243
FXUS63 KGRB 301911
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
211 PM CDT Tue Jun 30 2026

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- Dangerous heat peaks this afternoon. An Extreme Heat Warning is
  in effect through 7 PM today for central, east-central, and far
  northeast Wisconsin, with a Heat Advisory is in effect
  elsewhere. Additional heat headlines may be needed Wednesday.

- Scattered storms late this afternoon into this evening, with
  isolated severe storms. Greater chances for severe weather for
  Wednesday, especially during the afternoon into evening hours.
  This is now looking like a higher-end severe threat with most
  area now in an Enhanced Risk (level 3 of 5).

- Heavy rain potential Wednesday into Wednesday night. At least
  isolated instances of flash flooding possible.

- Additional chances for thunderstorms through Friday, with
  chances beginning to diminish for the holiday weekend.

- Gusty onshore southerly winds will result in high waves and
  dangerous currents on Lake Michigan beaches through late this
  evening.

&&

.DISCUSSION...
Issued at 210 PM CDT Tue Jun 30 2026

Precipitation/Thunder Chances...Morning round of storms has exited.
Given minimal large scale forcing interacting with MLCAPEs up to
4000J/kg, expect scattered storms to form late this afternoon
through this evening, potentially emerging from blossoming cu
field from northeast IA into southern WI. With a very moist
atmosphere (dew points in the low 70s) resulting in extreme
instability, deep-layer shear of 30-40 kts, severe thunderstorms
may occur. Coverage of storms is the main question due to the
capping across our area that was only enhanced by the morning
convection. Large hail, damaging winds, and heavy rainfall will
be the main threats.

Late tonight into Wednesday morning, good signal that shortwave on
leading base of plains trough, along with increasing 850mb-500mb
jets will trigger scattered storms in the morning lifting across
the area. There may be a break, then additional severe storms are
expected to develop due to interaction with outflow from morning
storms and approaching trough. These triggers will interact with
MUCAPEs up to 3000J/kg and effective shear 50+ kts. Overall a
higher-end severe risk is emerging with bowing structures and
potential supercells. Thus, there is a potential for all hazards,
including wind gusts over 70 mph, hail to golf-ball sized, and
isolated tornadoes. At this point, the greatest risk for high-end
severe would be mid afternoon into the evening. The storms in the
morning could produce severe wind gusts as well, though there is
not that great of a signal for that. SPC has placed most of the
area in an Enhanced Risk for Day2/Wednesday-Wednesday Night.

Another aspect for Wednesday is very heavy rain. PWATs up to 1.8
inches (97th percentile), with slower individual storm motions and
increased covergence combined with such high instability point to
favoring the high-end QPFs (max of HREF) which suggests at least
isolated 4+ inch rainfall amounts could occur if there are repeating
rounds of storms. Uncertainty in how storm evolution occurs precludes
the issuance of a flood watch attm, but will be adding heavy rain
to the forecast to get this messaging out there.

Models indicate additional convection may develop on Thursday as
the upper-level ridge breaks down; however this will depend
greatly on the evolution of convection on Wednesday. A greater
chance for showers and thunderstorms appears to be on Friday with
the passage of a mid-level shortwave, but it is too early to
determine the severe potential. There are signs of an additional
shortwave sometime over the 4th of July holiday weekend, but there
are timing and placement differences. In general, probabilities
from models/ensembles for showers gradually diminish while working
through the weekend. We`ll have to see if that trend holds up.
Into early next week, chances for rain go down further as high
pressure settles overhead.

Dangerous Heat...Despite the morning storms, temperatures have
already recovered to around 90. Those readings along with super
high dewpoints in the upper 70s to near 80 is already resulting in
apparent temps pushing past 105, with more heating to go during
the day. Though technically we will stay below extreme heat
criteria over central WI, not going to change heat headlines given
how oppressively humid it is and how there was no relief last night.
Also not going to extend the headlines through tomorrow with all
the uncertainty on how storms will impact temperatures on Wednesday.
Will let mid shift assess and re-issue heat headlines for Wednesday
if needed. Beyond Wednesday, unsettled weather and temperatures
not as warm and dewpoints coming down (including into July 4th)
slightly cast doubt on need for additional heat headlines. Upshot
is after the heat today, going to take it day-by-day to see gage
the needs for additional heat headlines.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1247 PM CDT Tue Jun 30 2026

Morning round of storms has exited. Additional storms expected
late this afternoon into this evening. Have kept it simple and
framed out prevailing thunder for a few hours, though it will
likely not be widespread and prevailing as this. Conditions will
be VFR, though like this morning, a direct hit from a TSRA would
result in brief IFR vsby. After a lull late evening through
overnight, another round of storms is expected to arrive over the
entire area on Wednesday morning. Gusty winds could occur with
that round of storms, but will address this with later updates
when confidence increases.

Southwest winds will increase today, with gusts up to 20 kts.
Winds subside late this evening and stay remain less than 10 kts
on Wednesday morning.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 7 PM CDT this evening for WIZ005-010-011-018-
019.

Extreme Heat Warning until 7 PM CDT this evening for WIZ012-013-
020>022-030-031-035>040-045-048>050-073-074.

Beach Hazards Statement through late tonight for WIZ022-040-050.
&&

$$

DISCUSSION.....JLA
AVIATION.......JLA

NWS GRB Office Area Forecast Discussion