619
FXUS63 KGRB 182330
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
630 PM CDT Tue Aug 18 2026

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Isolated to scattered showers and storms (40 to 60% chance)
  today. Marginal risk (level 1 of 5) for severe storms over
  portions of central and east-central Wisconsin.

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

&&

.DISCUSSION...
Issued at 133 PM CDT Tue Aug 18 2026

Shower/Storm Chances Today...
Initial showers have begun to develop across west, central, and
southern Wisconsin this afternoon but are largely from the mid-
level clouds visible on satellite, with limited rain reaching the
surface. As we get into the late afternoon to evening, convection
is expected to become more surface based, supported by a weak
upper jet and approaching cold front. Expect the rain coverage to
expand eastwards over the next few hours, especially across
portions of central and eastern Wisconsin along the lakeshore,
where temperatures are warmest. Active weather will be fast-moving
today, leading to fairly scattered coverage (40-60% at any one
location) with frequent on and off breaks through the evening. A
marginal severe threat will also be present, generally located
south of Highway 29 and along the lakeshore of Lake Michigan,
where instability is greater. Hail will be a threat for any storms
given organizational shear and the potential for some splitting,
especially with the relatively low wet-bulb zero heights.
Additionally, any storms could also bear a wind concern, mixing
down some of the strong winds aloft. Heavy rainfall will be
somewhat limited by the progressive nature of any storms and
somewhat limited moisture pool, but probabilistic guidance does
produce a 40% chance of a quick 0.50-0.75 inches of rain under the
stronger storms. Active weather then quickly dissipates going
into the overnight hours.

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. Some
rain and showers may linger on into Friday as well, continuing to
produce light showers through the daytime hours. Severe potential
may again be limited by a lack of instability but may also depend
on rain coverage in the 24 hours preceding. 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 00Z TAF Issuance
Issued at 600 PM CDT Tue Aug 18 2026

Main concerns to aviation continue to revolve around lingering
shower/storm chances as a cold front tracks south through the
evening. Thunder chances will continue to diminish after 00Z,
though SCT showers could still be around the area through 04Z. As
such, have opted to only include PROB30 mentions for SHRA at the
east-central terminals, with thunder chances remaining sub-30.
Conditions should largely remain VFR, though brief reductions of
both cigs and vsbys to MVFR will be possible if a shower moves
directly over a TAF site. More stable air then filters in behind
the front on Wednesday, leaving clearing skies and good flying
conditions in its wake.

CAMs still showing a signal for patchy fog developing overnight,
though would likely be limited to areas that recently received
rainfall. Have retained VFR vsbys for this TAF issuance due to
uncertainty of fog coverage.

Northwest winds gusting to 15 knots will be possible during the
first couple hours of the TAF period, eventually subsiding to
light and variable overnight.

&&

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

&&

$$

DISCUSSION.....Uhlmann
AVIATION.......Goodin