National Weather Service Raw Text Product

392 
FXUS63 KGRB 171946
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
246 PM CDT Tue Jun 17 2025

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- Moderate, heavy at times, rain is expected Wednesday morning
  through late afternoon for eastern WI. A few embedded
  thunderstorms are also possible, risk for severe storms is low.

- Thunderstorms will be possible at times from Thursday through
  Saturday. Best window for severe thunderstorms is Thursday
  afternoon and evening, then again Friday evening into Saturday
  morning.

- Hottest temperatres of the year are forecast Saturday and
  Sunday. Heat index values may reach the middle 90s to around 100
  degrees each afternoon.

&&

.DISCUSSION...
Issued at 246 PM CDT Tue Jun 17 2025

Rest of Today and This Evening...

Dry conditions are expected to prevail for the remainder of today as 
a stalled frontal boundary near the WI/IL border keeps the chance for 
thunderstorms south of the region. As the shortwave that is driving 
these thunderstorm chances lifts further north into NE IA this 
evening there is a chance a few showers could push into the central 
WI around midnight, but by this time any linger instability becomes 
weak. 

Moderate to Heavy Rain Likely and a Few Storms Possible Wednesday...

Showers will become more widespread Wednesday morning across central 
and east-central WI as a surface low strengthening over far SE WI with 
the help of a developing mid-level jet. There is still some 
variation amongst the CAMs with the exact track of this low which 
keeps confidence relatively low in how much rain any location may 
see. That said with the region likely residing on the northwest side 
of the low a narrow band of heavier rain is possible within the 
deformation zone. Rainfall totals within this zone may range from 1 
to 2". There is some minor concern this could cause some urban or 
low-land flooding, particularly in more flood prone areas in the Fox 
Valley. The other caveat with rainfall amounts is if there are any 
embedded thunderstorms. MUCAPE is not partiucluar impressive ~ 500-
750 J/kg, however, there is about 30kts of deep layer shear 
extending east to the lakeshore from a Green Bay to Appleton line. 
Should any storms develop it is not out of the question that some 
locations see 2-3" of rain, however, the likelihood of this is only 
about 5-10%.

Second area to watch Wednesday afternoon is across north-central WI. 
If this area remains mostly clear of cloud cover from the low to the 
southeast CAMs do some decent destabilization ~1000 J/kg SBCAPE. 
Recent HRRR runs show a line of storms develop along a lake breeze 
dropping south off Lake Superior during the mid to late afternoon. 
Don't expect these storms to become severe as the are further 
removed from the better shear profiles, but some brief gusty winds 
and small hail are not out of the question. 

Increasing Risk for Strong Storms Thursday and Friday...

Current guidance suggest the best windows for stronger thunderstorms 
across the forecast area are Thursday afternoon/evening and much of 
the day Friday. However, there will be quiet periods both days. 

The impetus for storms Thursday is weak low-level WAA coinciding 
with a left front exit region of a 40-45kt 500mb jet pushing over 
northwest WI. Because forcing will be not particularly focused the 
timing and location of any storm development is still uncertain. 
Should storms develop they will likely become widely scattered 
across much of the forecast area during the late afternoon and 
evening. LREF mean soundings show storms developing in an 
environment with 1200-1800J/kg SBCAPE and 25-35kts of 0-6km shear. 
Hodographs are generally straight and elongated. These parameters 
support at least a marginal (1 of 5) risk for severe wind gusts and 
1" or greater hail. PWATs are expected to be around 1" Thursday 
which is near average for mid-June. This should limit any flood risk 
to any urban or low-laying areas that see multiple rounds of 
thunderstorms.

Friday long-range guidance shows the chance for multiple rounds of 
thunderstorms. An initially push of WAA/steeper lapse rates in 
response to a developing cyclone over the Northern Plains may bring 
a round of storms during the morning/early afternoon. Depending on 
how quickly the boundary layer destabilized these storms could 
become severe wind and hail threats given favorable deep layer shear 
with a strong jet over the region. A second window for severe storms 
storms may come Friday evening/night as conditions for MCS 
development become favorable. LREF mean soundings show 1500-2500J/kg 
of SBCAPE by Friday evening along with surface dew points increasing 
into the upper 60s. Development of an MCS would likely be focused 
along the noise of a 40+kt LLJ. As the LLJ strengthens hodographs 
become more curved in the low-levels indicating increasing low-level 
helicity which may lead to more organized convection. There is still 
much uncertainty with where this MCS may track, but recent AI/ML 
severe products favor a corridor from near Duluth eastward toward 
Rhinelander.
  

Heat Building this Weekend... 

Expect the hottest temperatures of the year thus far this weekend as 
a high pressure system centered over the Tennessee Valley develops. 
NBM probs show 50-70% chances for highs over 90 degrees Saturday 
south of a Green Bay to Stevens Point line. Sunday looks a few 
degrees warmer with much of the area having a 50-80% chance of 
exceeding 90 degrees. Dew points in the upper 60s to low 70s both 
days will lead to head index values in the low to upper 90s at most 
locations.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1231 PM CDT Tue Jun 17 2025

Low stratus clouds have largely scattered out this morning with low-
end VFR developing across much of the region early this afternoon. 
Cigs will continue to gradually rise this afternoon with VFR 
conditions prevailing through the evening.

With a frontal boundary settling south along the WI/IL border think 
the risk for any thunderstorms this evening and during the early 
overnight hours is very low. Scattered showers may start to impact 
the east-central WI TAF sites ATW/GRB/MTW early Wednesday morning, 
however, initial aviation impacts should be minimal. More widespread 
moderate rain is expected to push into central and east-central WI 
mid-morning Wednesday. This will bring MVFR cigs back into the 
region along with periods of reduced visibility as periods of 
heavier rain move over the terminals. 

Light northwest winds will prevail for the rest of today before 
veering around and becoming easterly Wednesday morning.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION.....GK
AVIATION.......GK