Mattoon, Wisconsin Weather Forecast Discussion

879
FXUS63 KGRB 181102
AFDGRB
Area Forecast Discussion
National Weather Service Green Bay WI
602 AM CDT Thu Sep 18 2025
Updated aviation portion for 12Z TAF issuance
.KEY MESSAGES...
- Clusters of thunderstorms are forecast to develop over western
  Wisconsin and may track into north-central Wisconsin late this
  afternoon and this evening. The risk for severe weather is low.
- Periodic chances of scattered showers and thunderstorms are
  expected from Friday into early next week. The severe weather
  threat will remain low.
- Conditions hazardous to small craft will be possible on Lake
  Michigan south of Two Rivers on Friday night and Saturday morning.
  There will also be an increased risk of rip currents on Lake
  Michigan beaches along this section of shoreline.
&&
.DISCUSSION...
Issued at 234 AM CDT Thu Sep 18 2025
Short Term...Today...Tonight...and Friday
An amplified pattern is in place across North America early this
morning, with a broad upper low situated over the northern Plains
between ridging over the central Great Lakes and western Canada.
At the surface, a shallow cold front is draped across far northern
Wisconsin. South of this boundary, temperatures remain warm, in
the mid to upper 60s across parts of central and east-central
Wisconsin. A deformation zone associated with the upper low is
streaming mid and high-level clouds across the northern portion of
the state. These clouds have limited fog development overnight,
despite light winds and recent rainfall of 1-2 inches in the
northwoods. The primary forecast challenge revolves around shower
and thunderstorm development as the front slowly moves south and
the upper low inches closer to the region.
The forecast for the next 30 hours is complex due to weak large-
scale forcing interacting with an anomalously moist airmass. A
precipitable water axis of 170-190% of normal will be nearly
stationary across far northern Wisconsin through Friday afternoon.
The associated cold front will gradually sag south, with a slight
increase in speed expected over northeast Wisconsin due to
developing northeast winds.
Thunderstorms: Widely scattered light rain showers are possible
this morning and into the afternoon, mainly over northern
Wisconsin, as moisture is lifted over the frontal boundary. There
is a lower probability of showers further south along the surface
front. By late afternoon, expect the development of thunderstorm
clusters over western Wisconsin where instability is greater, with
MUCAPE values of 1000-1500 J/kg. Some of these storms may track
into north-central Wisconsin this evening, but they are expected
to weaken as they encounter a more stable environment.
Consequently, the risk for severe weather is low.
Later tonight, decaying showers could move into areas west of
I-39/US 51 after midnight, with chances less than 30%, as a
shortwave trough moves into the northern Mississippi Valley. A
better chance for rain is anticipated for Friday afternoon as
southerly flow increases over the stalled front and another
shortwave moves across southwest Wisconsin. Some thunderstorms
will be possible over central Wisconsin late Friday afternoon, but
with modest instability (MUCAPE up to 800 J/kg), the severe
threat remains low.
Long Term...Friday Night Through Wednesday
The extended forecast period will be characterized by a slow-
moving upper-level trough over the northern Plains and Upper
Midwest. This feature will be the primary driver for periodic
chances of scattered showers and thunderstorms this weekend and
potentially into early next week.
Thunderstorms: A second round of showers and storms is expected
Friday night as a piece of energy rotates out of the main trough
and lifts northeastward. With some minor elevated instability
(200-400 J/kg) present, a slight chance of thunderstorms will
persist through Saturday. There is uncertainty in how subsequent
shortwaves within the trough will evolve. Current model guidance
suggests another wave of energy will cross the region on Sunday
and Sunday night. Instability is forecast to be slightly higher on
Sunday, with most unstable CAPE values ranging from 400-800 J/kg.
If cloud cover and precipitation do not inhibit daytime heating,
instability could exceed 1000 J/kg. This suggests that Sunday may
have a higher probability of showers and thunderstorms compared to
Saturday. The overall risk for severe weather remains low through
the weekend due to weak instability and deep layer shear.
Looking ahead to early next week, a potent trough is forecast to
dig into the central Plains by Monday or Tuesday. This system is
now projected to have less of an impact on our weather than
previously anticipated. Confidence in the forecast details for
Monday and Tuesday is lower due to increasing spread among model
solutions. Some guidance suggests the weekend upper trough will
linger, while ensemble solutions push it northeast of the area on
Monday. Despite the chances for precipitation, temperatures at
850mb are expected to remain well above normal, leading to surface
temperatures that are near to slightly above normal for the
period.
Marine: For marine interests, breezy southeast winds of up to 20
knots are expected to create hazardous conditions for small craft
on Lake Michigan, south of Two Rivers, late Friday night through
Saturday morning. Waves of 3 to 5 feet will be possible. This has
been a consistent signal in the models, and the potential for a
Small Craft Advisory and a Beach Hazard Statement will continue to
be monitored.
&&
.AVIATION...for 12Z TAF Issuance
Issued at 602 AM CDT Thu Sep 18 2025
Scattered showers have developed over the past 1-2 hours across
areas generally north of HWY 29, which have already impacted RHI.
Favorable conditions exist for widely scattered to scattered
showers to develop this morning. Probabilities are highest at RHI,
but at about 20% or less. Have therefore refrained from adding
prob30s or tempos with the 12z issuance, but trends will have to
be monitored. Have also backed off on MVFR/IFR visibilities as
clouds and winds have kept ground fog very patchy.
Otherwise, broken mid-level clouds are forecast across the area
today. Some of the convective allowing models (CAMS) develop
clusters of showers and storms late this afternoon into this
evening, primarily over north-central WI. Confidence on
occurrence, timing, and location is generally low, so have
refrained from adding a prob30 at RHI. Trends will be monitored.
Statistical and numerical models develop low end MVFR or IFR
ceilings across many of the TAF sites after 06z tonight. Trended
down with ceilings, but kept ceilings at MVFR for starters. Later
shifts can lower ceilings if models continue to show this trend.
&&
.GRB WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
DISCUSSION.....MPC
AVIATION.......MPC

NWS GRB Office Area Forecast Discussion