566
FXUS63 KGRB 201149
AFDGRB

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

Updated aviation portion for 12Z TAF 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 12Z TAF Issuance
Issued at 649 AM CDT Mon Jul 20 2026

Smokes continues to sit over much of the region the morning. Vsbys
are generally between 2-5SM, with the lowest of those being in
central WI. HRRR/RRFS are in good agreement that smoke will clear
from west to east this morning into the afternoon as SW winds
increase to 10-15kts and gusts reach 20-25 kts.

The other aviation impact for this morning as a decaying line of
showers/storms pushing into central WI as of 1130Z. Satellite is
showing a decreasing lightning trend as line moves east into more
stable air so only have TSRA at CWA and AUW, with SHRA at all other
TAF sites. These showers should be exiting the area by 15Z this
morning.

This afternoon after 20Z expect another round of storms, some strong
to severe, to blossom over central/north-central WI and spread to
the southeast through the afternoon and evening. CAMs have been
trending toward 2 potential rounds this afternoon and evening
between 20Z and 04Z so have framed out the best timing for these
rounds in each TAF site, however, have TEMPO groups with no storms
to account for possible lulls in storms activity. Any individual
storm will be capable of producing severe winds over 50kts, large
hail and heavy rain.

.KOSH...

A decaying line of showers/storms currently west of a AUW to Y51 is
on track to reach OSH just after 13Z this morning. As this line
moves east it will be running away from the better instability so
don`t expect any lightning this morning.  Showers may drop vsbys to
around 5SM at times, but this will be short lives with OSH drying
out by 14Z.

The other aviation impact at OSH for this morning is lingering MVFR
vsbys due to smoke. The ob has been holding at 5SM early this
morning and with winds now out of the south don`t expect that to
decease. By late this morning smoke should be stating to clear with
VFR vsby returning.

For this afternoon OSH may now be looking and one or two rounds of
thunderstorms between about 22Z and 03Z. There is enough confidence
that at least one round will occur to include TSRA in the prevailing
group, but do have a TEMPO group from 22 to 02Z with no TSRA as it
will not be storming for the entire afternoon and evening. Any
individual storm that does pass over OSH will be capable of
producing severe winds over 50kts, large hail and heavy rain.

&&

.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.......GK
MARINE.........GK

NWS GRB Office Area Forecast Discussion