613
FXUS63 KGRB 041659
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1159 AM CDT Fri Jul 4 2025

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- There is a chance of thunderstorms today. A strong storm is
  possible that could produce small hail and gusty winds.

- A better chance for widespread rain and storms will occur
  Saturday afternoon and Saturday evening. A few severe storms are
  possible that could produce damaging wind gusts. Heavy rainfall
  is also possible that could produce localized urban and low-
  lying flooding.

- Very warm and muggy conditions will result in heat indices in
  the middle to upper 90s over central and northeast Wisconsin
  this afternoon, and over eastern Wisconsin on Saturday. Those
  with outdoor plans should take precautions for the heat.

&&

.DISCUSSION...
Issued at 312 AM CDT Fri Jul 4 2025

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

The latest RAP analysis and satellite/radar imagery show a surface
warm front extending from around Grand Forks to just north of the
Wisconsin Dells and Madison beneath a building 500mb ridge early
this morning. Modest 15 to 20 kts of southwest flow over the warm
front is leading to a band of deep moisture convergence and a slow
moving clusters of thunderstorms from northwest to south-central
Wisconsin. Several Severe Thunderstorm Warnings have been issued
with these thunderstorms, which have remained just west of the GRB
forecast area. More concerning is the 2-5" inches of rain that has
fallen along the I-94 corridor due to the training/slow moving
storms. Most convective allowing models (CAMS) have these storms
just clipping southwest Marathon and Wood counties early this
morning before making more headway into central WI in the 5-7 am
time frame. By this point, the CAMS show the large reservoir of
instability (2000-3000 j/kg) and moisture convergence weakening
and less intense convection. Therefore, any severe threat is
likely to occur before 7 am and primarily to western Marathon and
Wood Counties.

Focus of this forecast then shifts to thunderstorm potential for
the rest of today, hot/humid conditions this afternoon, then
severe/heavy rain potential on Saturday.

Thunderstorm Potential after 7 AM Today: As the mid-level ridge
builds overhead, the warm front will lift northeast today. Weak
moisture convergence will accompany the front. More concerning is
the 2000-3000 j/kg of instability that builds with the heat of the
day. Forecast soundings indicate an uncapped atmosphere this
afternoon after debris clouds from overnight convection have time
to dissipate/thin out. CAMS generally show the thunderstorm
activity gradually weakening this morning and little to no
thunderstorms developing this afternoon despite the ample
instability. Can`t say I`m entire confident going dry this
afternoon given the boundary and instability across the area, so
kept the theme of low chances of storms that slowly shift east
through the day. If a rogue storm develops, 20-25 kts of deep
layer shear would support intensities near severe levels. The
threat of storms should diminish by early this evening as the
warm front lifts northeast of the area. Relatively high confidence
that the area will be dry for firework events this evening.

Dangerous Heat Potential: Once debris clouds dissipate, low level
temperatures would support surface readings warming into the
middle 80s to lower 90s across the area. With dewpoints in the
lower 70s, heat indices in the middle 90s (perhaps briefly into
the upper 90s) can be expected this afternoon over much of
central and northeast WI away from the Bay and Lake. These heat
indices would fall shy of the threshold for a Heat Advisory so
will refrain from issuing headlines.

Heat indices in the lower to middle 90s can also be expected on
Saturday over northeast WI.

Severe/Heavy Rain Potential on Saturday: A positively tilted
shortwave trough will push a weak low pressure system northeast
across the northern Plains and western Lake Superior on Saturday.
A cold front will then slide east across northeast Wisconsin on
Saturday afternoon and evening, during peak heating. Expect the
chance of showers and storms to rapidly increase during the early
to middle afternoon hours over north-central WI before spreading
to northeast WI late in the afternoon or evening. An unstable
airmass (1000-2000 j/kg of instability) will remain present ahead
of the front. Deep layer shear of 25-30 kts and precipitable water
values near the climatological max will be present ahead of the
front. Deep warm cloud depths and downdraft capes to 800 j/kg will
support a heavy rain/damaging wind threat. HREF Probability Match
Mean indicate potential for 1-3" of rain over north-central WI on
Saturday afternoon and increased rainfall amounts from the NBM.

Long Term...Saturday Night Through Thursday

Focus of this forecast continues to remain thunderstorm/heavy rain
threats on Saturday night before the weather pattern settles down.

Thunderstorms/Heavy Rainfall Saturday Night: A positively tilted
shortwave trough will continue to push a cold front across
northeast Wisconsin on Saturday night. Lingering instability of
500-1000 j/kg combined with precipitable water values upwards of
2.00 inches (99-99.5 percentiles) will lead to a marginal severe
weather (strong, damaging winds) and heavy rain threat continuing
through the evening. Instability will gradually wane through the
night so impacts will likely transition to a strictly heavy rain
threat with time on Saturday evening. Low MBE velocities (under 10
kts) and high warm cloud depths (up to 13,000 ft) will lead to
efficient rainfall production and potential for training cells.
While latest rainfall forecasts are not impressive, heavy rainfall
ingredients tell a different story so will continue to advertise
in the HWO. Storms will taper off to light rain overnight before
ending late Saturday night or Sunday morning.

Rest of the forecast: Behind the front, temperatures will return
closer to normal through the end of the next week. Despite a
reprieve from the warm and humid airmass, a shortwave will be
moving across the region during the Tuesday timeframe which could
interact with diurnal instability upwards of 1000 j/kg for a
chance of strong thunderstorms. This will be the next period to
watch.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1159 AM CDT Fri Jul 4 2025

Debris and convective clouds will affect the TAF sites at times
through this evening. While some clouds will clock in at MVFR at
times, the majority of the time should stay at VFR. Isolated
showers and thunderstorms will be possible through the early
evening; however, confidence in coverage and timing is too low to
include in this set of TAFs. An approaching cold front will bring
showers and thunderstorms from west to east on Saturday, only
affecting KRHI in this TAF issuance but will include in the rest
of the TAF sites with subsequent issuances. Low level wind shear
will increase over north-central and central Wisconsin tonight,
and dissipate Saturday morning.

&&

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

&&

$$

DISCUSSION.....MPC/MPC
AVIATION.......Kurimski