842
FXUS63 KGRB 181120
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
620 AM CDT Tue Aug 18 2026

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Isolated to scattered showers and storms (30 to 60% chance)
  today. Marginal risk (level 1 of 5) for severe storms over the
  far south.

- Dry mid-week then next round of showers/storms arrives late
  Thursday into Friday.

&&

.DISCUSSION...
Issued at 144 AM CDT Tue Aug 18 2026

Shower/Storm Chances Today & Late This Week...Overnight showers
and storms have waned during the early morning hours as
instability decreased and the LLJ weakened/veered, with only a
few lingering showers remaining as of 1 AM. Look for the remaining
light showers to spread across the region through the early
morning hours with stray rumble of thunder possible. A break is
expected behind this first area, then look for redevelopment in
the late morning and afternoon hours as the front/shortwave
arrives and instability builds to 1000-1500 J/kg, highest south.
CAMs still not in the greatest agreement on coverage, possibly due
to weaker convergence along the front. An isolated strong/severe
storm will be possible during the early/mid afternoon along/south
of Hwy 29, where the higher instability will coincide with the
best shear (0-6 km bulk shear up to 40 kts). Gusty winds look to
be main threat, with some hail possible in the stronger cores. SPC
kept a marginal risk for severe weather over the far southern
CWA. As PWATs climb to near 1.5" ahead of the front, any storm
could produce some brief, locally heavy rain, but this looks to be
pretty isolated, with the highest chance over the far south. Any
storms would also be progressive enough to limit the heavier
rainfall/flooding potential, but some minor urban flooding would
be possible.

Next chance for organized/widespread showers and storms arrives
Thursday into Friday, as another cold front and shortwave drop
across the state. Timing has sped up, with 20-40% PoPs arriving
Thursday afternoon across central and north-central WI, with
50-70% PoPs spreading across the region Thursday evening into
Thursday night. Some locally heavy rainfall will be possible with
this system as PWATs climb to at least 1.5" and soundings showing
tall/skinny CAPE. Won`t be a drought buster type of event with a
widespread soaking rain, but some spots should pick up some much
needed rainfall. Latest timing would suggest severe threat will
remain low as the majority of the activity does not arrive until
late in the day/overnight, but some stronger storms will be
possible if the 1000 to 1500 J/kg CAPE materializes, with ML
guidance continues to show a broad 5-15% prob severe threat. Will
monitor timing/instability trends in the coming days.

Temps & Humidity Levels...Temps will remain fairly steady through
the week with highs mainly in the mid 70s to low 80s, which is
pretty close to seasonal norms. A couple chilly nights are
expected over northern WI, especially tonight where lows in the
lower 40s are forecast in far north-central WI (maybe a few upper
30s in the typical cold spots?). It will remain a little on the
humid side today, especially south of Hwy 29 as dewpoints climb
into the low to mid 60s ahead of the front. Less humid air arrives
on Wednesday behind the front as dewpoints drop into the 40s to
low 50s. Humidity levels will climb on Friday as the next system
moves across the region.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 620 AM CDT Tue Aug 18 2026

Mid-clouds will continue to spread across the area this morning. A
few light showers or sprinkles will remain possible over eastern
WI as well. Then, additional showers and a few storms are expected
this afternoon as a cold front works across the area. Mainly
MVFR/VFR conditions are expected, with locally lower flying
conditions, along with some gusty winds, possible in/near the
stronger storms. Will use a mix of TEMPO/PROB30 groups to cover
the shower/storm threat. As skies clear out tonight, some patchy
ground fog is possible, but confidence too low to include.

Light west winds will slowly increase this morning, up to around
10 kts by noon, with gusts around 15-18 kts, then shifting
northwest/north and decreasing behind the cold front tonight.

&&

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

&&

$$

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