Janesville, Wisconsin 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

801
FXUS63 KMKX 191952
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
252 PM CDT Sat Jul 19 2025

.KEY MESSAGES...

- Isolated showers and thunderstorms are possible late this
  afternoon and evening along a cold front, especially across
  central Wisconsin. Storms are expected to produce gusty winds
  up to 30 mph and small hail.

- Chances for storms develop Tuesday night, and again Wednesday
  night into Thursday. Exact timing and intensity remains in
  question, with the most confidence in Thursday seeing
  widespread convection.

- Heat builds back into the region, with highs in the upper 80s
  to lower 90s Tuesday into the weekend. Heat index values are
  likely to rise as high as 100 degrees and may rise as high as
  105 degrees on Wednesday.

&&

.SHORT TERM...
Issued 253 PM CDT Sat Jul 19 2025

Tonight through Sunday night:

Thunderstorm intensity forecasts continue to trend downward for
this afternoon and evening`s cold front. Although the
atmosphere has been able to recover somewhat in southwestern to
central Wisconsin, the front remains quite diffuse and is
struggling to produce widespread convection. CAMs remain less
than enthusiastic about the presence of widespread convection as
well, lending confidence to the forecast of general
thunderstorms with gusts around 30 mph and some small hail.
MLCAPE output on CAMs is around 2000 J/kg for the afternoon, but
a very dry layer aloft will likely keep storms from realizing
the full potential of this instability. Expecting isolated to
widely scattered storms to progress southeastward through the
region late this afternoon into this evening (25-35 percent
chance). Storms will slowly exit to the southeast while
decaying late this evening, with dry conditions then expected
across the majority of the area.

Late tonight, an MCS will traverse central Iowa into northern
Illinois, again developing along the nose of the 850 mb LLJ.
The far northern edge of precipitation from this system may
nudge into far southern Wisconsin, but meso models continue to
trend farther south. Still, a few storms may cross the border
(15-25 percent chance) and bear watching for lightning.

Aside from lingering convection in far southern Wisconsin in the
morning, dry weather is expected Sunday with highs in the upper
70s to near 80 degrees. Gusty northeasterly winds may create a
moderate swim risk from Milwaukee southward at Lake Michigan
beaches during the morning hours. Breaking waves and currents
are expected. Stay away from dangerous areas like piers,
breakwalls, and river outlets.

Sunday night, the LLJ develops once again across southeastern
Iowa to northern Illinois, with yet another chance (15-25
percent) for a few storms to cross the Illinois border into far
southern Wisconsin, but as the LLJ continues to slowly sag
southeastward these chances diminish.

MH

&&

.LONG TERM...
Issued 253 PM CDT Sat Jul 19 2025

Monday through Saturday:

Dry conditions are expected throughout Monday and Monday night
underneath high pressure. Winds shift southeasterly into Tuesday
and low pressure develops in the lee of the Colorado Rockies and
builds ridging/WAA across the southeastern U.S. and Mississippi
Valley. Ridge-riding shortwaves begin to traverse the Upper
Midwest Tuesday night, with several 500 mb impulses through
Thursday from a semi-stationary 500 mb low in the lee of the
northern Canadian Rockies. The majority of these impulses will
primarily progress across northern to central Wisconsin (Tuesday
night, Wednesday evening/night), but still may produce
scattered storms across southern Wisconsin (15-30 percent
chance).

With continued WAA and a building upper level ridge, expecting
temperatures to rise into the upper 80s and lower 90s Tuesday
through the end of the week. Models continue to support high
heat and humidity, with Wednesday seeing the most intense
ridging. Heat index values are expected to rise above 100
degrees on Wednesday, and may rise above 105 degrees. Although
ridging begins to break down on Thursday, still expecting heat
and humidity to continue.

Ridging begins to break down on Thursday as the 500 mb low
progresses eastward through northern Canada, allowing for
enhanced MCS activity shifting southward into southern
Wisconsin. Although it is too early to tell specific timing or
intensity, Thursday afternoon into Thursday night sees the most
likely chances for widespread thunderstorms (~40 percent based
on ensembles, but trending upwards). Southern Wisconsin remains
on the northern edge of ridging through the weekend, with
precipitation chances (20-30 percent) lingering due to timing
differences in shortwaves from model to model.

MH

&&

.AVIATION...
Issued 253 PM CDT Sat Jul 19 2025

MVFR to VFR conditions expected to continue through the
remainder of today, with isolated to widely scattered
thunderstorms developing along a cold front in northern
Wisconsin expected to push southeastward through tonight. Gusts
up to 25 kt are possible underneath these storms. Current winds
are southwesterly, and are expected to shift to become
northwesterly behind the front. A weak low pressure system then
develops in Iowa tonight, lowering ceilings to 800-1000 ft as it
approaches and draws in cooler, more moist air off Lake
Michigan with northeasterly winds. Ceilings will rise and break
apart into late Sunday morning. In addition, chances (15 to 30
percent) for showers and storms develop across far southern
terminals early Sunday morning as low pressure pushes eastward
across Illinois.

MH

&&

.MARINE...
Issued 253 PM CDT Sat Jul 19 2025

Scattered thunderstorms will impact mainly the southern half of
Lake Michigan into the evening. Winds will become northerly
overnight as a cold front moves across the lake. High pressure
will move across Lake Superior on Sunday. Winds will shift to
the east on Monday as the high moves into Ontario.

Winds will increase to around 20 knots and become southeast on
Tuesday as low pressure approaches from the Central Plains.
There are chances for storms Wednesday through Friday along a
frontal boundary that sets up across the lake. South to
southwest winds will remain elevated Wednesday and Thursday
ahead of the front.

Marquardt

&&

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

$$

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