Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 200330

Area Forecast Discussion
National Weather Service Green Bay WI
1030 PM CDT FRI AUG 19 2016

Updated aviation portion for 06Z TAF issuance

.SHORT TERM...Tonight and Saturday
Issued at 405 PM CDT Fri Aug 19 2016

Showers moved across northern WI this afternoon as a surface
frontal boundary over NW WI slowly worked toward the area. A few
showers also popped up over far northeast WI. Otherwise, thick
cloud cover and little forcing has keep things dry. Due to the
morning rain and cloud cover through the day, temps struggled to
climb, with most of north central WI still in the mid to upper
60s and 70s elsewhere.

Hi-res models weaken this area of showers associated with the
frontal boundary as it pushes east into Upper Michigan late this
afternoon and early evening. Larger scale models hang on to a
broad area of precip as the front sags / washes out across the
area. There is an area of showers and storms over southeast MN and
northern IA associated with a weak shortwave that may make a run
at central WI later this evening. CAPES are building across
southern WI, so this activity should sustain itself as it moves
ENE. So after a dry afternoon, look for more activity across
central WI by late evening. Severe weather threats is very low
with little to no instability, but heavy rainfall will be
possible as precipitable water values will be between 1.5 and 2

All of the activity tonight will be ahead of the main upper
trough and developing low pressure system. These will combine to
bring the main show on Saturday as the unseasonably deep upper
trough swings across the area along with the deepening low
pressure system. Model consensus is to bring the low pressure
system over Green Bay early Saturday afternoon, placing the
heaviest rainfall and best frontogenetical forcing over central
and north central WI. A widespread 1 to 2 inch rain event is
expected precipitable water values remain between 1.5 and 2 inches
so heavy rain will be the greatest threat, with localized flooding
possible and rises in rivers and streams. The heaviest rain the
last 36 hours fell over central WI, so that area looks to be at
the greatest risk of flooding. That said, the heaviest totals fell
just south of Wood and Portage counties, so do not feel a flash
flood watch was necessary. Over eastern WI, heaviest rain totals
will be more dependent on where/if storms form, although overall
forcing will also produce showers for most/all of the area.
Showers and storms end from SW to NE Saturday afternoon into the

The severe weather threat is best across far eastern WI from
Saturday morning into the early afternoon, even with minimal
instability. Any sunshine (which is very unlikely) will only
enhance the threat that is already there due to the impressive
dynamics in place. Damaging winds and hail would be the greatest
threats with any storms that can form.

.LONG TERM...Saturday Night Through Friday
Issued at 405 PM CDT Fri Aug 19 2016

Model Guidance is somewhat more in agreement with respect to the
seasonably strong shortwave trough and associated surface wave
that will begin to exit the forecast area Saturday evening. By
00Z Sunday, consensus of the guidance suggests that deepening
surface wave should be located somewhere over northern Lake
Michigan with attendant cold front moving just off the eastern
Wisconsin shoreline. Strong lift and area of moderate to heavy
rainfall just to the north and east of the surface low should be
exiting the far northeast corner of Wisconsin shortly after
00Z...with mid-level dry slot pushing its way northeast across
much of central and eastcentral Wisconsin. Maintained likely pops
across much of central and northcentral Wisconsin Saturday night
into Sunday morning as upper trough still deepening just east of
the area along with a couple subtle embedded impulses forecast to
quickly dive southeast across the area Sunday morning. Gradually
tapered pops into the slight category over eastcentral Wisconsin
Sunday afternoon as drier airmass begins to infiltrate the area.
Will be cooler and quite breezy on Sunday with strong subsidence
and modest pressure gradient over the area.

High pressure quickly builds into the state Sunday night and
Monday as upper heights begin to rebound in the wake of the
departing shortwave trough. Looks like a pleasantly cool night
Sunday night with lows generally in the low to middle 50s. After a
sunny start on Monday, return flow on the back side of the
departing high pressure system will quickly develop late in the
day. Most of the model guidance suggests there will be just
enough moisture in concert with stout WAA to at least generate a
band of mid-level clouds mainly across northern Wisconsin Monday

More disparity in the model guidance for Tuesday with respect to
how widespread shower or thunderstorm activity will be over the
northern half of the forecast area in WAA zone ahead of
shortwave developing over the northern plains. GFS and NAM seem
to be more robust with the precipitation while the ECMWF remains
essentially dry through at least Tuesday night. For now went with
the blend of model guidance in this regard with low chance pops
for Tuesday mainly across northcentral Wisconsin. Primary
shortwave trough will then approach the western Great Lakes
Wednesday night and Thursday with more widespread shower and
thunderstorm activity possible.

.AVIATION...for 06Z TAF Issuance
Issued at 1030 PM CDT Fri Aug 19 2016

Shower and storm threat ramps up after midnight into early
Saturday morning as low pressure approaches from the SW and
seasonably strong upper level system pushes in from the west.
Look for poor flying conditions for much of Saturday as
widespread showers and fog expected along with the chance for




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