459
FXUS63 KGRB 201149
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
649 AM CDT Fri Jun 20 2025

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Scattered thunderstorms are expected through this evening. A
  few severe storms may occur with large hail and gusty winds.
  More storms will be possible late tonight through Saturday
  morning as a complex of storms crosses as close as northern
  Wisconsin.

- Multiple days of dangerous heat are forecast from Saturday into
  Monday. Heat index values are expected to reach 95 to 105
  degrees each afternoon on Saturday and Sunday, and in the lower
  to middle 90s on Monday across east-central Wisconsin. Low
  temperatures in the 70s will provide minimal relief to the heat
  and humidity. A Heat Advisory has been issued, started at noon
  on Saturday. Cooler temperatures are forecast on Tuesday.

- The chances of thunderstorms will continue at times next week.
  Strong or severe storms are possible Monday into Monday night
  with damaging winds and large hail. It is too early to determine
  the risk of severe weather after Monday.

&&

.DISCUSSION...
Issued at 308 AM CDT Fri Jun 20 2025

Short Term...Today...Tonight...and Saturday

NW flow is active this morning. Couple of stronger waves within
the flow along with warm air advection aloft will continue to
support clusters of storms, gradually shifting east into more of
our forecast area. MUCAPEs have fallen since earlier, mainly in
the 500-700J/kg range. Instability with 50+ kts of effective shear
has led to occasional severe with stronger storms, but most storms
staying sub-severe. As most well developed complex early this
morning should remain southwest focused along nose of 850mb jet,
expect this general regime to persist through 12-14z following the
pocket of elevated instability and as stronger wave over central
MN shifts east. Later this morning, northern edge of complex
tracking over far southwest WI may result in a few storms to
develop over central WI, then as additional wave over northern MN
shifts southeast, may see another round of mainly elevated showers
and storms form over northern WI in area of increasing MUCAPE.
Eventually, warmer air builds in from the west at 800mb-700mb
which should begin to diminish convection. May remain mostly
cloudy for the day though as clouds are trapped beneath the
inversion aloft.

Storms/Severe Potential Tonight into Saturday Morning: After
general lull into early this evening, conditions still look to
change rapidly later this evening as warm front surges northeast
across the state. Low- level jet 50 kts will push a large
reservoir of instability (3000-4000 j/kg) into northern WI.
Genesis region for convective initiation to the north of capping
will be over central and northern MN ahead of sharp shortwave
trough and sfc low, along and north of warm front. Majority of
models keep the bulk of complex north of even northern WI closer
to track of sharp shortwave trough. Unlike last couple nights,
core of highest instability is now closer to the shortwave trough.
Thus, though some extension of storms may try to build south into
northern WI into higher instability, idea that models have been
showing which is now more or less supported by higher res
guidance, has more merit and will keep highest pops restricted to
northern fringe of cwa. Still a very present severe potential for
large hail and damaging winds given the degree of instability and
deep layer shear. SPC continues to highlight our area in Slight
Risk (level 2 of 5) for both Day1 and Day2 (far northeast WI due
to exiting complex of storms). Lingering thunderstorms remain on
track to exit on Saturday morning. Given degree of capping
building in, think additional storms on Saturday will be hard to
come by.

Dangerous heat begins on Saturday: 850mb temps rising to 23-24c
along with increasing capping due to anomously warm 700mb temps
will support high temps rising to around 90 north to the low to
mid 90s elsewhere away from the immediate Lake Michigan shore. The
heat combined with dewpoints reaching into the lower 70s will result
in heat index values reaching the mid 90s north, to the upper 90s
to around 100 away from the immediate Lake Michigan shore. The
highest heat index values will occur from central to northeast WI.
This will be the first day of multi-day dangerous heat. Though
some heat index values are marginal on Saturday in terms of strict
criteria, opted to keep it simple and issue a Heat Advisory over
all areas except Door County and to run this through Monday to
capture the higher heat index values Sunday and Monday, along with
the very warm nights Saturday night and Sunday night which will provide
minimal relief from the heat. The Heat Advisory begins at noon on
Saturday.

Long Term...Saturday Night Through Thursday

Intense heat will be the focus Saturday night into Monday
as very warm air at 925/850mb noted on the models. For next
week, the ring of fire will continue periodic chances of showers
and thunderstorms, locally heavy rain and stronger storms at times.

For Saturday night, southerly winds should remain in the 5 to 15
mph range with lows in the middle to upper 70s at most places.
On Sunday, hot and humid conditions will prevail with highs in
the 90s at most places. Forecast 850/925mb temperatures off the
models were supportive of the forecast high temperatures. One
item of interest to watch is the GFS is currently depicting the
warmest 850/925mb temperatures across north-central Wisconsin.
This would mean the hottest temperatures will be across the far
north. The other models do not show this distinct temperature
anomaly. Heat index values of 95 to 105 degrees are expected
across much of the area Sunday afternoon. A Heat Advisory
is likely with the possibility some of the area could reach
Extreme Heat Warning. Another very warm night is in store Sunday
night across the area with lows in the 70s, with the warmest
readings in the upper 70s over east-central Wisconsin. As the
previous forecaster noted, "The last time we had two consecutive
nights of min temps greater than 75 at GRB was July 1-2 of 2002.
Record warmest min of all time for GRB is 79 (last time in 1916).
This forecast has not changed much, so much of Wisconsin remains
in the major to extreme impact to very high category for heat
risk.

Increasing clouds and the chances of showers and storms arrive
Monday. Eastern portions of the state will see another day in the
90s, with readings only in the lower to middle 80s across north-
central Wisconsin due to cloud cover and rain. Strong or severe
storms are possible Monday afternoon into Monday night as the
cold front moves across the area.

Beyond Monday night, models continue to show the area in the ring
of fire as there will be periodic chances of showers and storms
through the end of the work week. There will be breaks in the
precipitation, but timing them this far out is difficult. Cooler
temperatures return on Tuesday as the frontal boundary will be
draped across or south of the area through the end of the forecast
period. Locally heavy rain and the possibility of stronger storms
exists.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 648 AM CDT Fri Jun 20 2025

Showers and isolated thunder cross east-central WI through 13-14z.
A few showers will linger in wake of this into early afternoon.
Pop- up shower or storm is possible later in the afternoon over
northern WI near the Upper Michigan border. Otherwise, skies will
remain BKN-OVC with generally MVFR to lower VFR prevailing.

After minimal chances of showers and storms this evening, late
tonight, scattered showers and storms are expected to cross
especially northern WI. There is uncertainty in how far south
storms will track. Isolated gusty winds and large hail could
occur. However, if there is more of a direct impact from a line of
storms, widespread winds over 40 kts and heavy rain would occur.
VFR conditions until late tonight when the risk of lower cigs and
visibility develop, especially if a complex of storms tracks
across the area.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from noon Saturday to 7 PM CDT Monday for WIZ005-
010>013-018>021-030-031-035>040-045-048>050-073-074.

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

$$

DISCUSSION.....JLA/Eckberg
AVIATION.......JLA