Janesville, Wisconsin 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

606
FXUS63 KMKX 220900
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
400 AM CDT Fri Aug 22 2025

.KEY MESSAGES...

- Fog/low stratus potential remains through early this morning,
  especially west of Interstate 90. Patchy dense fog may occur.

- Next chance of showers/a few storms is tonight into early
  Saturday morning with a cold front.

- Much cooler, less humid, and breezy from later Saturday into
  early next week.

&&

.SHORT TERM...
Issued 400 AM CDT Fri Aug 22 2025

Today and Tonight:

Some upper level clouds have been slow to mix out and skies may
never truly become clear early this morning as addition circus
streams in from the northwest. This cloud cover will sparse
and/or thin in areas is doing just enough to insulate that fog
is struggling to develop across southern Wisconsin. Primarily
areas west and south of Madison are those that developed fog
prior to the upper level clouds moving in and thus fog remains.
Low lying areas such as river valleys and marsh lands will be
the most susceptible. Temperatures will continue to slowly
decrease bringing the dewpoint depression values closer and
closer to zero, which will promote an expansion in patchy fog.
Its likely that the upper level clouds will just delay this
development and will potentially shield some areas. This cloud
cover should also prevent/slow down the development of patchy
dense fog which does remain likely for areas south and west of
Madison. Any fog and subsequent low stratus will burn off
shortly after sunrise.

Any lingering stratus should rise and dissipate overtime late
this morning and into the afternoon. Light southwest winds are
expected today ahead of an approaching cold front. High
temperature are expected to climb into the upper 70s to low
80s. The cold front is expected to gradually move southeast
throughout the day and should move through southern Wisconsin
tonight into Saturday morning. The looks to largely hold off
until after midnight. A few CAMs, like the NAM and WRF, continue
to overdue the rainfall a bit. Many of these hold outs have
come down in the most recent model runs which has resulted in
some slightly lower POPS of 15-35%. Even with the more
aggressive of the models rainfall seems scattered with central
Wisconsin having a better potential for rain than far southern.
Forcing along the frontal boundary looks fairly weak and
instability is expected to be pretty low (MUCAPE around 400-500
j/kg). Which should be more than enough with even a little CIN
to get a few claps of thunder and some lightning, but overall
rain and storms along the frontal boundary look fairly weak.
Unless the more aggressive models (which are few in number)
become reality there are many people who may not see any rain as
showers would be widely scattered to isolated.

Showers chances linger into Saturday morning around 10-15% for
far southeastern Wisconsin. Once the front has passed, steady
cold air advection will be underway. West northwest winds will
develop and the high temperatures will drop to around the low to
mid 70s Saturday. Cooler temperatures are expected as we head
into the extended.

Patterson

&&

.LONG TERM...
Issued 400 AM CDT Fri Aug 22 2025

Saturday night through Thursday:

As low pressure occludes in the Hudson Bay Saturday night, expect
northwesterly breezes to continue to bring a cooler airmass to
southern Wisconsin. Lows in the lower 50s are expected. A few
shortwaves may still try to rotate through the 500 mb flow over
the course of Sunday and Monday, but modeling currently shows a
lack of midlevel moisture and therefore a lack of precipitation
potential; favoring general cloud cover rather than showers within
the northwesterly flow regime.

Highs in the mid to upper 60s and lows in the upper 40s to lower
50s are expected each day through Tuesday, with a recovery to
highs in the 70s and lows in the 50s Wednesday and Thursday as low
pressure finally exits to the east and ridging from the Plains
builds into the Great Lakes region.

MH

&&

.AVIATION...
Issued 400 AM CDT Fri Aug 22 2025

Fog has begun to develop mainly west and south of Madison. Fog
is expected to expand across southern Wisconsin overnight
leading to visibility around 3-5 SM. Patchy dense fog, primarily
from JVL to MSN and west, will be possible. The sparse (FEW to
SCT) upper level clouds has prevented fog from spreading across
southern Wisconsin quickly and will likely prevent the spread
of patchy dense outside of river valleys. Fog and any
subsequent low stratus should dissipate shortly after sunrise.

Light and variable winds will become southwesterly by mid to
late morning ahead of an approach cold front. The cold front is
expected to pass tonight into early Saturday morning. Scatted to
isolated showers and a few storms will be possible as the front
passes. Winds will shift to the west northwest behind the front.

Patterson

&&

.MARINE...
Issued 400 AM CDT Fri Aug 22 2025

High pressure will linger over the Great Lakes Region and begin
weakening today. Light and variable winds early this morning
will become southwesterly by mid to late morning. The high will
continue to weaken and push east overtime as a low pressure
system in northeastern Ontario strengthens/deepens. Winds will
increase ahead of an approaching cold front this afternoon. The
cold front will move southeast across the lake tonight into
Saturday morning. Scattered showers and a few thunderstorms are
possible along the front. Winds will shift to west northwest
behind the front and remain brisk Saturday through Monday as the
low pressure in Ontario continues to deepen. These gusts may
reach Small Craft Advisory levels at times during this period
for the nearshore waters.

As temperatures drop behind the cold front through the weekend,
delta T values will grow and the convective cloud depth looks to
increase over the northern half of the lake. Both of these
values increasing and the prescience of a deep upper level low
to the northern will lead to the potential for some waterspouts
both Sunday and Monday.

Patterson

&&

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

$$

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