Area Forecast Discussion
Issued by NWS Green Bay, WI

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837
FXUS63 KGRB 231950
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
250 PM CDT Sun Sep 23 2018

Forecast discussion for routine afternoon forecast issuance

.SHORT TERM...Tonight and Monday
Issued at 249 PM CDT Sun Sep 23 2018

The latest RAP analysis and satellite/radar imagery show a cold
front stalling out over far northern WI and the Upper Peninsula
early this afternoon. This front has brought in some low stratus
over Vilas county, and scattered to broken cumulus over northern
WI. Meanwhile, clear skies are prevailing across east-central WI
all the way to a low pressure system over eastern South Dakota. As
the front over the U.P. border returns northwards, forecast
concerns mainly revolve around clouds and temps in the short term.

Tonight...As an elongated low pressure system moves from eastern
SD to northern MN, the stalled cold front will return north across
the U.P. and Lake Superior. Some clouds above 15 kft will likely
push into northern WI ahead of the low, but most areas will
continue to see mostly clear conditions persist. The pressure
gradient will tighten across the region, and the resultant
increase of boundary layer winds should limit fog potential. Did
lower temps a tad in the cold spots. But otherwise, lows should
fall into the middle 40s to lower 50s.

Monday...The low pressure system will continue to move north into
Ontario while pushing a cold front across Minnesota.
South/southeast winds will increase ahead of the front, and
should push highs into the upper 60s to middle 70s. However, the
south winds will also start to draw in increasing amounts of Gulf
moisture. This should result in more cloud cover and humid
conditions arriving for the afternoon. But precip chances should
remain south and also west of the region through early evening.

.LONG TERM...Monday Night Through Sunday
Issued at 249 PM CDT Sun Sep 23 2018

A cold front will move through the area Late Monday night into
Tuesday, bringing showers and a chance of thunderstorms. On
Monday night, pops will be highest in the northwest, near
the cold front. Models have trended east with a surge of tropical
moisture during the evening, so most of that rain should miss us.
On Tuesday, the main focus should be over the southeast half of
the forecast area, where the slow frontal passage will still be
occurring. Instability is progged to be fairly weak during most of
the event, except Tuesday afternoon in the Fox Valley/Lakeshore
region, when CAPE may reach 500-1000 j/kg. With deep layer shear
of 45-55 knots and upper divergence with the RRQ of a 120 knot jet
streak coincident with the instability axis, a few strong storms
cannot be ruled out.

The cold front should be east of the region by Tuesday night,
but the RRQ will linger over eastern WI during the evening and
a strong upper level trough is forecast to move through the
region late Tuesday night into Wednesday morning. Will need to
keep some small pops in eastern WI Tuesday evening, and across
far northern WI into Wednesday.

Another cold front is forecast to arrive late Wednesday night
into Thursday, along with the next chance of showers. Other than
mid-level lapse rates of 6.5 to 7 C/KM over northern WI Thursday
afternoon, models show little instability to work with. Will
only mention a slight chance of storms over north central and far
northeast WI Thursday afternoon, aided by the passage of a short-
wave trough.

High pressure may bring a brief dry period for the end of the
work week, but models strongly disagree on the arrival time of
precipitation for the weekend. The ECMWF model keeps dry weather
in place through Friday night, while the GFS has a much wetter
look through the entire weekend.

Temperatures will be above normal Monday night into Tuesday,
then below normal for the rest of the extended period.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1051 AM CDT Sun Sep 23 2018

Generally good flying weather across central and northern WI
through the taf period. Low clouds near the U.P. border have made
little progress south. Otherwise, may have few to sct fair
weather clouds build by early this afternoon only to dissipate
around sunset. Mid and high clouds increase some tonight at the
same time as the surface pressure gradient tightens. These trends
should limit ground fog potential. Low level wind shear increases
late over north-central WI, but not quite enough to include in the
tafs at RHI.
&&

.MARINE...Increasing southeast winds and favorable fetches will
lead to building waves on Lake Michigan late tonight into Monday
morning. Waves are expected to peak along the Lake Michigan
shoreline in the 09-13z time period at 4 to 4.5 ft. A small craft
advisory has already been issued, and will issue a beach hazard
statement for the morning hours north of Manitowoc.

Gusty southwest winds could also generate conditions that are
hazardous to small craft on Thursday and Thursday night.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement from 1 AM CDT Monday through Monday
morning for WIZ022-040.
&&

$$

SHORT TERM.....MPC
LONG TERM......Kieckbusch
AVIATION.......MPC



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