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Area Forecast Discussion (AFD)

Issued by Green Bay, WI (GRB)

FXUS63 KGRB 251127

Area Forecast Discussion
National Weather Service Green Bay WI
627 AM CDT Sun Aug 25 2019

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Monday
Issued at 338 AM CDT Sun Aug 25 2019

Quiet weather continues today before becoming more unsettled 
overnight through the day Monday. The main focus in the short-term 
will be on shower and thunderstorm potential and timing tonight and 
Monday. Some of the storms may produce heavy rainfall.

Today: The surface ridge that has been in place across the are over 
the past few days will continue to gradually slide off to the east 
of the area today. In fact, the surface high pressure center is 
expected to be over southeastern Quebec this morning, before sliding 
into the New England states/New Brunswick by this afternoon. This 
will be ahead of a developing low pressure system and cold front 
approaching from the northern Plains. Aloft, a ridge axis will shift 
eastward into the eastern Great Lakes through this afternoon as much 
of the western Great Lakes sits roughly on the inflection point 
between the ridge to the east and a trough to the west. Plenty of 
dry air in place across the area, will lead to a continued 
precipitation-day for much of WI. There will be some late morning to 
early afternoon CU development along with increasing high clouds 
during the afternoon hours. High temperatures today will be similar 
to Saturday with highs warming into the mid 70s for most locations, 
with a few upper 70 degree readings possible over the central and 
east-central portions of WI. 

Tonight: Clouds will further increase across the area and thicken 
overnight. Again, this will be well ahead of the aforementioned 
developing surface low and cold front. Current thinking is that most 
of the overnight hours will remain dry; however, an initial 
shortwave sliding across the Plains is expected to shift eastward 
into the area overnight. The system is expected to split into two 
pieces of energy, one is associated with an MCS, shifting eastward 
across the central Plains into Missouri this morning. The northern 
portion of this wave, over eastern Nebraska/western Iowa is the most 
likely portion of the wave to slide through at least the south half 
of Wisconsin. Moisture transport with this wave is not overly 
impressive and it will be fighting quite a bit of dry air entrenched 
across the area. That, along with only weak forcing (show by 
overlaying Q-conv and 1000-500mb RH), will lead to only a small 
chance of an isolated shower toward daybreak across portions of 
central into east-central Wisconsin. Very little instability will be 
available, so thunder chances look to be very low. Again, this is 
only associated with the first shortwave to slide through the area. 
Overnight lows will likely be kept a bit warmer with the added cloud 
cover and increased southerly flow. Lows are expected to be in the 
mid to upper 50s to around 60, warmest central and east-central.

Monday: The much deeper shortwave axis will dig across the northern 
Plains Monday morning and then push eastward into western WI by 00Z 
Tuesday. At the surface, the aforementioned surface low will deepen 
further as vort max rounds the base of the upper-level trough. The 
cold front is forecast to reach central Wisconsin between 21Z/26 and 
00Z/27. Along and ahead of the cold front, forcing and moisture will 
increase, allowing for better chances of showers and thunderstorms 
during the mid to late afternoon hours. It is possible that most of 
the day could be dry or at most have some isolated showers through 
the morning hours with the first shortwave. Instability looks to be 
very weak, especially with the front sliding through the area during 
the evening and overnight hours. 

Models are generally painting out around 500-1000 J/kg of MUCAPE in 
the late afternoon/early evening. Wind shear values are expected to 
be around 20 to 30 kts along and just behind the front. The better 
shear is expected to remain over southern Wisconsin into northern 
Illinois, where a 50kt mid-level jet is expected to develop. With 
limited instability and marginally favorable shear values, it looks 
like strong to severe thunderstorm potential is fairly low across 
northeast Wisconsin throughout the day Monday. Shear vectors are 
expected to become parallel to the front during the late afternoon 
and early evening hours, which may lead to some training 
thunderstorm potential and will likely lead to some heavy rainfall 
as the eastward motion will be defined by the speed of the 
front/forcing. Precipitable water values are expected to be nearly 
double the average for this time of year, with values in the 1.5 to 
2.0 inch range. This may lead to some ponding of water on roadways 
and potentially some localized flooding of poor drainage areas. High 
temperatures are expected to be in the low to mid 70s.

.LONG TERM...Monday Night Through Saturday
Issued at 338 AM CDT Sun Aug 25 2019

Monday night is the main concern during this part of the forecast
as a mid level trough and surface cold front traverse Wisconsin. 
Timing remains a bit uncertain as the models seem to be a little 
slower bringing the system across the state, but thunderstorms
look like a pretty sure bet. Models didn't show a lot of 
instability in the forecast area, but SPC has the southern half 
of the area in a marginal risk of severe storms.

The storms may produce heavy rainfall, but since it has generally
been dry across the area for at least a week, this is not as much
of a concern as it would have been earlier in the month. There is
just a marginal risk of excessive rainfall across Wisconsin.

Once the approaching system departs expect below normal 
temperatures with no widespread significant rainfall into early 
next weekend.

.AVIATION...for 12Z TAF Issuance
Issued at 627 AM CDT Sun Aug 25 2019

A few lake clouds/upslope clouds have pushed inland this morning
with bases around 4 kft to 6 kft. Fair weather cumulous clouds 
will become more widespread again by around 15z today as cirrus
clouds also increase from the south and southwest. The best chance
of seeing some broken clouds and perhaps a sprinkle will be across
the western TAF sites, near CWA and AUW specifically. Clouds will
steadily increase and thicken tonight as the next system
approaches the area, with widepspread rainfall expected by late
Monday morning and especially into the afternoon hours. Otherwise,
MVFR conditions can be expected for this issuance.



SHORT TERM.....Cooley