128
FXUS63 KGRB 201805
AFDGRB

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

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Showers and thunderstorms are likely across northern WI this
  afternoon. Severe storms are not expected, but locally heavy
  rainfall may occur.

- A chance of showers and thunderstorms will be possible at times
  from Sunday afternoon through the middle of next week. The
  severe thunderstorm potential is low for Sunday and Monday, but
  is too early to determine for Tuesday and Wednesday. Locally
  heavy rain is possible.

&&

.DISCUSSION...
Issued at 356 AM CDT Sat Jul 20 2024

Short Term...Today...Tonight...and Sunday

High pressure brought quiet conditions to the forecast area
early this morning, with partly cloudy skies and patchy fog in a
few locations. A cold front was dropping south through southern
Ontario and northern Lake Superior. This front will become the
focus for shower and thunderstorm development as it moves through
GRB CWA this afternoon and tonight.

The back door cold front will sag south through northern WI this
afternoon. A weak upper trough over the region won`t provide any
forcing, as the main short-wave will remain well to our west.
However, increasing moisture (dew points in the lower to middle
60s) and instability (CAPE around 750-1000 j/kg) along the front
should lead to numerous showers and scattered thunderstorms across
north central and far northeast WI. With PWATs increasing to 1.25
to 1.75 inches and weak flow (5 to 10 knots) aloft leading to
slow movement of any storms, there is potential for locally heavy
rainfall. Severe weather is not anticipated, as deep layer shear
is expected to be weak (5 to 15 knots). Highs will be mostly in
the upper 70s to middle 80s.

The cold front will move through the rest of the forecast area
tonight, but with the diurnal decrease in instability and a lack
of any other forcing mechanism, suspect that precipitation coverage
will be isolated to scattered at best. Low temperatures will range
from the lower to middle 50s far north to the lower to middle 60s
in C/EC WI.

On Sunday, the weak front should be situated south and west of
the forecast area, but models show scattered showers and storms
redeveloping in the southwest part of the forecast area during
the afternoon. The best chance should be southwest of a line from
Merrill to Oshkosh, where CAPE values of 750-1000 j/kg will
reside. Weak shear and light steering winds aloft will again
support a localized heavy rainfall threat with any storms. Farther
northeast, plenty of sunshine and a more comfortable air mass will
make for a nice day. Highs will be in the 75 to 80 range.

Long Term...Sunday Night Through Friday

Northeast Wisconsin will remain on the west side of a broad upper
trough through midweek, and then transition to weak ridging by
the end of the week.

A surface boundary residing over the area on Monday, combined with
daytime heating, will bring a chance for scattered showers and
isolated thunderstorms. MLCAPES are as high as 1000 to 1500 J/kg,
but weak shear and lack of upper forcing results in a low severe
thunderstorm potential. Pwats of 1.0 to 1.5 inches could bring
locally heavy rain.

There are higher chances (40-70%) for showers and thunderstorms
on Tuesday and Wednesday as a mid-level shortwave approaches from
the northwest, along with the sfc front still over the area.
CAPE values are similar, but 0-6km shear increasing to 15-30 kts.
Pwats are in the 1.0 to 1.5 inch range. May need to keep a closer
watch for a few isolated stronger storms, but this will also
depend on the position of the front and timing of the shortwave.
Models are not in great agreement on this at the moment.

Models continue to diverge late in the week, with the GFS quick to
move the front out and allow upper ridging to build. The ECMWF and
Canadian are slower with this. Sometime late Wednesday-Thursday
any remaining precipitation should clear out with dry days to
follow to end the week.

Temperatures will be near to slightly below normal, with daily
highs in the mid 70s to low 80s, and daily lows in the 50s to low
60s.

&&

.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.....Kieckbusch/KLJ
AVIATION.......GK