673-675-777-779-868-
870-872-874-876-878-190730-
Lake Michigan Michigan City IN to St. Joseph MI 5 NM offshore to
mid-line of lake.-
Lake Michigan from Seul Choix Point to Rock Island Passage 5NM
offshore to Mid Lake-
Lake Michigan South of a line from Seul Choix Point to the
Mackinac Bridge and North of a line from Charlevoix MI to South
Fox Island 5NM offshore-
Lake Michigan from Charlevoix to Point Betsie MI 5NM Offshore to
mid lake-
Lake Michigan from Point Betsie to Manistee MI 5NM offshore to
Mid Lake-
Lake Michigan from Rock Island Passage to Sturgeon Bay WI 5NM
offshore to mid lake-
Lake Michigan from Sturgeon Bay to Two Rivers WI 5NM offshore to
Mid Lake-
Lake Michigan from Two Rivers to Sheboygan WI 5NM offshore to Mid
Lake-
Lake Michigan from Sheboygan to Port Washington WI 5NM offshore
to Mid Lake-
Lake Michigan from Port Washington to North Point Light WI 5NM
offshore to Mid Lake-
Lake Michigan from North Point Light to Wind Point WI 5NM
offshore to Mid Lake-
Lake Michigan from Wind Point WI to Winthrop Harbor IL 5NM
offshore to Mid Lake-
Lake Michigan from Winthrop Harbor to Wilmette Harbor IL 5NM
offshore to Mid Lake-
Lake Michigan from Wilmette Harbor to Michigan City in 5NM
offshore to Mid Lake-
Lake Michigan from Pentwater to Manistee MI 5NM offshore to Mid
Lake-
Lake Michigan from Whitehall to Pentwater MI 5NM offshore to Mid
Lake-
Lake Michigan from Grand Haven to Whitehall MI 5NM offshore to
Mid Lake-
Lake Michigan from Holland to Grand Haven MI 5NM offshore to Mid
Lake-
Lake Michigan from South Haven to Holland MI 5NM offshore to Mid
lake-
Lake Michigan from St Joseph to South Haven MI 5NM offshore to
Mid Lake-
223 AM CDT Fri Apr 18 2025

This Hazardous Weather Outlook is for the open waters of Lake Michigan.

.DAY ONE...Today and Tonight

Thunderstorms are expected at times today into this evening. A few
storms may approach severe levels, particularly across the
southern half of the lake.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

Gusts to 30 knots are possible on Monday.

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