Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 291804
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
104 PM CDT Mon May 29 2017

Updated aviation portion for 18Z TAF issuance

.SHORT TERM...Today...Tonight...and Tuesday
Issued at 335 AM CDT Mon May 29 2017

A cold front will slowly make its way through the western Great
Lakes today. The front will bring a cooler air mass to the region,
as daytime highs are limited to the upper 50s across north-central
Wisconsin, with lower to middle 60s across the rest of the cwa
where a later arrival of the front will allow some higher
temperatures. The front will also bring a return of showers and
isolated thunderstorms as a shortwave spins around the upper low
tracking through Ontario, taking advantage of modest instability
from daytime heating.

Showers will diminish during the overnight hours with the loss of
daytime heating. However rain chances will return on Tuesday as a
secondary cold front tracks through the region coincident with
yet another shortwave spinning around the same persistent low
located to the north. The secondary push of cold air will keep
highs on Tuesday down into the middle 50s across north-central
Wisconsin, with highs in the upper 50s to lower 60s across the
rest of the cwa.

.LONG TERM...Tuesday Night Through Sunday
Issued at 335 AM CDT Mon May 29 2017

Mean flow depicted by the models to consist of upper troughing
over southeast Canada/northeast CONUS, weak upper ridging over the
High Plains and another upper trough over the Pacific Northwest.
This pattern would keep a northwest flow aloft into WI into next
weekend with timing issues of weak systems moving through the mean
flow. The main problem for next weekend will be a shortwave trough
that the models rotate southward into the Great Lakes with an area
of low pressure running along the southern periphery of this
trough. All in all, a dry period is expected for Wednesday/early
Thursday and Friday with rain potential for Saturday/Sunday.
Temperatures to warm close to normal into Friday, then cool for
the upcoming weekend.

The chance for showers will continue into Tuesday evening until
daytime heating wanes and what appears to be the last shortwave
trough that pivots southeast through the western Great Lakes.
Expect mostly cloudy skies for the remainder of the night with
min temperatures in the lower 40s north (perhaps a couple upper
30s if clouds break), to the upper 40s east-central WI. A ridge of
high pressure is forecast to stretch from the mid-MS Valley
northwest into south-central Canada on Wednesday and send drier
air eastward into WI. This should bring mostly sunny skies and
warmer air into northeast WI, although a gusty west wind may
negate some of this warming. Nevertheless, max temperatures should
be able to reach the middle 60s north-central/lakeshore, upper 60s
to around 70 degrees elsewhere (still a touch below normal).

The surface ridge axis shifts east Wednesday night and extend from
the TN Valley northwest into the western Great Lakes region.
Mostly clear to partly cloudy skies with diminishing winds will
bring another somewhat cool night with min temperatures in the
lower to middle 40s north, upper 40s to around 50 degrees south.
The surface ridge is progged to weaken over the western Great
Lakes on Thursday, mainly due to one of those weak systems
dropping southeast in the northwest flow aloft. Most of the models
focus the better precipitation chances across IA and IL, however
cannot rule out the northern periphery of any shower activity from
reaching central/east-central WI by Thursday afternoon. Will have
more clouds south than north, which would bring fairly uniform
temperatures to the forecast area. Look for max temperatures to be
in the upper 60s near Lake MI, lower 70s north-central and lower
to middle 70s south.

It now appears that central/east-central WI will reside on this
northern periphery of precipitation through Friday as a quasi-
stationary front to be located across southern sections of the
Great Lakes. This is one of those situations where a slight shift
north or south of 50 miles can spell the difference between partly
sunny skies/no precipitation or mostly cloudy skies/chance of
showers and thunderstorms. The latest trend is for enough dry air
on northeast winds will keep this front south and the bulk of
Thursday night/Friday would be dry (but stay tuned!). Max
temperatures for Friday will range from the middle 60s lakeside,
around 70 degrees north and lower to middle 70s south.

Plenty of questions exist headed into next weekend, primarily
focused on the timing/location of a surface low that is progged to
swing southeast ahead of a south-southeast moving shortwave
trough. The CMC has been optimistic with high pressure building
south into WI and keeping all precipitation to our west and south.
The ECMWF has pulled a 180 and gone from sweeping the surface low/
associated cold front through WI on Saturday, to keeping this
system now similar to the CMC. Meanwhile, the GFS does bring the
low pres/cold front through WI, but not until Sunday. Such a vast
array of possibilities, plus model run-to-run inconsistencies,
all point to a low confidence/high uncertainty forecast for next
weekend. Have followed the consensus solution which keeps chance
pops in the forecast both days, but can see a need to lower pops
as we get closer to the weekend, especially if the GFS joins the
other models. Temperatures will cool to below normal levels with
maxes both Saturday and Sunday in the lower 60s near Lake MI,
middle 60s north-central and upper 60s elsewhere.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 104 PM CDT Mon May 29 2017

A cold front pushed east of Wisconsin today and showers were
scattered across the area early this afternoon and ceilings were a
mix of VFR and MVFR. Lightning was not indicating any thunder as
of 1730Z, but surface based CAPE was in excess of 800 J/kg in
north central and parts of northeast Wisconsin so would expect
thunderstorms to develop. Unsure of the when/where for storms so
have not included them in TAFs. Thunderstorms may reduce ceilings
and/or visibility to IFR.

Expect MVFR ceilings across central and north central Wisconsin
late tonight through Tuesday morning.
&&

.MARINE...
Issued at 335 AM CDT Mon May 29 2017

Southwest winds early this morning will turn westerly and gust to
near 25 knots later this morning and this afternoon behind a cold
front. Winds will subside to 10 to 20 knots during the evening
hours and continue into the overnight hours.
&&

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

$$

SHORT TERM.....Kurimski
LONG TERM......Kallas
AVIATION.......MG
MARINE.........Kurimski


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