Coleman, Wisconsin Weather Forecast Discussion

638
FXUS63 KGRB 192343
AFDGRB
Area Forecast Discussion
National Weather Service Green Bay WI
643 PM CDT Fri Sep 19 2025
Updated aviation portion for 00Z TAF issuance
.KEY MESSAGES...
- Multiple rounds of showers and thunderstorms are expected this
  afternoon through the weekend. Isolated strong storms with gusty
  winds and small hail will be possible during this time.
- Heavy rain and localized flooding will be possible with any
  shower or storm activity this weekend. The greatest chances for
  heavier rain will be over central Wisconsin on Saturday.
- Breezy southeast winds will result in hazardous marine and beach
  conditions south of Sturgeon Bay late tonight through early
  Saturday afternoon.
&&
.DISCUSSION...
Issued at 249 PM CDT Fri Sep 19 2025
Short Term...Tonight and Saturday
This afternoon... GOES water vapor imagery shows driving upper-
level trough currently residing over the northern Plains. Low-
level moisture transport continues to flow out ahead of a quasi-
stationary boundary to the southwest as instability builds across
our southern tiers of counties. As a result, additional storm
activity is expected to start popping up in central Wisconsin
within the next hour or two. Showers over portions of central to
north-central Wisconsin have a history of producing heavy rain as
of early this afternoon.
Forecast through Saturday remains low predictability as CAMs
struggle to get a hold on the convoluted synoptic pattern. Looking
at the big picture, the 500 mb wave train ramps up heading into
the weekend, and is forecast to interact with plentiful moisture
aloft. Result will be several rounds of rain/storms across
northeast Wisconsin through Saturday evening. However, given the
complex nature of the current pattern paired with poor CAM
performance, would feel low confidence in adding any finer details
to the forecast.
Rain/storm chances... Scattered showers and storms remain possible
through the remainder of the afternoon into this evening as
instability continues to nose into central Wisconsin. Main
concern will be pockets of heavy rainfall as low pressure over
northeast South Dakota ushers in a conveyor belt of PWATs
approaching 200% of normal. Heavy rain/flooding threat would be
further evidenced by light winds below around 700 mb, which would
be conducive to training/slow-moving cells. Showers and storms
then spread north and east overnight into Saturday morning as more
robust shortwave energy pivots over the upper Mississippi Valley.
Some CAMs suggest that there could be a brief lull in activity
later Saturday morning before more diurnally-driven storms pop
along the instability gradient (~1,000 to 1,200 J/kg MUCAPE) early
Saturday afternoon. Heavy rain and localized flooding would once
again be the main concern with these storms, though any stronger
cells could produce gusty winds and small hail. This being said,
deep layer shear below 20 knots should prevent any organization of
stronger storms. Greatest risk for heavy rainfall remains over
central Wisconsin.
Marine hazards... Increasing southeast winds and waves building 3
to 5 ft will result in conditions hazardous to small craft along
Lake Michigan nearshore waters on Saturday. As such, there will
also be increased potential for rip currents along Lake Michigan
beaches in Kewaunee and Manitowoc counties. Marine and Beach
Hazards headlines have been issued, and small craft are encouraged
to exercise caution on the Bay late this afternoon into early
Saturday morning.
Long Term...Saturday Night Through Friday
Unsettled weather continues Saturday night into early next week as
a slow moving upper trough moves across the Upper Midwest and
Great Lakes. This will keep periodic rain chances (30-70%) in the
forecast as shortwaves rotate through the upper trough. Rain
chances will decrease Saturday night as one of the shortwaves
exits, with another one forecast to arrive Sunday afternoon/evening,
with the showers once again becoming more widespread. A couple
more shortwaves, along with a weak frontal boundary, are forecast
to move across the area Sunday night into Monday, continuing the
chances for rain. Thunder chances will be limited due to cloud
cover and marginal instability and deep layer shear. But with up
to ~1000 J/kg of MUCAPE, isolated storms are expected at times. A
couple could be on the stronger side, but the risk for severe
weather is low. The showers and storms will also bring an isolated
flooding threat into early next week. PWATs will be running
between 1.0-1.30" (125-175% of normal), so any training cells
could pose a flood risk, especially for any areas that receive the
heavier rainfall today and Saturday. But with the longer range
ensemble guidance still showing under 20% chances of 1"+ in any
one 24-hour period, along with breaks in the heavier bouts of
precip, the overall flood threat will be low.
The lingering trough could keep some lighter showers in the area
on Tuesday, especially south, then a drier and less unsettled
stretch of weather is expected during the middle of next week as
an upper low becomes cut-off just south of the Great Lakes. Some
disagreement how long it will be far enough to our south to keep
the area dry, but should get at least a day or two of dry
conditions. Then, as the low drifts north/east, chances for
showers and a few storms returns to the region late in the week
and into next weekend. Temps through the period look to be near or
a little above average.
&&
.AVIATION...for 00Z TAF Issuance
Issued at 642 PM CDT Fri Sep 19 2025
Main aviation concern will be the ongoing active weather in the
region as showers and storms cross the region this evening,
tonight, and again on Saturday.
This evening and for the first half of the night, active appears
limited to central Wisconsin, bringing MVFR to IFR ceilings and a
few rumbles of thunder to AUW and CWA. For the Fox Valley, went
dry during this period, but kept a PROB30 for after midnight as
some models do suggest some showers for Saturday morning. The
added moisture will likely develop a low cloud deck for most
locations by Saturday morning, so kept IFR conditions in place for
the time being.
Rain will not be constant, with intermittent breaks at any one
location, especially on Saturday. Predictability for a longer dry
break is still uncertain, but would expect it to be a window in
the late morning to early/mid afternoon from west to east before
widespread active weather resumes Saturday evening. Thunder will
could return towards the end of the TAF period as well.
&&
.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement from 1 AM CDT Saturday through Saturday
afternoon for WIZ040-050.
&&
$$
DISCUSSION.....Goodin/Bersch
AVIATION.......Uhlmann

NWS GRB Office Area Forecast Discussion