567
FXUS63 KGRB 171124
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
624 AM CDT Mon Aug 17 2026

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Patchy ground fog early this morning, otherwise less humid
  today.

- Scattered showers (20-40% chance) and an isolated storm late
  tonight.

- Scattered showers and storms (30-60% chance) are back on for
  Tuesday. Marginal risk (level 1 of 5) of severe storms in the
  afternoon. Greatest risk still to the south.

- Rain chances return Thursday night into Friday (30-60% chance),
  but overall the rain this week will not make much of a dent into
  growing drought conditions.

&&

.DISCUSSION...
Issued at 157 AM CDT Mon Aug 17 2026

Near term through tonight...High pressure over the plains is
resulting in N flow over WI this morning. Just enough weak
convergence near Lake Michigan early this morning to result in
enhanced stratocu and even a few passing sprinkles at MTW. Over
central and north-central, cooling temps and lingering low-level
moisture has led to patchy thin ground fog. Temps today will be a
couple degrees less than Sunday. It will be noticeably less
humid.

Shortwave trough and cold front over southern Saskatchewan makes
it to northwest Ontario and northern MN this evening. Scattered
storms will form on these features in axis of CAPEs 500-1000
J/kg. Remnants of this (20-30% chance - greatest northern WI) will
make it into WI late tonight as LLJ veers more W, but not expecting
much for thunder as this activity will be running out ahead of
instability axis.

Scattered showers and storms with small severe risk on Tuesday...
Though parent trough in northern stream heads across Ontario and
Quebec on Tuesday, cold front is not as quick to move through,
back to a solution shown a few days ago. At current, seems like
potential for front interacting with peak heating, CAPEs of at
least 1000J/kg and effective shear over 40 kts favors not only
increasing chances for showers and thunderstorms, but also
pushes the severe risk back upward. SPC Day2 pointed that way,
with marginal risk now back into far southern areas. If trends
continue (not sold on that given the variability seen with the
speed of this frontal passage), could see more of our southern and
eastern areas in a risk eventually. Of note, various machine
learning output is not thrilled with severe probs here on Tuesday.
Wind, with a nod to hail given longer hodographs, would be primary
hazards. Heavy rain is a lower risk with highest PWATs staying
south and soundings showing more progressive storm motions, lessening
the risk for training, backbuilding storms. Scattered showers and
storms exit quickly Tuesday evening, and Wednesday still looks
dry.

Extended...Next chance for showers and some storms returns
Thursday night through Friday with sharpening nw flow bringing
shortwave/front through. More consensus with this in the models
compared to 24hr ago, so pops have ramped up to near 60% over
northern areas. Better instability still southwest of the state,
but could be sufficient enough return flow ahead of front to lead
to stronger storms. Though storm predictability is low this far
out, Machine learning guidance continues to paint broad 5 to 15%
prob severe, so will continue to monitor. Behind the front,
imagine Saturday could be a day with plenty of clouds and some
scattered instability showers given the broad troughing settling
across the Great Lakes. It then turns drier for Sunday as high
pressure moves in. Nothing too notable in terms of temperatures
with highs in the upper 70s to lower 80s much of this week,
trending more solidly into the 70s next weekend.

Even though we do have at least some rain chances for the week,
no widespread rain chances are expected given this kind of splitting
pattern we are stuck in, with stronger jet dynamics staying north
and greatest instability gradient persisting south. We could
certainly use some rain. Rainfall deficits in the last 30 days
are now running over 3 inches for much of north-central WI, with
mainly 1 to 2 inch elsewhere. As of early July not even 50% of our
forecast area was in a D0 (Abnormally Dry) classification. As of
the latest drought monitor (8/11), over 90% of our area is within
D1 (Moderate Drought), while parts of Oneida and Vilas counties
are into D2 (Severe Drought).

&&

.AVIATION...for 12Z TAF Issuance
Issued at 624 AM CDT Mon Aug 17 2026

Other than early morning patchy ground fog at RHI and lingering
MVFR stratocu at MTW, scattered VFR cu this morning will give way
to just some passing cirrus this afternoon at all the TAF sites.
Mid clouds will increase late this evening from the west ahead of
a cold front, with a small chance of some showers mentioned at
RHI, AUW, and CWA after 06z as the front approaches. Though there
will likely be scattered thunderstorms not that far upstream this
evening near the front, opted to not include any thunder in the
TAFs for now as the activity should be falling apart as it moves
into the area.

Light NW winds this morning will shift to SW-S this afternoon,
with strongest winds (10-12 kts) at MTW due to a lake breeze.
SW winds will occur all areas tonight.

&&

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

&&

$$

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

NWS GRB Office Area Forecast Discussion