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Area Forecast Discussion (AFD)

Issued by Green Bay, WI (GRB)

FXUS63 KGRB 220012

Area Forecast Discussion
National Weather Service Green Bay WI
712 PM CDT Sat Sep 21 2019

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Sunday
Issued at 251 PM CDT Sat Sep 21 2019

The latest RAP analysis and satellite/radar imagery show a very
moist airmass surging northeast across Wisconsin ahead of a cold
front pushing east over Minnesota. Despite most unstable capes
upwards of 600 j/kg across the area, widespread low clouds
continue to shift northeast over northeast WI thereby limiting the
amount of instability that can develop. Some showers and drizzle are
developing over the area early this afternoon, but the area has 
been thunderstorm free so far. The best severe weather parameters 
reside over far western WI and Minnesota closer to the front. But 
thunderstorms cannot be ruled out, and short range models 
indicate at least isolated coverage for the rest of the afternoon.
A strong storm is possible, particularly over north-central WI 
towards late afternoon. Forecast concerns revolve around precip 
trends and potential for strong or severe storms. 

Tonight...The cold front will slowly move east tonight and
approach northeast WI by 12z Sunday. Thunderstorms will be
possible ahead of the front as severe weather parameters peak over
north-central WI during the evening. Most unstable capes up to
1500 j/kg and effective shear around 30 kts may result in strong 
or isolated severe storms over that area. HREF max reflectivity
paintball suggest the highest potential for strong storms will
occur during the 8 pm to midnight time period. With pwats upwards
of 1.80 inches, strong gusty winds and heavy rainfall will be the
primary threats in the strongest of storms. Though the potential
for storms will diminish early overnight, spotty showers and
drizzle will be possible until the front clears. Since a
warm/humid airmass will be in place for the most of the night,
temps will remain summer-like, and range from near 60 in the north
to the upper 60s over the Fox Valley.

Sunday...The front will remain over eastern WI for most of the
day, which will lead to on and off showers at times. A
strengthening shortwave will then move into the northern
Mississippi Valley in the afternoon, at the same time as a wave of
low pressure slides into northern Illinois. Rainfall is expected
to return north on the northern flank of this low generally from
mid to late afternoon. So will show increasing rain chances during
this time, highest across east-central WI. Temps are not expected
to rise much due to the clouds and rain.

.LONG TERM...Sunday Night Through Saturday
Issued at 251 PM CDT Sat Sep 21 2019

Sunday night...surface trough/cold front will complete its passage
across the area by early Sunday evening, with the upper trough
digging across the Great Lakes. Precip associated with the surface
features look to be pushing east of the area by mid-evening, but
increasing lift and forcing from the deep upper trough will
continue showers through much of the night. Prospects for thunder
are looking low as instability is almost nil but can't rule out
some isolated rumbles of thunder. Patchy fog will be possible, 
especially if clearing takes place before sunrise, but winds in 
the boundary layer look to remain above 5-10 mph and should keep 
things mixed up enough to limit fog potential. Temps will be 
cooler than previous nights as cold air advection begins. Lows 
look to drop into the upper 40s to middle 50s. The muggy 
conditions will also come to an end through the night. 

Monday and Monday night...cyclonic flow and the departing upper 
trough could linger a shower or two, mainly across northern WI in 
the morning, but the majority of the day/area looks to stay dry as
drier air advects into the area. Dry weather is expected Monday
night as surface high pressure and mid level ridging builds into 
the western Great Lakes. Light winds and clear skies will likely
lead to some fog development. Temps look to be near normal.

Tuesday into Wednesday...dry conditions will prevail for most of 
Tuesday as ridging slides east of the area. Return flow and strong
warm air advection will ramp up ahead of an approaching cold 
front. Strong cold front will swing across the area Tuesday night
into Wednesday morning. Timing not favorable for severe weather,
but instability and bulk shear values of 40+ kts could lead to
some stronger storms. PWATS climb to near 1.5" so heavy rain will
be possible, but flooding issues should be minimized by the 
progressive nature of the system. Will continue chance POPs into 
Wednesday to both cover if the system slows down and for any 
lingering post-frontal/cyclonic flow showers.

Rest of the forecast...dry weather returns Wednesday night into
Thursday as high pressure builds into the region. Another system
will impact the area later in the week.

.AVIATION...for 00Z TAF Issuance
Issued at 712 PM CDT Sat Sep 21 2019

A cold front will produce showers and a few thunderstorms as it 
moves slowly across the area tonight and early Sunday. MVFR/IFR 
flight conditions expected. Ceilings and visibilities should 
improve to VFR Sunday afternoon over most of the region.


Beach Hazards Statement until 6 AM CDT Sunday for WIZ022-040-050.


LONG TERM......Bersch