274
FXUS63 KGRB 142343
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
643 PM CDT Mon Jul 14 2025

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Near-surface Canadian wildfire smoke is most likely over north-
  central WI through this evening.

- Isolated showers and storms into early this evening. Scattered
  storms across northern WI late Tuesday afternoon into Tuesday
  night. Severe storms are possible far north central during the
  evening. Heavy rain also a risk.

- Staying active with showers and a chance of storms Wednesday
  through Wednesday evening. Marginal risk of severe storms and
  heavy rainfall. Lingering scattered showers on Thursday, then
  dry for the end of the week.

- Very warm and humid through Wednesday. Heat index values
  peaking in the lower 90s on Tuesday. Cooler and less humid late
  this week.

&&

.DISCUSSION...
Issued at 250 PM CDT Mon Jul 14 2025

Lingering smoke and thunderstorm chances and impacts are main
focus points through midweek.

Smoke: Today opened up with smoke farther south than was projected
by HRRR/RAP smoke models last 24 hours, with many 5-7 mile vsby
observed as far south as east-central MN into central WI. Even
with the trends, visibility is not as low as yesterday and air
quality is not as poor. Using the HRRR/RAP guidance as guide, but
adjusting it, kept mention of patchy to areas of smoke. Most of
the smoke later this afternoon into early this evening that could
result in poorer air quality will be advected into far northern
WI from northeast MN. This is where an Air Quality Advisory is in
effect until 6 pm this evening, though only for Vilas County in
our forecast area. Still looks like approaching frontal boundary
over northern Plains and associated southwest flow ahead of it
will eventually push the smoke to the north later tonight.

Thunderstorm Chances and Impacts: Weak ripple showing up as
scattered mid clouds shifting across northern WI along with weak
wind shift will interact with MLCAPES over 1000J/kg to yield a
few showers and perhaps a storm over especially far northeast WI
this afternoon. No severe weather is expected. 12z PWAT on GRB
sounding not even reaching the 75th percentile, so heavy rain
seems like minimal threat as well, so long as storms keep moving
at decent clip.

Next chance for storms arrives late Tuesday as weak low pressure
system pushes cold front from South Dakota to northern Minnesota.
Strongest dynamics and wind energy aloft stays to the north which
could limit organization and coverage, but there is a modest low-
level jet intersecting the approaching front along with building
instability as MLCAPEs reach 1500-2000J/kg over northwest WI
during peak heating on Tuesday afternoon. Ahead of the front,
effective shear is 25-35 kts which could support loosely organized
storms, marginal severe risk with damaging wind gusts and small
hail. Storm motions will be slower and PWATs will be over 1.75
inches which is well over the 90th percentile for the day. Warm
cloud depths of 11-12kft are also favorable for warm rain
processes. Overall it seems there could be a few storms late
Tuesday over northwest WI that could produce damaging wind gusts
and heavy rain. Eventually these will reach north-central WI
early to mid Tuesday evening, though the severe risk should wane
later into the evening as instability lowers and the low-level jet
weakens. Heavy rain could still be an issue though with training
storms over the north even into the overnight hours as the showers
and storms shift east. WPC is highlighting northern WI for a
Slight Risk of Excessive Rainfall.

Later Tuesday night, shortwave over the Dakotas, maybe combining
forcing with convectively generated shortwave out of storms that
form on Tuesday afternoon over the Dakotas, will be shifting
toward Upper Mississippi Valley. By daybreak Wednesday, the wave
(kind of looking like an MCV in some of the guidance) will be
running into what looks to be a complex diffuse frontal boundary
somewhere over Wisconsin. Gradient of MLCAPE will be over or just
south of the forecast area. Storms motions will be from southwest
to northeast, so either by track of upstream complex of storms
and/or developing storms ahead of shortwave, portions of area will
see showers and storms on Wednesday. Severe potential will depend
on how far building instability extends from the south, but given
the presence of decently defined wave, think there is a potential.
Latest SPC Day3 outlook has shifted the risk farther north, which
looks good based on location of instability gradient.

Heavy rain a continual issue with PWATs at least as high, if not
higher, pooling ahead of approaching system and along the frontal
boundary. NBM probabilities for rainfall can be on the low side
when dealing with qpf with summertime storms, but 90th percentile
which offers reasonable high-end scenario, shows 24hr total QPF
for Wednesday and Wednesday night up to 1.75 inches over central
and north-central WI with amounts over 2 inches not that far off
to the west. Trends on this have come up over the last couple
days. As Tuesday night looks more active mainly over the north,
Wednesday into Wednesday evening are looking more active over much
more of the area, including the urban areas of the Fox Valley
which can be more prone to flooding. WPC shows all our area in a
Marginal Risk for Excessive Rainfall.

Temperatures and Heat Impacts Through Wednesday: Ahead of the
approaching front which brings the more active weather late
Tuesday through Wednesday, a very warm and humid airmass will
surge into the region. Highs reach the mid 80s today, with
readings nearing 90 on Tuesday. These temps combined with
dewpoints 69F to 71F will yield heat index values into at least
the lower 90s, if not mid 90s locally on Tuesday. Nighttime lows
will only drop into the mid to upper 60s tonight, with most areas
staying in the upper 60s to near 70 on Tuesday night just ahead of
the front. Exception Tuesday night will be over the far north
with more rain cooled air. Though it will remain muggy on
Wednesday, more clouds and chances for showers and storms will
keep highs in the mid 80s. Overall, heat index values into
Wednesday will remain below criteria and there is no enhanced
signal showing up when looking at the heat in terms of relation to
this time of year climatology and/or duration of the heat (NWS
HeatRisk output). Thus, no headlines issued for the heat at this
time.

Outlook: Frontal boundary shifts east by Thursday. Could still be
a few lingering showers on Thursday morning, but risk of severe
and heavy rain will be east of the area by that time as high
pressure settles over the western Great Lakes. The high will bring
cooler and less humid, more comfortable conditions to the region.
The break in the weather will not last long as west-northwest flow
aloft and series of weak fronts will result in chances for showers
and at least isolated storms. Its early, but at this point, greater
instability is forecast to stay mainly west and south of the area,
keeping severe potential on the lower side for the coming weekend.
Temperatures look seasonable with highs upper 70s to lower 80s and
lows in the mid 50s to lower 60s.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 632 PM CDT Mon Jul 14 2025

Satellite still showed patchy smoke over the forecast area at
issuance time, but this had evidently become more elevated, as
sfc vsbys were only 7-10sm. Based on current observational and
smoke model trends, do not plan to include smoke in this set of
TAFs. We could see some minor vsby restrictions due to fog
overnight, mainly from central into far northeast WI, but will
only mention MVFR vsbys at AUW/CWA for now.

Isolated showers and possible a storm developing in a moisture/
instability gradient near a weak front will dissipate early this
evening, leaving mostly clear skies overnight. SCT cumulus clouds
may redevelop during the heating of the day on Tuesday. A line of
thunderstorms is expected to reach NC WI Tuesday evening, and
this will be addressed with the 06z TAF issuance.

Light S-SW winds expected tonight, then becoming gusty during the
day on Tuesday.

&&

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

&&

$$

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