Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 231116

Area Forecast Discussion
National Weather Service Green Bay WI
616 AM CDT Sun Jul 23 2017

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Monday
Issued at 340 AM CDT Sun Jul 23 2017

Main forecast concerns include: potential for additional
thunderstorms this afternoon/early evening and extent of fog
tonight which could become dense.

The 07z MSAS surface analysis indicated a weak area of low
pressure over eastern Upper MI with a weak cold front arcing back
southwest through eastern WI. There was also an inverted trough
that stretched from northwest WI northwestward into northern MN
(associated with a mid-level shortwave trough). Radar mosaic
showed the last storms of the night exiting Manitowoc county with
pockets of fog left behind over northeast WI.

This shortwave trough is expected to move southeast into the Great
Lakes region today, accompanied by a couple of embedded shortwaves
within this trough. The air mass over northeast WI will still be
warm/moist which will lead to moderate instability when the
increased forcing from the shortwave trough arrives. Model show
the strongest shear to be over eastern WI (30 kts) which may be
just strong enough to sustain updrafts as thunderstorms attempt to
fire over the area. Will focus pops mainly over the east today
where the shear/instability to be highest. The risk of severe
appears to be more focused to our south and east, but an isolated
severe storm cannot be ruled out. Max temperatures to range from
the upper 60s to lower 70s north, around 80 degrees south.

A lingering shower or storm may linger over eastern WI into early
this evening, however an area of high pressure is forecast to
begin building into WI tonight, bringing cooler/less humid
conditions to the forecast area. Skies will attempt to clear
overnight, but with low-level moisture slow to depart with
light/variable winds, any clearing may simply turn into a stratus
deck with fog developing. Some of this fog may become dense and
will need to watch for possible advisory conditions. Min
temperatures to range from around 50 degrees north, around 60
degrees east-central WI.

Once the morning fog burns off, the rest of Monday looks mostly
sunny with below normal temperatures and comfortable humidity
levels as the area of high pressure moves overhead. Max
temperatures to be in the lower 70s near Lake MI, middle to upper
70s elsewhere.

.LONG TERM...Monday Night Through Saturday
Issued at 340 AM CDT Sun Jul 23 2017

Main focus for the extended period will be severe threat late
Tuesday into Wednesday. Otherwise, pretty quiet weather is
expected with temperatures hovering near or slightly above normal.

Dry and quiet weather is expected Monday night as surface high
pressure and mid-upper level ridging push across the Great Lakes.
Clear skies and light winds should allow some fog to develop,
especially across north central and far northeast WI where
boundary layer winds look the weakest. It will be another cool
night across the area, especially across the north, where some
spots could drop into the upper 40s.

Tuesday looks to be dry for most/all of the daylight hours as
models have started to converge on a solution that holds off the
precip associated with an approaching frontal boundary and upper
level disturbance until Tuesday night. Will increase POPs across
central and northern WI as shower/storm coverage looks to increase
overnight as a complex of storms is likely to develop over
Minnesota late in the day and push eastward overnight. Big
question is how this complex will behave as it works eastward late
Tuesday. The severe weather threat looks a little iffy late
Tuesday night and early Wednesday as any activity approaching from
the west/northwest will begin to encounter more stable
conditions, especially the farther east it works across WI, with
little to no instability forecast over the eastern half of the
state. However, can`t really argue with SPC day 3 outlook which
brings a slight risk west of a Rhinelander to Wausau line as a MCS
could make a run at the area overnight before weakening.

On Wednesday, models only differ slightly on the timing of the
front, showing the front across northwest WI at 12z Wednesday and
sweeping across the area through late afternoon. Timing not
perfect for severe weather, but assuming we get some sunshine,
CAPE values are forecast to climb between 800-1300 J/kg on the
GEM/ECMWF, with the GFS showing beefy values over 3000 J/kg. 0-6
km bulk shear of 35-45 kts looks to be sufficient to support some
organized convection. Dewpoints in the mid to upper 60s and
precipitable waters between 1.5 and 2.0 inches will bring some
heavy rain threat to the area as well, but the storms look to be
progressive. That said, any training of storms could lead to
localized flooding, especially in urban areas. How widespread
this event becomes will likely come down to if we can clear out
from the overnight convection to build instability ahead of the
front. If the front speeds up or an outflow sets up out ahead of
the front from the overnight convection, our area could miss out.
On the flip side, if the overnight convection dies out early and
the front slows down, we could be in for a more significant severe
weather event.

Showers/storms will exit by late Wednesday evening as the front
and upper level disturbance push out of the area. High pressure
then builds back into the region, bringing dry conditions late
this week into the weekend.

.AVIATION...for 12Z TAF Issuance
Issued at 616 AM CDT Sun Jul 23 2017

Patchy low clouds and fog are expected to dissipate by mid-
morning, although cigs/vsbys until then will be in the LIFR/IFR
range. Except for the RHI TAF site where cigs are forecast to
remain in the MVFR range today, the rest of the northeast WI
should see VFR conditions return. There is a chance for afternoon
showers and thunderstorms mainly over eastern WI as a mid-level
shortwave trough swings through the region. The bigger problem may
be tonight as the trough exits, skies try to clear and winds go
light and variable. Anticipate low clouds and fog to redevelop
later tonight with cigs/vsbys anywhere from LIFR to low-end MVFR
conditions. This fog should also gradually dissipate Monday
morning as an area of high pressure builds into the region and VFR
conditions return.



SHORT TERM.....Kallas
LONG TERM......Bersch
AVIATION.......Kallas is the U.S. government's official web portal to all federal, state and local government web resources and services.