487
FXUS63 KGRB 200728
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
228 AM CDT Mon Jul 20 2026

Forecast discussion for routine morning forecast issuance

.KEY MESSAGES...

- Level 3 of 5 risk of severe storms today. Damaging winds, large
  hail, and a few tornadoes all possible. Highest risk is mid
  afternoon to early evening (3 to 8PM).

- Smoke visibility and air quality impacts continue this morning
  with improvements this afternoon.

- Drier and cooler for mid-week.

&&

.DISCUSSION...
Issued at 223 AM CDT Mon Jul 20 2026

Thunderstorm/Precip Chances:

Forecast for a round of severe thunderstorms this afternoon
through this evening remains on track. WV imagery shows a potent
short-wave over southern Manitoba this morning that will move
southeast toward Lake Superior through the day. Accompanying the
upper wave is a seasonally strong jet streak (300 mb winds near
100kts). At the surface, winds veering to the southwest this
morning will bring a plume of upper 60s to low 70s dew points to
the region creating a highly unstable airmass by early to mid
afternoon. With favorable thermodynamic and kinematic fields have
increasing confidence in a higher end severe weather event taking
place across the region this afternoon into this evening.

One potential hindrance to this event realizing its full potential
is a round of morning showers/storms withing the warm advection
regime. CAMs continue to suggest a cluster or broken line of
showers/storms will develop over MN early this morning ahead along an
instability gradient and track into western WI around sunrise. These
storms would likely be expiating a downward trend they reach
central/northern WI as they out run the better instability, but
the remnant cloud debris may delay/modify building instability.
CAMs generally have this activity dying out by 10AM this morning.
These trends will bear watching as any differential heating
boundary/outflows may serve as initiating boundaries later into
the day. By mid morning attention will shift to an area of rapidly
destabilizing air east of the incoming cold from southern IA to
the WI/UP border. As the aforementioned upper-short wave starts to
impinge upon this unstable air expect storms to start initiating
across northwest an  nort-central WI around 3-4PM (could be as
early as 1PM). Initially expect a broken line of super cells as
0-6 shear vectors are angled about 30 degrees off the cold front
which should help to keep cells discrete initially. Think these
initial cells will pose the greatest strong tornado (EF2+) and
large hail (2+ inch) threat as the deep shear will promote well
organized and persistent updrafts, especially any storms that
deviate to the right of the mean wind. With time and eastward
extent forcing along the front and increase storm coverage should
lead to cold pool aggregation and the development of one or move
bowing segments that will pose a damaging (70+ mph) wind threat as
well as brief spin up tornados. The severe weather threat is
expected to persist all the way to the lakeshore with storms
exiting the region around 8-10PM tonight.

In addition to the severe threat high rain rates are expected with
these storms, however, the scattered nature of the storms and
progressive movement likely will limit widespread higher rain
amounts, but localized 2 inch amounts could occur. Urban areas or
poor drainage areas will be susceptible to higher rain rates and
localized flooding.

Behind the front Tuesday scattered diurnally diver showers may
pop-up across northern WI with northwest winds over Lake
Superior. Forecast sounding show this should be very shallow
convection so don`t expect much if any lightning with these
showers. Beyond Tuesday a building ridge should keep the middle of
the week dry with a possible return in showers/storms late in the
week into next weekend, but predictability is low.

Smoke Trends:

Surface obs in central WI are showing slowing decreasing visibility
early this morning with smoke trapped under an inversion. These
trends should reverse by mid-morning as the boundary layer starts to
mix. Increase southwesterly surface winds today should also greatly
help with clearing surface smoke from west to east. HRRR/RRFS do
show another possible plumb of smoke pushing into the region Tuesday
and Wednesday as winds turn northwesterly, however, there is a high
degree of uncertainty with how dense smoke may be.

Temp Trends:

Today will be the warmest day of the week with highs in the middle
to upper 80s. Cooler temperatures arrive behind the cold front and
linger through much of the week.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1028 PM CDT Sun Jul 19 2026

Smoke trends main issue through late tonight. Given steady SW
winds and smoke not as thick as Saturday, expect vsby 2-5 SM,
lowest over central WI. First chance of showers and storms arrives
over central/north-central WI on Monday morning. Expect these
storms to be on weakening trend, but brief IFR vsby could occur.
Remnants of these storms could reach eastern WI TAF sites late
morning, but with expected weakening, continue to only mention
showers, with no TS. Models still pretty locked on to morning
convective evolution, though less than ideal handling of upstream
showers and storms this evening doesn`t mean these trends are a
lock. Otherwise, MVFR vsby from smoke will slowly improve by
midday over central WI, then over eastern WI mid to late afternoon.
Lowest vsby will be 3-4SM.

Late afternoon after 22z, expect showers and storms to blossom
over central/north-central WI, before spreading to eastern WI
through 03z-05z. In this iteration of the TAFs have tried to frame
out 3-4 hr time frame of when TS are the most favored at each TAF
site. Details for the timing and specifics for impact with TS can
be added in later updates.

.KOSH...

Dry rest of tonight with smoke trends main concern. Enough wind
tonight and less concentrated smoke to prohibit vsby lower than 3
SM most of the night.

On Monday, the first cluster of showers and thunderstorms will
approach OSH between 15z-18z Monday. Still no TS mentioned as
storms will be on the weakening trend and outrunning better
instability to the west. Yet, even though weakening, the showers
could still produce brief gusty outflow winds from the NW. Decent
agreement that primary window for showers and storms at Oshkosh
has shifted later now, not until after 00z. Ran with a prevailing
for TS starting at 21/01z. Similar to other eastern WI TAF
sites, did not include specifics yet with the TS mention in terms
of heavy rain, gusty winds, hail, as these details can be added
in later updates. A direct hit from TS would result in IFR
conditions, but not enough confidence to include any notable
reduction to vsby yet.

&&

.MARINE...
Issued at 223 AM CDT Mon Jul 20 2026

South to southwest winds will increase this morning, with gusts to
30 kts likely this afternoon. Waves on Lake Michigan will build to 4-
7 ft, and to 2-4 ft on the bay. Small Craft and Beach Hazard
Statements are in effect today.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement from 10 AM CDT this morning through
Tuesday morning for WIZ022-040-050.
&&

$$

DISCUSSION.....GK
AVIATION.......JLA
MARINE.........GK

NWS GRB Office Area Forecast Discussion