940
FXUS63 KGRB 210002
AFDGRB

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

Updated aviation portion for 00Z TAF 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 00Z TAF Issuance
Issued at 702 PM CDT Sat Jul 20 2024

As expected line of slow moving thunderstorms along a cold front
is drifting south across northern WI. Thus far, RHI is the TAF
site mainly impacted with brief IFR visibility. Some storms may
continue through 01z at RHI. Appears that these storms will mainly
shift into central WI late this evening and early overnight. Have
included a mention of showers at AUW and CWA, but for now have
kept any mention out of the Fox Valley to the lakeshore, though an
isolated pop-up shower could occur as the cold front shifts south
through the area late this evening and overnight. RHI, AUW and
CWA may see some fog late tonight with greatest risk of sub IFR
visibility at RHI where there is best chance for partial clearing
and cooler temps setting up low-level inversion after the added
bonus of wet ground from the rain this afternoon. Smoke will be
around rest of tonight at all TAF sites, but at this point the
smoke appears elevated.

On Sunday, any fog will burn off quickly leaving scattered to
broken cu across much of the area. The elevated smoke layer will
persist as well. A few showers or storms may develop in the
afternoon as close as central WI. Confidence not high enough to
put any mention in the TAFs as a lot of model guidance keeps main
focus for these south and west of the TAF sites.

.OSH..VFR conditions will prevail tonight. A mix of cu and
blowoff mid clouds from the showers and storms over northern WI
will persist this evening. There is a also an elevated smoke layer
present. As a cold front drops south across the airfield between
03z-06z tonight, plan on winds to shift to northeast less than 10
kt. THere could be an isolated pop-up shower but it remains a very
low probability and has not been included in the TAF.

On Sunday, most of the day will feature scattered to at times
broken cu with bases around 050. The elevated smoke layer will
persist but at this point is not expected to reduce visibility.
There is a small risk of a shower or storm mid afternoon onward on
Sunday, but better chances look to remain farther south and west
over central WI, though it is possible they could impact the
approach into Oshkosh.

&&

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

&&

$$

DISCUSSION.....Uhlmann/Bersch
AVIATION.......JLA