114
FXUS63 KGRB 312331
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
631 PM CDT Sat May 31 2025

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Wildfire smoke returns for the rest of the weekend. Main impact
  will be hazy, milky skies.

- Very low relative humidity north-central and far northeast WI
  results in elevated fire weather conditions this weekend.

- Temperatures in the lower 30s with frost north central and far
  northeast WI late tonight. A Frost Advisory has been issued.

- High temperatures surging much above normal by Monday, when
  readings could reach around 90 in some locations. Highs in the
  mid 80s on Tuesday. Building waves will result in dangerous
  swimming conditions for Lake Michigan beaches Monday night
  through Tuesday night.

- Showers and thunderstorms Tuesday into Tuesday night. Risk of
  heavy rain and at least some severe storms.

&&

.DISCUSSION...
Issued at 216 PM CDT Sat May 31 2025

Short Term...Tonight through Sunday Night

NW flow aloft between ridge over the western CONUS and trough over
eastern Canada prevails this weekend. Ridge of high pressure
dominates at the sfc. Plumes of elevated smoke will drift across
the area in the short term, most widespread Sunday and beyond.
Low RH values expected this weekend which along with gusty winds
over 15 mph will lead to elevated wildfire potential. Already
issued SPS for today and with similarly low RH values on Sunday,
if winds are a bit stronger likely will need another SPS.

Quiet otherwise, though it looks chilly again tonight over north-
central and far northeast WI. Given the ridge, mostly clear skies
and light winds and a dry airmass, see no reason to not go with
lower (and recently better performing) MOS guidance for min temps
tonight. Result is lows in the lower to middle 30s from Lincoln to
Marinette counties north to the Upper Michigan border. Have issued
a Frost Advisory for these areas. Farther south, a few spots could
dip into the upper 30s, but risk for widespread temps in the mid
30s and areas of frost looks lower. The wildfire smoke could impact
high temps so have trimmed those a touch on Sunday. Return flow
kicks in on Sunday night as ridge shifts east, thus not looking as
cold as tonight.

Long Term...Monday Through Saturday

Broader trough over western Canada tracks east by midweek sweeping
up a southern stream wave across the western Great Lakes Tuesday
into Wednesday. Cold front at the sfc moves through Tuesday night
into Wednesday. Very warm and humid ahead of the front, then as
the front crosses, expect widespread showers and storms. High
pressure builds in later Wednesday/Wednesday night, then southern
stream systems could clip the area to close the workweek.

Severe storms/heavy rainfall: At first glance, Tuesday could be
severe weather day with front approaching during peak heating and
running into a very warm/moist air mass and winds aloft and dynamics
would support. Forecast instability not as high though as there
is hint of some capping and there is also potential for late night
and morning convection to the west to impact extent of heating
and instability. Still fair to say there is a risk of some severe
storms given the stronger 850-500mb winds overhead ahead of the
cold front. SPC Day4 slight area pokes into southern half of WI
which looks pretty similar to Machine Learning (ML) probabilities
for severe from CIPS/NSSL/NCAR. Heavy rain has looked like potential
for a while given the high pwats pushing over 1.75 inches, nearing
200-225 percent of normal. ECWMF EFIs show higher signals for QPF
and ECMWF/NAEFS show pwat values nearing climatological maxes for
this time of year. Storm motions should be high given stronger
winds aloft, but training storms could lead to localized flooding
potential. This is in line with WPC, which highlights all or part
of the area Tue and Wed in marginal risk of excessive rainfall
(ERO). Will continue to highlight heavy rain and severe potential
in HWO.

Temperatures/humidity: Summerlike warmth is expected Monday into
Tuesday with high temperatures rising into the upper 80s to around
90 on Monday, and highs well into the 80s on Tuesday. Local lake
breezes Monday, but gradient S/SW flow will keep lake and bay
modification limited. Low humidities this weekend will be replaced
with summer like humidities Monday into Tuesday as dewpoints rise
into at least the mid 60s both days. Ahead of the front, it will
remain very warm on Monday night with lows remaining in the 60s
area wide, and even in the mid to upper 60s in some spots. NWS
Heat Risk output for Monday is showing moderate values, indicating
that the heat will affect those individuals that are already
sensitive to heat, especially those without effective cooling
and/or adequate hydration. Looks like a relatively short hit of
the heat as in wake of the front, high temperatures will cool
back into the 70s for the rest of the week.

One item to note is that as the summer heat and humidity occurs
early this week, building waves will lead to a high swim risk and
dangerous swimming conditions on Lake Michigan starting Monday
night and persisting through early Wednesday morning. Thus, Tuesday
is not looking like ideal beach day despite the warmth and humidity
due the higher swim risk and the potential for storms later in the
day.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 631 PM CDT Sat May 31 2025

VFR conditions expected through the TAF period. Elevated wildfire
smoke will continue to spread across the region. The smoke on
Sunday looks to be quite thick, making for a very hazy sky, but
no reductions to surface visibility are expected. A few high-based
cumulus clouds will be possible in the afternoon, mainly across
central and north central WI.

Winds will become light and variable this evening with west
winds, generally under 15 kts, expected on Sunday. A lake breeze
will shift the winds to the south/southeast at MTW.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Frost Advisory from 1 AM to 8 AM CDT Sunday for WIZ005-010>013-
018-019-021-073.
&&

$$

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

NWS GRB Office Area Forecast Discussion