867
FXUS63 KGRB 220505
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1205 AM CDT Tue Jul 22 2025

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- A Beach Hazard Statement has been issued for Door, Kewaunee, and
  Manitowoc Counties for Tuesday as persistent south winds build
  waves to 3 to 5 feet (high swim risk).

- An active pattern will set up from Tuesday afternoon through
  Thursday night, with occasional showers and thunderstorms.
  Strong to severe storms and heavy rainfall will be possible at
  times. At least a low probability of flash flooding exists.

- Temperatures and humidity will rise through the middle of the
  week. The hottest day will be on Wednesday, when highs will
  reach the upper 80s to lower 90s, and heat index values will
  soar to 95 to 100 degrees for central WI.

&&

.DISCUSSION...
Issued at 342 PM CDT Mon Jul 21 2025

Short Term...Tonight Through Thursday Night

A 700 mb shortwave trough will bring some cloudiness late this
afternoon and evening to nrn and central WI, with any showers
over nw WI expected to dissipate in a drier airmass. Otherwise the
Hudson Bay high pressure area will shift to Quebec and the nern
USA on Tuesday, while a sfc trough and warm front extends from
the central high plains to nrn or central MN. Aloft, a MCV is
expected to track from ND to nrn Lake Superior on Tue.

Over nrn and central WI, weak low to mid level warm, moist
advection will develop early on Tue and increase into Tue nt via
sly winds and eventually a swly low level jet later Tue nt.
MLCAPE will likely build to 2000 J/KG or greater as steep mid
level lapse rates of 7-7.5 C/KM shifts into the region. The warm,
moist advection will bring shower and storm chances from 40-50
percent toward north central WI to a dry forecast for east
central WI on Tue. As the low level jet veers into nrn and central
WI Tue nt, a MCS is expected to track sewd from nrn MN into nrn
and central WI. Forecasting the srn extent of the MCS is
challenging as it will be running into warm 700 mb temps of 12C,
but Corfidi Vectors are nly and abundant low level moisture is in
place. Believe much of the area will see showers and storms with
50 percent chances or less along and south of a line from
Wisconsin Rapids to Green Bay. This MCS could easily linger into
Wed AM and inhibit some heating over the area. Mdt deep layer wind
shear will be in place along with mdt values of CAPE so a severe
MCS with damaging winds is a possibility.

The clouds and rain will be dissipating from Wed AM into the
afternoon with very warm and humid conditions developing, but
confidence is low at this time for heat headlines. The sfc
trough/stationary front will be draped from nrn MN to Upper MI or
nrn WI on Wed into Wed nt with a persistent swly low level jet of
25-30 kts so redevelopment to some degree is likely. Higher
confidence for widespread showers and storms would be for Thu into
Thu evening when a weak shortwave trough will kick a slow moving
cold front across the area. PWs of 1.8-2.00 inches will be in
place for Tue nt-Thu eve and cannot rule out areas that may see
repetitive convection and locally a few inches or more of rain
during this time. Urban and small stream flooding would be
possible but also at least a small potential for flash flooding.

Long Term...Friday Through Monday

High pressure will finally settle over the nrn Great Lakes for Thu
nt-Fri while dry conditions are forecast through the weekend. Very
warm and humid conditions will return during the weekend, while
weak shortwave troughs in wnwly flow aloft will bring a return of
small chances of showers and storms for Sun nt-Mon.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1152 PM CDT Mon Jul 21 2025

Aviation concerns include precipitation chances and potential for
MVFR ceilings later tonight into Tuesday morning.

Increasing moisture and elevated instability will arrive over the
western part of CWA overnight, and could lead to widely scattered
showers and storms. These should be limited due to a building
upper level ridge and only weak return flow/WAA, so don`t plan to
mention these in the TAFs. The building ridge and increasing
capping may also hinder any significant development during the day
on Tuesday, so despite there be being small pops in the forecast,
will hold off in the TAFs. There is a possibility that a short-
wave/MCV may brush through the NW part of the CWA late Tuesday
evening, leading to potential thunderstorm development in NC WI
(and RHI) toward the end of the TAF period.

Low-level moisture is expected to increase from SW to NE overnight
into Tuesday, and this seems reasonable given the wide expanse of
low clouds (mostly MVFR) over MN/IA/western WI. Will bring
stratocumulus into the western TAF sites overnight, and into the
eastern TAF sites early to mid Tuesday morning. Have opted to
mention MVFR ceilings at RHI/AUW/CWA/OSH/ATW during the morning
hours, with daytime heating causing ceilings to rise to VFR by
late morning/mid-day. The stratocumulus should persist until late
afternoon or early evening before scattering out. Low clouds may
move into the northwest part of the forecast area late tuesday
evening, so have introduced these to the western TAF sites after
04z/Wed.

Light southeast winds are expected overnight, with southerly winds
becoming a bit gusty during the late morning/afternoon on Tuesday.

.KOSH...Some stratocumulus clouds will arrive late tonight/early
Tuesday, then linger until sunset. Have opted to mention MVFR
ceilings during the early to mid morning hours. Think OSH will
remain dry through this TAF period, as elevated instability
remains to our west overnight, and upper ridging and capping
limits potential during the day and evening hours. Cannot rule
out some thunderstorm development as a warm front approaches later
Tuesday night/early Wednesday, so this will need to be watched.

Winds today will remain light from the southeast overnight, then
become SSE-S and a bit gusty during the late morning/afternoon
on Tuesday.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement from 9 AM CDT Tuesday through Tuesday
evening for WIZ022-040-050.
&&

$$

DISCUSSION.....MG
AVIATION.......Kieckbusch