973
FXUS63 KGRB 211200
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
700 AM CDT Sun Jul 21 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Isolated to scattered showers and thunderstorms are expected
  across parts of central WI and the southern Fox Valley this
  afternoon, and over north central and central WI Monday
  afternoon. More widespread storms expected on Tuesday. Locally
  heavy rainfall will be possible.

- Hazy conditions from elevated smoke should improve from the
  northeast today into tonight, then return to the northwest part
  of the forecast area on Monday.

- Seasonal temperatures continue into next week.

&&

.DISCUSSION...
Issued at 405 AM CDT Sun Jul 21 2024

Short Term...Today...Tonight...and Monday

The cold front had shifted just SW/S of the forecast area early
this morning, and only an isolated shower or storm was ongoing
in central WI. Some fog and low clouds had formed in parts of
north central WI, where heavier rains occurred yesterday evening.
Drier air was moving into far northern WI, where dew points were
dropping into the 50s.

Fog in NC WI will mix out quickly this morning. The cold front
will be situated south and west of the forecast area, but models
show CAPE increasing to 500-1000 j/kg in central WI and the far
southern Fox Valley this afternoon, so isolated to scattered
showers and storms may redevelop there. Farther northeast,
clearing skies and much drier air will lead to a very pleasant
day. Highs will be in the 75 to 80 range.

Any lingering afternoon convection in central WI should quickly
wane by evening, and weak high pressure and a drier Canadian air
mass will support a dry forecast through the night. Fog will
likely develop in the Wisconsin River Valley (NC/C WI) overnight.
Lows will range from the upper 40s and lower 50s north to 55 to 60
in the south and along the lakeshore.

On Monday, a short-wave trough and associated low pressure system
will approach the region from the northwest in the afternoon.
CAPE will increase to 500-1000 j/kg across the northwest half of
the CWA, leading to a chance of showers and storms in the
afternoon. Highs will be in the upper 70s and lower 80s.

Smoke Trends: Elevated smoke will impact the region today, but
the hazy conditions should improve from the northeast today into
tonight. The hazy/smoky conditions are expected to return to the
northwest part of the forecast area Monday afternoon. At this
point, it doesn`t appear there will be any significant impacts at
the surface.

Long Term...Monday Night Through Saturday

Somewhat active weather for the first couple days of the long term
forecast, and then transitioning into a dry and quiet stretch.

Surface front remains draped across Wisconsin Monday night and
Tuesday. Some lingering showers or thunderstorms are possible near
the front Monday night, but the better opportunity arrives Tuesday
afternoon as a mid- level shortwave approaches from the
northwest and daytime heating leads to increased instability. The
increased forcing will likely expand coverage of showers and
thunderstorms, with a 60-85% chance of receiving at least a trace
of precipitation within the GRB forecast area. CAPES are in the
800 to 1300 J/kg range, but effective layer shear remains weak,
around 10 to 20 kts. Therefore, the severe weather threat remains
low, but some gusty winds could accompany any storms. Pwats
around 1.5 in and the slower movement will bring an isolated
threat for locally heavy rain.

Models are trending towards the surface front moving out a little
earlier than originally depicted. However, with the shortwave
still to pass through Wednesday, some showers or isolated thunder
are still possible, especially the first half of the day.
Instability is very limited at this point, so any storms which do
form are not expected to be severe.

Surface high pressure and upper ridging take over from there,
bringing dry weather and seasonable temperatures for the rest of
the work week. Temperatures may trend above normal next weekend.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 640 AM CDT Sun Jul 21 2024

A variety of ceiling heights were observed across GRB CWA this
morning, but once the low stratus/fog mix out in NC/C WI early,
most locations will vary between MVFR and low-end VFR through the
morning. The exception will be across far NC/NE WI, where an
influx of drier Canadian air was already bringing some clearing.
There should be enough lingering moisture and instability to
support isolated to scattered showers and thunderstorms southwest
of a line from MDZ-PCZ-OSH this afternoon, but these should end
and clearing should arrive by evening. High pressure will bring
mostly clear skies tonight, though areas of fog should develop in
the Wisconsin River Valley of NC/C WI overnight.

Elevated smoke over the region should thin out from NE to SW today
into tonight, and is not expected to have any significant impact
on surface vsbys.

.OSH...Expect ceilings to vary between MVFR and low-end VFR
through midday/early afternoon, with VFR conditions and eventual
clearing occurring thereafter. There is a small chance (20%) of
showers or a storm, mainly between 18z-21z, but confidence is too
low to mention in the TAF. Clear skies expected overnight, with a
small chance of MVFR fog late. Light NE-E winds prevail through
this evening, then become light and variable overnight.

&&

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

&&

$$

DISCUSSION.....Kieckbusch/KLJ
AVIATION.......Kieckbusch