710
FXUS63 KGRB 021146
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
646 AM CDT Wed Jul 2 2025

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- There is a chance of thunderstorms this afternoon, especially
  over northern WI. A few storms may become strong to marginally
  severe, especially in far northeast WI between 2 pm and 8 pm.

- Additional chances for rain and storms will be possible Thursday
  through the holiday weekend, with the best chance for widespread
  showers and thunderstorms occurring Saturday and Saturday night.
  There should be a break in the thunderstorm chances for 4th of
  July fireworks displays (Friday evening).

- Temperatures will be above normal through the holiday weekend,
  with very warm and humid conditions (heat indices in the upper
  80s to middle 90s) arriving for the 4th of July.

&&

.DISCUSSION...
Issued at 346 AM CDT Wed Jul 2 2025

Clear to partly cloudy skies were observed across GRB CWA early
this morning. Temperatures ranged from the middle to upper 50s
northwest to around 70 southeast. WV imagery showed a short-wave
trough moving through SW Ontario and northern MN, accompanied by a
cluster of showers and embedded thunderstorms. These showers
should reach north central WI toward midday.

Thunderstorm and Severe Weather Potential this Afternoon: The
short-wave over northern MN will track through northern WI this
afternoon, generating showers and scattered thunderstorms. There
will be some ingredients in place for severe weather, including
CAPE of 1-2K j/kg, deep layer shear of 30 to 35 knots and
inverted-V soundings. However, the strongest forcing will overlap
with the lowest CAPE values over northern WI, so overall
confidence in severe storms is low. SPC HREF 40+ dBZ paintballs
and max wind speeds, and simulated reflectivities off the CAMs,
showed the most organized convection occurring in far NE WI, in
the vicinity of a weak convergence zone/surface trough. Will focus
messaging for severe storms mainly over that region between 2 pm
and 8 pm, even though the SPC Marginal Risk covers the entire CWA.
Coverage of the showers and storms should be less farther south
due to weaker forcing. The main area of showers and storms should
shift east of the region by early evening, but models suggest
continued weak development occurring along a weak frontal boundary
as it sags south during the evening and early overnight.

Thunderstorm Chances through the Holiday Weekend: The weak front
is expected to sag southwest of the region by Thursday, then stall
out through Thursday night. Despite a moist and unstable air mass
over the southwest part of the forecast area, models don`t show
much convection developing, likely due to very weak low-level flow
ascending the frontal zone and overall weak forcing. Will only
carry slight chance/chance pops for the southwest CWA during this
period.

The warm front is finally expected to lift northeast through the
forecast area on the 4th of July, though convective coverage will
be limited due to a building upper level ridge over the western
Great Lakes. After the warm front lifts through, generally dry
conditions are expected for evening fireworks displays.

Southwest flow, a stronger short-wave trough and cold frontal
passage will bring a more widespread period of showers and storms
Saturday and Saturday night. Instability (800-1500 j/kg) and deep
layer shear (20-30 knots) will be marginal, and widespread cloud
cover and showers could limit strong to severe thunderstorm
development. PWATs in excess of 2 inches suggest potential for
very heavy rainfall during this period. There is disagreement
on how quickly the cold front will clear the region, but once it
does, there should be a quieter/drier period for at least a day or
two.

Temperatures: Above normal temperatures are expected through
Saturday, followed by near to a little below normal conditions
through the middle of next week. Very warm and muggy conditions
arrive for the 4th of July, with heat indices rising into the
upper 80s to middle 90s during the afternoon. Mild and muggy
conditions will persist through the evening fireworks displays.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 630 AM CDT Wed Jul 2 2025

Aside from a few patches of fog in NC/C WI, VFR conditions
prevailed early this morning. Radar showed showers approaching
from northern MN.

Cumulus clouds will develop later this morning, but mid-level
clouds will also increase over northern WI as a short-wave trough
arrives. This short-wave will generate scattered showers and
thunderstorms during the afternoon and early evening, with the
greatest coverage expected over far NE WI. The storms will be
capable of producing gusty winds, and a few of the stronger storms
could also produce some hail. Have attempted to time the storms
with Prob30 groups in the TAFs. A secondary line of showers could
develop as a weak boundary sags south during the evening, but
confidence is too low to add a mention of showers with this
feature. Later tonight, clearing skies and light winds will allow
fog to develop, especially over NC WI. Have added IFR conditions
at RHI, with a TEMPO group for LIFR.

West winds will gust to around 15 kts later this morning and
afternoon, and gusts to 30 to 40 kts will be possible with any
storms. Light and variable winds develop this evening and
continue overnight.

&&

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

&&

$$

DISCUSSION.....Kieckbusch
AVIATION.......Kieckbusch

NWS GRB Office Area Forecast Discussion