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