Forecast Discussion for GRB NWS Office
FXUS63 KGRB 191141

Area Forecast Discussion
National Weather Service Green Bay WI
641 AM CDT Sun Aug 19 2018

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Monday
Issued at 329 AM CDT Sun Aug 19 2018

Upper ridge and surface high pressure will keep
mostly sunny and warm weather across the area today. Highs will be
a few degrees above normal.

Clouds will increase tonight in warm advection ahead of an upper
trough approaching from the central Plains states. Some ground fog
is possible depending on how quickly clouds arrive.

Upper level diffluence, low level warm advection and the arrival
of Gulf moisture will bring an increasing chance of showers and
thunderstorms Monday. There might not be much rain across eastern
Wisconsin until late in the afternoon. Clouds and precipitation
will keep high temperatures close to normal.

.LONG TERM...Monday Night Through Saturday
Issued at 329 AM CDT Sun Aug 19 2018

Main focus for the long term will be the showers, storm
chance, and heavy rain threat Monday evening/night associated
with a low pressure system. Otherwise, pretty quiet late summer
weather is expected with another chance for storms arriving Friday
into Saturday. Temps will start off below normal on Tuesday, then
return to near to above normal for the rest of the week.

GFS/GFS Ensemble/Canadian/NAM are in decent agreement showing a
low pressure system tracking from northern MO Monday morning to
Lake Huron by Tuesday afternoon. The low pressure is only expected
to slowly deepen as it approaches the area. The ECMWF is also
showing this track, but a little slower. A weak surface boundary
should also reside over far northern WI or Upper Michigan. Aloft,
models all show a mid-upper trough swinging across the Great
Lakes along with a coupled jet. These features will create an
large area of showers and embedded storms (mainly south) across
much of the area Monday night. Instability at the surface and
aloft looked limited north of the low pressure, under under 500
J/kg, so will keep only a slight chance for thunder. Heaviest and
most persistent rain still looking to be in the Fox Valley and
lakeshore, where up to an inch or two is possible, possibly
leading to some localized flooding. Rain should wrap up pretty
early on Tuesday as the system pulls away from the area. Secondary
shortwave, cyclonic flow and lingering moisture in place during
the late morning and early afternoon, so expect clouds to stick
around along with a small chance of some showers.

Northeast winds will become a little gusty late Monday night, then
turn to the north on Tuesday. With the system not expected to
deepen much over the Great Lakes, winds will remain in check, but
gusts could reach 20 to 30 mph on Tuesday as we mix up to
2000-3000 ft. Winds will slowly diminish Tuesday night.

Surface high pressure builds across the Great Lakes on Wednesday
and Thursday, bringing dry and quiet late summer conditions. As
the high shifts east, moisture will increase by late in the week
ahead of a cold front, low pressure, and upper trough, with a
chance for showers and storms arriving Friday into early

.AVIATION...for 12Z TAF Issuance
Issued at 640 AM CDT Sun Aug 19 2018

Ground fog with LIFR conditions will improve quickly
after 13z. Good flying weather expected after the fog clears, with
light surface winds and just a few clouds. Clouds will increase
tonight, with some low clouds or fog possible towards dawn in
central and northcentral portions of Wisconsin. Showers and some
thunderstorms will arrive from west to east during the afternoon
or evening Monday, with lowering ceilings and visibilities.

Issued at 329 AM CDT Sun Aug 19 2018

Northeast winds will become gusty late Monday night, then turn to
the north/northwest on Tuesday. Gusts of 20 to 25 kts are
expected on Tuesday, with locally higher gusts to 30 kts possible
in the afternoon and early evening. Offshore winds will limit
wave heights, but waves of 2 to 4 feet, possibly up to around 5
feet, are expected. The winds and waves will create conditions
hazardous to small craft. A small craft advisory will likely be



LONG TERM......Bersch

NWS GRB Office Area Forecast Discussion