909 
FXUS63 KGRB 250834
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
334 AM CDT Wed Aug 25 2021

Forecast discussion for routine morning forecast issuance

.SHORT TERM...Today...Tonight...and Thursday
Issued at 333 AM CDT Wed Aug 25 2021

A brief break in the thunderstorm activity can be expected through 
tonight, before the next chance arrives by Thursday afternoon. 
Otherwise, above normal temperatures and more humid conditions can 
be expected across the area.

Today and tonight: A surface low over Ontario this morning, is 
expected to shift east, reaching western Quebec late this afternoon 
into tonight as it steadily weakens. At the same time, the low will 
drag a weakening cold front across the area from west to east. This 
front is expected to slow and become nearly west-east oriented 
across central/east-central Wisconsin by late tonight as it slowly 
sags toward southern Wisconsin. Drier air aloft and a stout CAP 
across the area should limit any shower or thunderstorm potential 
through this time period. The lack of thunderstorms will allow 
temperatures to warm well above Tuesday's temps with most places 
warming into the mid to possibly upper 80s. Humidity levels will be 
elevated as well with dewpoints reaching into the upper 60s to low 
70s across central, east-central and far northeast Wisconsin. This 
will make it feel more uncomfortable this afternoon. Overnight lows 
are forecast to be in the low 50s across north central WI and in the 
low to mid 60s for much of the rest of the area.

Thursday: The aforementioned frontal boundary is expected to linger 
across southern Wisconsin as a shortwave approaches the area from the 
west. Current timing on the shortwave appears as though it would be 
closer to 18Z to 21Z before the shortwave arrives. This will lead to 
increasing chances of showers and thunderstorms from west to east 
throughout the day. The greatest instability is expected to remain 
south of the frontal boundary; however, some elevated instability 
will allow for the potential for some thunderstorms across northeast 
Wisconsin. The severe weather threat looks to be fairly low through 
Thursday afternoon and into Thursday evening. High temperatures will 
likely be kept a bit cooler on the north side of the boundary and 
with the increasing potential for showers and storms. Afternoon high 
temperatures will likely remain in the mid to upper 70s.

.LONG TERM...Thursday Night Through Tuesday
Issued at 333 AM CDT Wed Aug 25 2021

Will continue to see active weather through the weekend, with 
quieter and less humid weather early next week.

Thursday night through Friday...Latest models are suggesting this
period to watch for heavy rainfall potential across central and 
northeast WI, while the threat for severe thunderstorms appears a 
little less. Still dealing with a muddled forecast and uncertainty
on where various features and boundaries will be located. With 
that said, there seems to be at least some agreement that a 
surface boundary will be situation just south of the GRB forecast 
area Thursday night and stretching back towards southern MN and 
northern IA. This while a LLJ increases across northern IA and a 
complex develops on the nose of this jet Thursday night, possibly 
tracking into central WI early Friday morning and then moving east
throughout Friday. Pwats are around 1.6 to 1.9 inches on Friday. 
With the climatological max for GRB at 1.87 inches for this date 
heavy rainfall will certainly be possible. Forecast time sections 
also show a very saturated atmosphere to about 35K to 40K feet. 
Meanwhile, the thunderstorm potential is less certain. If the sfc 
boundary stays south as thought it will be tough to get much 
instability, especially across northern WI. The best chance for 
any storms will be across central into east- central WI Friday 
afternoon. Current SWODY3 includes parts of central WI in a 
marginal risk of severe storms, with the rest of the area in 
general thunder. While the WPC ERO has parts of the CWA in a 
slight risk Thursday night and Friday, and the rest of the CWA in 
a marginal. Both of these outlooks make sense given the current 
setup. 

Friday night through Sunday night...Another complex could impact
the area Friday night into Saturday, this time with the LLJ firing
across western WI. Instability initially remains on the lower end,
but increases quite substantially by Saturday afternoon as the
surface boundary shifts northward. Heavy rain and thunderstorms 
will remain possible Friday night into Saturday, but the timing 
details are still unclear. Heading into Sunday, the main upper
trough/shortwave approaches from the west and starts to clear
everything out, but not before a cold front sweeps across the
area, possible during peak heating on Sunday. This will lead to
another opportunity for strong storms and heavy rainfall. 

Monday through Wednesday...After the cold front clears the area
Sunday night cooler and drier air filters in. This will lead to
less humid conditions with highs generally in the 70s and lows in
the 50s (still a little above normal for the last couple days of
August). Flow becomes NW to W as an upper ridge builds across the
Upper Midwest. The days should generally be dry if this pattern 
holds.
&&

.AVIATION...for 06Z TAF Issuance
Issued at 1055 PM CDT Tue Aug 24 2021

VFR conditions prevailed during the evening hours, with mid- 
upper level clouds continuing to spread across the area. Models 
have come into better agreement for tonight, keeping things dry, 
so will stick with no showers/storms in the TAFs. Still some 
question how much fog will form tonight, as clouds are expected 
through much of the night. Expect at least patchy/areas of fog 
tonight into early Wednesday due to recent rain, light winds and 
high dewpoints. Locally dense fog also possible if/where some 
clearing occurs. Any low clouds and fog will lift Wednesday 
morning, with only scattered clouds possible the rest of the day. 
Winds will remain on the light side, shifting to the 
west/northwest Wednesday and to the north Wednesday evening.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM.....Cooley
LONG TERM......KLJ
AVIATION.......Bersch