969
FXUS63 KGRB 241921
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
221 PM CDT Wed Jul 24 2024

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

 - Areas of ground fog are possible late tonight into early
   Thursday morning, especially in central and north central
   Wisconsin. Locally dense fog is possible.

 - After a cool night tonight, temperatures and humidity levels
   will be on the rise from Friday into early next week.

 - The chance of thunderstorms will return from Sunday through
   Tuesday. The severe weather potential is low at this time.

&&

.DISCUSSION...
Issued at 220 PM CDT Wed Jul 24 2024

Short Term...Tonight and Thursday

The latest RAP analysis and satellite/radar imagery show upper
troughing centered across the Great Lakes early this afternoon. At
the surface, high pressure is building in from northern Ontario.
Moisture fluxes off Lake Superior and Green Bay thanks to north
flow has contributed to a widespread cu field across northern
Wisconsin. Ceilings have been gradually rising since about mid-
morning but still have a few ceilings below 3000 ft early this
afternoon. No showers have developed thus far thanks to mid-level
capping and think the warm air aloft will keep the cu field in
check and prevent spotty showers from developing. Scattering of
the cu field is expected later this afternoon due to mixing into
dry air aloft. Forecast concerns mainly revolve around cloud
trends and fog potential over the next 24 hours.

Cloud trends and fog potential: High pressure will settle across
the area tonight. With loss of heating, the fair weather clouds
will dissipate this evening, which will set up a good radiational
cooling night. Forecast temps are forecast to fall well below the
cross over temps at most locations with the exception of the Fox
Valley. Expanded the potential for ground fog and will add a
mention of patchy dense fog to the HWO. With a light east
flow, portions of the Fox Valley to the west of Lake Winnebago
will likely see a lower potential for fog.

The boundary layer looks considerably less moist on Thursday so
don`t expect nearly the cumulus field as today. But do expect
scattered coverage based on upstream conditions over Ontario. Therefore
looking a mostly sunny to partly cloudy day at most locations.

Temperatures: Cool temps are forecast tonight in the 40s and 50s.
These temps are about 5 to 10 degrees cooler than normal to the
north and west of the Fox Valley.

Pleasant temps in the mid and upper 70s are forecast on Thursday.

Long Term...Thursday Night Through Wednesday

Focus of this forecast include warming temperatures late this week
into the weekend as sub-tropical ridging pushes into the region from
the west.  This will be followed by rain and thunderstorm potential
during the Sunday thru Monday timeframe when several shortwave
impulses impact the region.

Precipitation:  While a stable high pressure system will ensure a
dry period across the region through Saturday, potential remains
periods of showers and storms on Sunday through Monday. Medium range
guidance indicates a shortwave will lift into the Great Lakes region
from the southwest on Sunday.  With the high pressure departing to
the east, the airmass will be growing more unstable by this time
although it`s nothing impressive (up to about 1000 j/kg). Therefore,
the potential remains for thunderstorms on Sunday. Despite the more
unstable conditions, weak deep layer shear around 15-20 kts
indicates the potential for severe weather is low.

Additional shortwave energy will arrive from the west on Monday. The
airmass will become increasingly unstable (1000-2000 j/kg of cape)
and looks like the period of highest precip chances.  Deep layer
shear remains relatively weak (15-25 kts), but precipitable water
values peak around 1.75 inches during this timeframe so the threat
of heavy rainfall will increase.  The relatively weak shear should
keep the risk of organized severe weather low.

For both days, confidence in the details like timing and precip
probabilities are relatively low.

Temperatures:  Once the high passes to the east, the airmass will
become warmer and more humid.  While excessive heat is unlikely,
highs will be warming into the upper 80s to lower 90s from Sunday
through the middle of next week.  Combined with dewpoints in the
lower 70s, heat indices in the low to mid 90s will be possible
during the afternoon hours.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1245 PM CDT Wed Jul 24 2024

Shallow moisture below 850mb has led to an extensive
blanket of MVFR to VFR stratocumulus over Northeast Wisconsin early
this afternoon. Strongly anticyclonic flow trajectories associated
with surface high pressure north of Lake Superior and relatively
warm temperatures between 6.0 and 12.0 kft agl will keep stable
conditions in place this afternoon. A subset of model guidance
suggests rogue shower activity will be possible along a Bay of Green
Bay convergence boundary during the late afternoon and early
evening, however, the models appear to have initialized too rich of
near surface moisture. VFR conditions this evening.

A very favorable radiative cooling environment is anticipated
tonight as surface high pressure continues to build into the region.
Shallow but dense ground fog will be possible for most/all of the
taf sites around daybreak Thursday. A period of IFR to LIFR
conditions is possible after 09z, but expected to mix out quickly
Thursday morning.

.OSH...MVFR to VFR stratocumulus this afternoon. Little
confidence exists in the modeled depiction for a shower chance at or
to the north of the airfield in the 6-9pm time window. A scattering
trend this evening with a potential for dense ground fog at/near the
airfield 4-8am Thursday. A light east flow overnight may allow the
relatively warm waters of Lake Winnebago to influence the formation.
Ceilings are expected to mix quickly Thursday morning with
prevailing VFR conditions at the end of the period.

&&

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

&&

$$

DISCUSSION.....MPC/MPC
AVIATION.......CB