500
FXUS63 KGRB 202012
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
312 PM CDT Sat Jul 20 2024

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- Isolated to scattered showers and thunderstorms are expected
  across northern WI this afternoon/evening. The chance of showers
  and thunderstorms continues at times Sunday afternoon through
  the middle of next week. Severe storms are not expected, but
  locally heavy rainfall may occur.

- Hazy skies are expected to continue this weekend as wildfire
  smoke remains across the Great Lakes. Minimal impacts are
  expected at the surface.

- Seasonal temperatures continue into next week.

&&

.DISCUSSION...
Issued at 312 PM CDT Sat Jul 20 2024

Short Term...Tonight and Sunday

Scattered showers and a few rumbles of thunder developed this
afternoon along a slow- moving, backdoor cold front. As this
front continues to drop southwards through the late afternoon and
evening, additional shower and thunder activity seems likely. The
main impact with any rain will be heavy localized rainfall, given
their slow moving nature. Severe and strong storms are not
expected, mainly due to an absence of both shear and upper level
support for more organized convection. Still, thunder will be
possible at times, at least until we lose diurnal instability. Any
active weather will likely diminish quickly in coverage and
strength overnight once the instability is gone, so removed
thunder for this period. An isolated shower or two will still be
possible, but most will likely see dry conditions return. Patchy
fog will be possible tonight as temperatures drop towards the
lower 50s to lower 60s. Any fog may be denser in areas that see
rain.

Smoke from Canadian wildfires may return to our region behind the
front. Fortunately, the thickest and densest plume of smoke
appears to remain to our west and minimal smoke is expected to get
near the surface. Still, the sky may take on a more hazy
appearance by the late evening tonight through portions of Sunday.

The front will be predominantly south and west of the region by
late morning Sunday, at which point precipitation will be largely
out of our area. That said, diurnal instability in most models
does redevelop across our southwestern counties, encompassing
portions of central WI to the western Fox Valley. Afternoon CAPE
values are a bit more modest, around 500-750 J/kg, but this may
still bring some isolated to scattered redevelopment behind the
front during the afternoon. Again, would expect no severe threat
but heavy rain will be possible for the slow moving nature of any
precipitation. Outside of the rain, temperatures on Sunday will be
in the middle 70s to lower 80s.

Long Term...Sunday Night Through Saturday

Main forecast concerns revolve around assessing best chances for
showers and storms early/mid next week, with a period of quiet
weather expected at the end of the week. Temps look to remain
mainly +/- 5 degrees of normal much of the period.

Can`t rule out a stray shower or storm Sunday evening/night,
especially across central WI, but most spots look to remain dry
as instability wanes/shifts westward early in the evening.
Chances for some showers and storms will increase again on Monday
as daytime heating will lead to some hit or miss activity. Best
instability (CAPE approaching 1000 J/kg) will be across central WI
as the northeast flow will keep more stable conditions closer to
the U.P. border and Lake Michigan. Showers and storms could linger
into Monday night, especially across central and north central
WI. Similar set up for Tuesday with a little more instability,
along with a weak front and shortwave energy dropping south
across the region, likely providing for more widespread activity.
The upper shortwave/trough will swing across/through the region on
Wednesday, keeping the chance for shower/storm activity in the
area, especially during the late morning and afternoon. No severe
weather is expected during the early/mid week period but with
PWATs around 1.0" Monday and Wednesday, and near 1.5" on Tuesday,
some locally heavy rainfall is possible where the slow moving
storms occur.

Dry conditions are expected Thursday and Friday as surface high
pressure and mid-upper level ridging slides across the Great
Lakes. Chances for showers and storms arrive sometime next weekend
as the ridge breaks down and a boundary approaches from the
northwest.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 105 PM CDT Sat Jul 20 2024

Generally VFR conditions will prevail through this afternoon and
evening. Exception being across northern WI were a slow moving line
of thunderstorms may drop vsbys to MVFR/IFR. Framed out a window of
21Z-00Z during which thunderstorms would be most likely in the RHI
TAF. Behind the cold front, driving the line of thunderstorms, winds
will become north to northeasterly.

For areas south of a RRL to NMN line this afternoon a developing
cumulus field will result in SCT to BKN cigs around 4.5-6kft. As the
aforementioned cold front pushes further south this evening and
overnight scatter showers are expected to move across central WI,
however, lightning active is expected to wane after sunset. As mentioned
above as the cold front moves through winds will veer and become
north to northeasterly through Sunday morning. Additionally, areas of
fog may develop across portions of central and northern WI after
midnight. Any fog that does develop should burn off by 12-13Z
Sunday. Lastly, there is an area of wildfire smoke moving in behind
the cold front as well, however, comparing current surface vsbys
with satellite trends it appears most of the smoke is remaining
elevated. Will need to monitor these trends as smoke arrives to see
if adjustments to vsbys will be needed.

An area of non-severe thunderstorms may return to central WI around
mid-day Sunday, but confidence in how far north and east this storms
will develop is too low to mention in the CWA and AUW TAF at this
time.

.OSH...A borderline SCT to BKN cumulus field has developed over OSH
early this afternoon with cigs around 5kft. The cumulus field will
persist over the area through this afternoon with cloud bases rising
to around 6kft. Depending on when the cloud deck developing ahead
of a slow southward advancing cold front arrives skies may clear out
for a period this evening, otherwise this cloud deck will bring BKN
cigs around 5kft tonight. Current trends suggest that any
precipitation associated with the cold front will stay north and
west of OSH. As the cold front moves through overnight winds will
veer from the west and become northeasterly for Sunday morning.
There is an area of wildfire smoke moving in behind the cold front
as well, however, compering current surface vsbys with satellite
trends it appears most of the smoke is remaining elevated. Will need
to monitor these trends as smoke arrives to see if adjustments to
vsbys will be needed.

&&

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

&&

$$

DISCUSSION.....Uhlmann/Bersch
AVIATION.......GK