482
FXUS63 KGRB 171121
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
621 AM CDT Tue Jun 17 2025

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Scattered storms tonight south of highway 29. Severe weather is
  not expected. Showers with a few storms on Wednesday,
  especially Fox Valley to the lakeshore. An isolated severe storm
  and heavy downpours are possible.

- Thunderstorms will be possible at times from Thursday through
  Saturday. Some potenti.al for severe weather on Thursday
  afternoon and evening, then again Friday afternoon into
  Saturday. Details concerning timing and location are still
  uncertain at this time.

- A hot and humid airmass will move into the region this weekend into
  early next week. Heat indexes could rise well into the 90s.

&&

.DISCUSSION...
Issued at 216 AM CDT Tue Jun 17 2025

Short Term...Today...Tonight...and Wednesday

Shortwave trough and sfc low is over Lake Superior early this
morning. Cold front tied to the low is shifting across WI.
Elevated instability with the lift/convergence along the front
has triggered loosely organized clusters of showers and some
storms through the night. Isolated feisty storms have shown MESH
values over 0.5 inch at times.

Showers and storms will shift east of the area early this morning
(4-6 am). Rest of the day in wake of the front looks generally
dry. The cold front is weak so high temperatures will end up similar
to Monday with readings into the upper 70s north, to low-mid 80s
elsewhere. West to northwest winds behind the front will limit any
lake cooling. Later today, expect instability to build again from
IA to the IL/WI state line along the cold front. Shortwave trough
emerging into this regime will spur on new round of showers and
storms this afternoon over IA eastward along the cold front into
southern WI. Eventually, northern fringe of this development
will bring some showers and possible storms to southern areas
this evening into the overnight, before exiting late. Stronger
shear and even gradient of instability remains south, so severe
potential looks low tonight.

Attention then turns to the shortwave trough moving into the western
Great Lakes on Wednesday. Expect the wave and a convectively induced-
compact sfc low to track across southern WI into lower Michigan
through the day. There are differences in models how far north the
shortwave and compact low will track.

Along and to the south of the low, enhanced vorticity and backed
low-level flow along with ample instability will result in
greatest severe weather threat. To the north of the low, sharp
cyclonic flow and well developed comma head of precip will result
in moderate to heavy rain. If this heavy rain tracks across areas
from Fox Valley to the lakeshore, would see at least a minor
flooding concern as the urban areas from Green Bay to Appleton and
Oshkosh are more prone to flooding with heavier rainfall rates. An
isolated severe storm is possible, especially if the convective
low tracks more into our area. SPC has most of northeast and
east-central WI within a Marginal Risk for severe (level 1 of 5).
Also will need to keep eye farther north and west of the low as
building daytime instability may result in additional swath of
showers and storms during peak heating over north-central WI.

High temperatures on Wednesday will depend on track of the compact
low. Utilized blend of NBM/MOS guidance for readings near the
low, though not as low as NAM MOS showed (mid to upper 60s Fox
Valley). Used NBM over north-central and central, farther away
from the low and shield of more widespread showers.

Long Term...Wednesday Night Through Monday

The main story for this period will be an active pattern
Thursday into Saturday with several rounds of thunderstorms
followed by summer time heat as a 500mb ridge builds across
the southeast United States.

For Wednesday night, the first system will exit the area.
Lingering chances of rain and a few storms should linger into
the evening. Some differences in the speed of the system, thus
took a blend of the quicker/slower models which matched up well
with surrounding offices. On Thursday, a warm front is expected
set up across the western Great Lakes. The combination of daytime
heating indicated at least 500 to 1,000 J/KG of CAPE and 0-6km
shear values of 30+ knots. A few strong or severe storms may
possible during the afternoon and evening.

The warm front will linger across the area Thursday night into
Friday, thus continued chances of thunderstorms. Better
instability is expected on Friday along with sufficient shear
values to support strong or severe storms along and north of the
warm front assuming there is a break in the precipitation from
Thursday night. A lot of uncertainty on where the front will end
up, thus the focus for highest chances of storms uncertain. The
models are in better agreement with another complex of storms
moving across the area, in response to increasing low level jet
Friday night. The biggest question is where? Have broadbrushed
rain chances for the time being until there is better resolution
on location and any adjustments to timing. Any linger
precipitation should end on Saturday as the warm front lifts
northward. Based on the GFS model, strong capping should limit any
convection Saturday afternoon. If any storms do develop Saturday
afternoon, they would become severe quickly as model soundings
showing at least 3,000 CAPE and still strong 0-6 km shear.

The other story will be the heat with highs in the middle 80s to
lower 90s, possibly a few middle 90s in some spots. Did edge up
high temperatures by a degree or two Saturday through Monday
based on 850mb/925mb temperatures. Dewpoints are expected to
climb into the middle 60s to lower 70s. This could put heat index
values into the lower to middle 90s Saturday through Monday
across central and east-central Wisconsin. This would be the first
significant hot spell of the summer for northeast Wisconsin, thus
an increased risk of heat impacts this weekend into early next
week. Along with the warmer weather, will need to be concerned
about dense marine fog along the Lake Michigan shoreline this
weekend, and will also need to watch for the increased risk of rip
currents this weekend along Lake Michigan beaches.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 620 AM CDT Tue Jun 17 2025

Showers and storms that crossed central to far northeast WI have
exited across Lake Michigan early this morning. Today into the
evening looks mainly dry. The next round of showers and storms
will occur over southern parts of the area later tonight, though
there is a potential most of this will stay south of even ATW and
MTW. More areas will see some showers and storms on Wednesday.
Strong to severe storms will be possible, along with heavy rain.
Confidence is still low in the details.

IFR-MVFR ceilings have formed from central to north-central WI
(AUW/CWA and RHI). The low clouds will persist through mid
morning, before VFR cu and/or mid and high clouds prevail late
this morning through tonight. Visibility may lower to MVFR later
tonight during any showers or storms.

&&

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

&&

$$

DISCUSSION.....JLA/Eckberg
AVIATION.......JLA