349
FXUS63 KGRB 220357
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1057 PM CDT Thu Aug 21 2025

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- Dry conditions persist through Friday morning, with shower and
  storm chances arriving Friday afternoon. An isolated strong to
  severe storm will be possible Friday afternoon through Friday
  evening, especially over northern WI.

- Turning fall-like this weekend into next week. Daytime
  temperatures Sunday through Tuesday 10 to 15 degrees below
  normal. Nighttime lows below 40 are possible Monday night and
  Tuesday night over northern WI.

&&

.DISCUSSION...
Issued at 212 PM CDT Thu Aug 21 2025

Primary focus of the forecast is risk of isolated storms Friday
afternoon into Friday night as cold front crosses, then the early
taste of fall that sets up behind the front for the weekend into
much of next week.

Near term through Friday morning...Plenty of stratocu and cu for
the afternoon. Clouds are shallow, so no showers or sprinkles
expected. Skies will clear out this evening, with just high
clouds approaching ahead of showers and storms over the Dakotas
and Minnesota. Enough cooling late tonight to lead to patchy fog
especially central WI. Dry weather will hold through Friday
morning.

Storm chances Friday afternoon into Friday night...Mid-upper level
trough and mid-level jet settle toward Upper Mississippi River
valley late Friday. Primary cold front will reach northwest WI
Friday evening. Ahead of the front, building instability (MLCAPES
around 1000 j/kg) and effective shear of 25kt may support isolated
strong to severe storms. Could be a lot of mid and high debris
cloud early in the day, so not sure if the instability forecast
will be realized. Soundings still point to tall, but skinny CAPE,
suggesting limited coverage of any strong to severe storms, which
is supported by limited clustering of HREF paintballs of 40+ dbz.
Does seem like there could be two times favored for storms, early
to mid afternoon over far northern WI as remnants of showers and
storms over MN on Friday morning shift into the area, then again
in the evening over northern WI as cold front moves in from the
northwest. Marginal risk for severe (level 1 of 5) remains in
place from SPC which covers the potential.

Turning fall-like this weekend into much of next week...Cold front
will clear southeast forecast area on Saturday morning, with broad
mid to upper level troughing expanding from James Bay to the western
Great Lakes by Sunday. Below normal temperatures are expected Sunday
through Tuesday with highs likely in the 60s. Monday night and
Tuesday night look chilly as high pressure settles overhead. ECMWF
Extreme Forecast Index (EFI) continues to show signal for temps
well below climo. Latest iteration of NBM min temp forecasts are
showing readings around 40 at favored cold spots in northern WI
Monday night and Tuesday night. NBM 10th/25th percentiles for min
temps are still as low as the lower to middle 30s, so expect to
see forecast temps trend cooler over time. One or two of these
nights could even see potential for frost in far northern WI.
Latest probabilities of temps less than 37F are 30-50% over Vilas
County, highest on Monday night.

In terms of sensible weather, plan on breezy northwest winds with
partly to mostly cloudy skies this weekend into early next week.
Greatest chances for scattered showers will be on Sunday, though
there could be a few showers or sprinkles at other times as the
cooler air (850mb temps 3-5c) crosses the area. Given the cooler
air crossing the water lakes (water temps of 68-72f/20-22c) there
will be a lake response, but nw-wnw wind direction will keep the
majority of enhanced lake effect to the north over Upper Michigan.
Coolest conditions will last into Tuesday, then gradual warming
occurs by late next week, but even then temperatures look to stay
below normal.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1057 PM CDT Thu Aug 21 2025

A few patches of clouds around 4000 ft and areas of high clouds
will prevail late this evening and overnight. Prospects for fog
are lower than last night, as the surface ridge slides to the east
and some more stratus/high clouds will be arriving from the west
late tonight (especially over central and north central WI). But
with light winds another low-level inversion setting up, along
with lingering moisture and a period of clear skies, at least some
patchy fog is expected. MET/MAV/LAV advertising VSBYs mainly in
the 1-6SM range, so will continue these in the 00z TAFs. SREF/HREF
probabilities of <3 mile VSBY only in the 10-50% range. But where
fog does form, we could get some locally denser fog as HREF
probabilities of <1/2 mile in the similar 10-40% range, highest
from the Fox Valley to central WI (where the arriving clouds look
to reach last). Patches of LIFR/IFR CIGs are possible overnight as
well, but these may stay closer to SW WI.

Dry conditions will prevail into Friday morning, then an isolated
to broken line of showers and embedded storms is expected to move
west to east across most of the area Friday afternoon and
evening. Some lower CIG/VSBYs are possible in the heavier
activity. Light winds are expected tonight, with southwest/west
winds gusting to 15-20 kts late Friday morning and afternoon.

&&

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

&&

$$

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