Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 311941
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
241 PM CDT WED AUG 31 2016

Forecast discussion for routine afternoon forecast issuance

.SHORT TERM...Tonight and Thursday
Issued at 240 PM CDT Wed Aug 31 2016

The main forecast challenge for the next 24 hours will be cloud
trends as the cool air mass moving over the warmer water of the
Great Lakes will generate lake effect clouds and even a few
sprinkles (showers?) across far northern or eastern WI.

The 19z MSAS surface analysis indicated a ridge of high pressure
extended from central Canada south into the Central Plains. We
resided in the cool, northerly flow with temperatures closer to
seasonal norms and low dew points. The radar mosaic had picked up
on a few very light rain showers or sprinkles over central
sections of Upper MI, otherwise the rest of the area was dry.

Northeast WI to remain under a northwest flow aloft tonight with
upper ridging over central NOAM and upper troughing over the
northeast CONUS. Models show a shortwave trough, embedded with
this northwest flow, will rotate through the Great Lakes overnight
and bring some passing clouds to the forecast area. This cool air
mass overhead will also develop additional lake effect clouds from
both lakes Superior and Michigan on north-northeast winds, thereby
keeping more clouds in the forecast near the Upper MI border and
along the Lake MI shoreline tonight. These clouds will play havoc
with temperatures as sky conditions could range from mostly clear
to mostly cloudy depending on where you are. For now, have gone
with a generic partly cloudy sky tonight with min temperatures
ranging from the middle to upper 40s north, to the middle 50s near
Lake MI.

An area of high pressure is forecast to build into the Western
Great Lakes on Thursday and veer the winds more to the northeast.
The air mass aloft is still rather cool (8H temps around +10C),
thus would expect another round of cumulus to develop with the
daytime heating. This wind direction should bring more clouds to
east-central WI and cannot completely rule out a stray shower or a
few sprinkles given the warm lake temperatures (mid to upper 60s).
Under partly to mostly cloudy skies, look for max temperatures to
be in the middle to upper 60s north-central/lakeshore, mainly
lower 70s elsewhere with a few middle 70s possible over central
WI.

.LONG TERM...Thursday Night Through Wednesday
Issued at 240 PM CDT Wed Aug 31 2016

Canadian high pressure will bring pleasant late summer weather to
the region through the weekend, along with a gradual warming
trend. Used a blend of the better performing guidance sets through
Saturday, with some adjustments to lower min temps in the typical
cold spots on Thu ngt and Fri ngt. Will probably see a couple
readings in the upper 30s Thu ngt.

Models continue to slow down the arrival of precipitation with an
approaching cold front, due to the blocking influence of the east
coast hurricane/tropical system. Have removed any precipitation
from the Sunday forecast, and will only carry chance pops for
Sun ngt/Mon. It is certainly possible that the lingering tropical
system will lead to an even later arrival of the precipitation.

Once the frontal system moves into the region, models suggest it
will be accompanied by abundant moisture (PWATs of 1.5-2 inches)
and instability (CAPE of 1500-2000 j/kg). Deep layer shear is
fairly strong during this period too. There is some evidence to
support a possible surface wave lifting northeast along the
stalled frontal boundary Monday night into Tuesday. This could
result in a period of heavy rainfall. Overall confidence in any
of the details is low due to uncertainty with the lingering
tropical system.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1227 PM CDT Wed Aug 31 2016

Strong CAA moving over Lake Superior on northerly winds will
continue to bring lake effect clouds to much of northeast WI this
afternoon before thinning somewhat this evening. Cigs are expected
to remain VFR with bases in the 4-6K ft range. A shortwave trough
is forecast to rotate through the Great Lakes region later tonight
and bring another round of clouds to northeast WI. Some of these
clouds could dip into the MVFR range, along with patchy fog over
the RHI TAF site. Even though high pressure is expected to build
into the region on Thursday, plenty of cool air aloft will once
again allow a cumulus deck to develop with cigs either at hi-end
MVFR or low-end VFR conditions.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM.....Kallas
LONG TERM......Kieckbusch
AVIATION.......AK


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