Area Forecast Discussion
Issued by NWS Green Bay, WI

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770
FXUS63 KGRB 271123
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
623 AM CDT Sat May 27 2017

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Sunday
Issued at 308 AM CDT Sat May 27 2017

An area of light showers and sprinkles will continue moving east
this morning. Precipitation amounts from this area of showers will
be fairly light, if it measures at all.

A weak boundary will exist over the area today, acting as a focus
for isolated showers and thunderstorms this afternoon and this
evening. Hi-res models keep the coverage fairly sparse, therefore
will keep POPs fairly low this afternoon.

Precipitation chances will be better later tonight and into Sunday
as a mid level trough and attendant pv anomaly track through the
western Great Lakes as a surface low tracks off to the southeast.
This round of precipitation will carry the risk of heavy rainfall
given the strong dynamics associated with it and the fairly low
storm motion vectors as soundings show fairly weak winds in the
mid and low levels. Although area rivers are currently below flood
stage, this fresh round of rain could push some of these rivers
back above flood stage.

.LONG TERM...Sunday Night Through Friday
Issued at 308 AM CDT Sat May 27 2017

The main weather feature that will impact northeast WI continues
to be a closed upper low/shortwave trough that is forecast to
slowly move east from western Ontario/western Great Lakes Memorial
Day, to southeast Canada/northeast CONUS next Friday. The forecast
challenge remains trying to time individual shortwaves that will
be moving through the trough to produce daily precipitation
chances through mid-week. Temperatures are expected to cool
through Wednesday, then begin to warm late week.

Chance of showers/early evening thunderstorms will carry over into
Sunday evening as daytime heating wanes and northern stream
shortwave trough shifts northeast away from the forecast area.
While a few showers may linger past midnight, expect most of the
overnight hours to be dry under mainly cloudy skies. Min
temperatures to be in the middle to upper 40s north, upper 40s to
lower 50s south.

On Memorial Day, the closed upper low is forecast to reach south-
central Ontario with the shortwave trough extended southeast
through the western half of the Great Lakes region. Cool air
aloft, coupled with daytime heating and the passage of an
individual shortwave, should allow shower chances to gradually
increase through the day with a few afternoon thunderstorms as
lapse rates steepen. Max temperatures will be cooler than previous
days as readings only are expected to reach the middle to upper
50s north-central WI, middle 60s eastern WI.

Much like Sunday night, most of the showers (early evening storms)
will diminish Monday evening with only spotty showers lingering
over northern WI through the rest of the night as additional
shortwave energy rotates around the closed upper low. Min
temperatures to range from the lower 40s north-central, to the
upper 40s to around 50 degrees east-central WI. Northeast WI to
reside on the cool, cyclonic side of the upper trough on Tuesday
with another day of pop-up showers expected. Lapse rates are not
as steep as in previous days, thus thunder potential looks too
minimal to carry in the forecast. Any precipitation would be light
and more miss than hit due to the lack of any shortwaves evident
from the 00z model output. Another unseasonably cool day on tap
with readings pretty close to those seen on Memorial Day (5 to 10
degrees below normal).

The passage of yet another shortwave trough may keep a small pop
in the forecast through Tuesday night. Wisconsin to be on the
western edge of the cyclonic flow headed into Wednesday and models
have backed down a bit on shower coverage. In fact, any shower
activity now is practically too isolated to even mention in the
forecast for the most part. Less precipitation coverage should
allow for an uptick in temperatures with readings in the lower to
middle 60s north-central/lakeshore, middle to upper 60s elsewhere.

We finally get out of the cyclonic flow by Thursday, however the
western Great Lakes region remains in a northwest flow aloft with
upper troughing to our east and upper ridging to our west. The GFS
continues to try and generate a stray afternoon shower, while the
ECMWF has come in dry on Thursday. May leave a token pop per
previous forecast, but confidence on any precipitation actually
occurring is rather low. Assuming any showers stay away,
temperatures will continue to slowly warm with lower 60s near Lake
MI, middle 60s north-central and upper 60s to around 70 degrees
elsewhere.

By the end of the work week, models begin to differ with the
handling of the upper ridge as it moves eastward. The GFS
maintains this ridge, keeping a northwest flow into WI, while the
ECMWF flattens the ridge and brings a near zonal flow from the
Pacific northwest to the Great Lakes. The extent of WAA into WI
will be the difference between shower/no shower chances into the
forecast area. Too much uncertainty at this range to discount the
precipitation threat, so will need to mention small pops once
again. Max temperatures on Friday should get close to normal and
be similar to readings from Thursday.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 623 AM CDT Sat May 27 2017

Some early morning MVFR/IFR fog will linger for an hour or so
before mixing dissipates the fog. Daytime heating and the
presence of a weak boundary in eastern WI may result in scattered
showers and isolated thunderstorms late in the afternoon/early
evening. Will keep the mention VCSH at GRB/ATW/MTW, but
confidence is not high enough to mention thunderstorms at this
time. A better chance will arrive late tonight as a shortwave
tracks through the region. At time time will include a prob30
group for the TAF sites, with an upgrade possible in future
issuances.
&&

.MARINE...
Issued at 308 AM CDT Sat May 27 2017

Gusty westerly winds are expected on Memorial Day through Tuesday
with gusts around 25 knots possible making for hazardous
conditions for small craft.
&&

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

$$

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



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