Phlox, Wisconsin Weather Forecast Discussion

204
FXUS63 KGRB 201738
AFDGRB
Area Forecast Discussion
National Weather Service Green Bay WI
1238 PM CDT Sat Sep 20 2025
Updated aviation portion for 18Z TAF issuance
.KEY MESSAGES...
- Multiple rounds of showers and thunderstorms are expected
  through the weekend. Isolated strong storms with gusty winds,
  small hail, and heavy rain will be possible at times.
- Breezy southeast winds will result in hazardous marine and beach
  conditions south of Sturgeon Bay late tonight through early
  Saturday afternoon.
&&
.DISCUSSION...
Issued at 217 AM CDT Sat Sep 20 2025
SHORT TERM (This morning through Sunday morning)
Broad warm air advection and a short-wave trough are driving
scattered showers and thunderstorms across the region this morning
ahead of a slow-moving low-pressure system centered over western
Minnesota. A few of these storms are producing fairly heavy
rainfall. Expect this scattered activity to continue through the
morning, however, there will also be lulls in the precipitation at
times.
Thunderstorms: An arc of stronger thunderstorms is possible this
afternoon and evening. A narrow band of MUCAPE values are expected
to increase to 1000-1200 J/kg ahead of a surface front lifting
into central Wisconsin. HREF 4-hr max updrafts indicates that
areas south of Highway 29 have the best chance to seeing stronger
storms this afternoon. The primary hazard with any stronger storms
would be hail, up to quarter sized. Deep layer shear is expected
to be weak, less than 25 knots, which suggests storms will be
pulse. Convective activity is expected to wane after 9 PM this
evening; however, with ample cyclonic lift, showers and weak
storms may linger into early Sunday morning.
Flooding: With Precipitable Water (PWAT) values near 1.3 to 1.4
inches, which are above the 90th percentile for this time of year,
there is a risk for pockets of heavy rain. Warm cloud depths are
also over 10,000 feet, which promotes efficient warm rain
processes and could lead to high rainfall rates (1-2 inches/hour).
Slow storm motions and potential training of storms may also
contribute to an increased risk for minor urban/street flooding
for areas that see multiple rounds of rain today. Some areas of
central WI have already received 1-3 inches of rain from
yesterday/overnights storms.
Marine Hazards: Increasing southeast winds and waves building to
3 to 5 feet will result in conditions hazardous to small craft
along Lake Michigan nearshore waters today. These marine and beach
hazard headlines are in effect through this afternoon. The
increased potential for rip currents exists along Lake Michigan
beaches in Kewaunee and Manitowoc counties.
LONG TERM (Sunday through next Saturday)
After a potential lull in precipitation on Sunday morning,
ensemble guidance suggests another short-wave and surface boundary
will move across Wisconsin, bringing showers and potential
thunderstorms Sunday afternoon into Monday. Instability is
expected to be less than Saturday, with MUCAPE values peaking
around 800 J/kg, which is sufficient for scattered thunderstorms
but greatly limits severe weather potential. The primary threat
with any storms will be isolated instances of locally heavy
rainfall due to slow-moving or training cells.
By the middle of next week, ensemble height anomalies indicate a
ridge will develop over central Canada while a low-pressure system
forms over the Ozarks. This pattern will help funnel drier, more
stable air into the region, shunting most of the precipitation
chances south of the area through Thursday. As a result, we expect
a stretch of generally dry conditions and more seasonable
temperatures.
Toward the end of the week, the low-pressure system over the
Ozark Plateau is forecast to lift northeastward toward the eastern
Great Lakes. This will increase rain chances (15-20%) for eastern
Wisconsin late in the week and into the weekend. While showers
will be possible, the greatest instability is expected to remain
well to our south, significantly limiting the risk for any severe
weather.
&&
.AVIATION...for 18Z TAF Issuance
Issued at 1224 PM CDT Sat Sep 20 2025
Latest model and radar trends support only isolated showers and
storms (20%) through 20-21z, then become scattered to numerous
(40-60%) during the remainder of the afternoon into the evening
before diminishing during the late evening and overnight hours
(15-30%). Scattered showers and storms (30-50%) are again
expected on Sunday. For later tonight into Sunday morning, IFR or
low CIGS are likely (60-74%) at KRHI/KAUW/KCWA. Still a lot of
uncertainty of CIGS at KGRB/KATW/KAUW due to model guidance
suggesting VFR/MVFR category. Decided to put a tempo group for
IFR or lower CIGS at these 3 sites. Also, areas of fog are
expected overnight. HREF probabilities are 30-60% for visibilities
of 1 mile or less north and west of the Fox Valley. Have not gone
that low in the 18z TAFS, but will pass on concerns to the next
shift that may need to lower visibilities in the KRHI/KAUW/KCWA
TAFS.
&&
.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement until 4 PM CDT this afternoon for WIZ040-
050.
&&
$$
DISCUSSION.....GK
AVIATION.......Eckberg

NWS GRB Office Area Forecast Discussion