543 
FXUS63 KGRB 062344
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
644 PM CDT Mon Sep 6 2021

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Tuesday
Issued at 313 PM CDT Mon Sep 6 2021

A very quiet and seasonal Labor Day ongoing with some high clouds
drifting across the area, along with some daytime cu. Temps were 
in the upper 60s to middle 70s.

For tonight and Tuesday...a quiet evening is expected as most/all
of the daytime cu will diminish late this afternoon and evening, 
as a few high clouds continue to drift across the area. As the 
evening progresses, a LLJ will strengthen to 40 kts over northern
MN, as a shortwave and cold front push southeast into the western
Great Lakes. Look for an area of showers/storms expected to 
develop ahead of these features. Some models have one main area, 
while others show two separate areas with one associated with the
stronger WAA. If the WAA activity gets going, the activity could 
arrive a little earlier, but the majority of the shower/storms 
will arrive after midnight, with the bulk of the activity 
along/north of Hwy 8 overnight into Tuesday morning. Activity 
along the front is possible across the entire area Tuesday 
morning, then will exit into Lake Michigan by 18-19z. Additional 
isolated-scattered showers and possibly a storm will be possible 
across northern WI as the upper trough swings into the area. As 
the surface low deepens over northern Lake Superior on Tuesday, 
gusty winds will develop, with gusts to 30 mph expected.

The organized severe weather threat still looks low, as the storms
look to be elevated overnight into early Tuesday. However, 
MUCAPEs will advect into the area as the LLJ moves east, with 
models ranging from 1500-3000 J/kg. This, along with steep mid-
level lapse rates and effective shear of 35-45 kts, will promote 
some stronger storms with a severe storm also possible near the MI
border. This matches with where the Marginal Risk resides. Wet- 
bulb heights will be running between 8,000-10,000 ft, so hail will
be the primary threat in any storm. Wind profiles support some 
wind gusts as well. The race will be on later Tuesday morning 
between the advancing front, the weakening cap, and building 
surface instability. Still a small chance some surface based 
convection could get going closer to the lakeshore and with decent
shear a strong or severe storm would be possible before the front
and storms push east into Lake Michigan. PWATs increase to near 
1.75" ahead of the front, so locally heavy rainfall will be 
possible.

Lows tonight will be warmer than last night with most spots
bottoming out in the 50s to around 60. Highs will be a little
tricky on Tuesday as clouds/precip trends could impact how warm
parts of the area will get. Lowered highs over the north (mid 60s)
as the most widespread showers/clouds are expected, with 70s
across the rest of the area.

.LONG TERM...Tuesday Night Through Monday
Issued at 313 PM CDT Mon Sep 6 2021

Tuesday night through Wednesday night: The front will quickly shift 
well east of the area Tuesday evening; however, the upper-level 
trough axis will shift slowly eastward through late Wednesday 
afternoon. Continued wrap-around moisture along with yet another 
shortwave passing through the area Wednesday into Wednesday evening, 
will lead to scattered showers and possibly a few thunderstorms with 
the cooler air aloft. The best chance for thunderstorms will be 
Wednesday afternoon during peak heating. 850mb temperatures will 
likely be cool enough with northwest flow to keep lake effect clouds 
and possibly rain showers into north-central WI through late 
Wednesday evening. Otherwise, the increased cloud cover will allow 
for cooler high temperatures, mainly in the mid 60s to around 70. 
Overnight lows are forecast to be in the 40s to around 50s.

Thursday into Friday, a surface ridge will build across the area, 
allowing for quiet conditions with precipitation-free conditions. 
Aloft, upper-level ridging will slowly build across the area. The 
combination of these features, will lead to warming temperatures, 
especially into Friday, when temperatures may be near to slightly 
above normal.  

Friday night through Saturday: The next low pressure system and 
associated cold front will approach the area late Friday night 
before the front passes through the area Saturday. This will lead to 
increasing chances for showers and thunderstorms with temperatures 
warming above normal, into the upper 70s.

Rest of the extended: Model solutions are generally pointing toward 
quieter conditions for the end of the weekend. At this point kept 
the forecast close to the blend of models for that time period. 
Temperatures are expected to be near normal.
&&

.AVIATION...for 00Z TAF Issuance
Issued at 622 PM CDT Mon Sep 6 2021

A strong system is expected to race through the area during the
next 24 hours. There is still considerable uncertainty related to
the timing, location, and strength of the convection that will
accompany it. 20Z SFC analysis indicated the primary cold front
was still way back in ND, which raises some doubt as to how 
quickly the models are sweeping it east across the area tomorrow 
morning. Opted to make a few minor adjustments to timing (slower)
with the 00Z TAF package, but otherwise stuck with the gist of 
the previous TAF issuance. May make some more substantial changes 
later this evening if the evolution of the convection becomes 
clearer.
&&

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

$$

SHORT TERM.....Bersch
LONG TERM......Cooley
AVIATION.......Skowronski