Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 211949
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
249 PM CDT Mon Aug 21 2017

Forecast discussion for routine afternoon forecast issuance

.SYNOPSIS...
Issued at 247 PM CDT Mon Aug 21 2017

Thunderstorms tonight, then turning cooler and less humid with
much quieter weather for the rest of the work week.

A band of westerlies across the northern CONUS and southern
Canada will undergo substantial amplification the next several
days, resulting in the development of a longwave trough over
eastern North America. The westerlies will gradually weaken during
the latter half of the period as the flow consolidates back to the
north across Canada. But at least some remnants of the upper
trough will linger near the area.

The developing northwest upper flow into the amplifying eastern
trough will bring a large anticyclone and associated dry airmass
southeast from Canada. So the warm and humid conditions of today
will be replaced by seasonably cool temperatures by mid-week.
Readings will begin to rebound by the weekend. The main
opportunities for rain will be early and late in the period, with
amounts depending primarily on what falls in the next 18 hours.
That will probably still be sufficient to result in AOA amounts at
most locations.
&&

.SHORT TERM...Tonight and Tuesday
Issued at 247 PM CDT Mon Aug 21 2017

Strong large scale forcing will sweep across the area tonight as
shortwave energy digs southeast and begins to carve out the
eastern longwave trough. In addition, the RRQ of an upper speed
max will pull through the region. Though the specifics of the
convective development area still a little unclear, those factors
should result in pretty widespread coverage of the storms tonight.
So took PoPs up to categorical across the area. PWATs rising to
near 2 inches combined with the large scale forcing will result in
a threat of heavy rainfall. There is also a risk for severe
storms, mainly through about midnight. Of most concern will be
the potential for damaging wind gusts due to merging cells
producing wet microbursts.

Cooler and drier air will begin to sweep into the area Tuesday,
but most locations will get off to a pretty warm start so readings
should still manage to warm within a few degrees of normal. Any
lingering showers in the east will end early in the day.

.LONG TERM...Tuesday Night Through Monday
Issued at 247 PM CDT Mon Aug 21 2017

Northwest flow will dominate the western Great Lakes during the
mid-late week period with a large area of high pressure at the
surface sliding down from Canada. This will provide for mainly dry
conditions through Saturday with temperatures generally slightly
below normal.

First chances for showers looks to be Wednesday night into
Thursday as a shortwave trough and upper jet speed max push into
the state. Some model timing and location differences, which
should be expected this far out which a weak feature like this.
Models favoring southwest WI for the best chances. Not much
moisture to work with, as surface winds are out of the NE, keeping
dewpoints in the 40s to low 50s. Will keep slight chance POPS
across mainly central WI for now, but not too fired up on shower
chances.

Next chance for showers/storms arrives this weekend as the next
shortwave approaches. Models differ on strength, timing, and
moisture, so hard to get much clarity at this point. GFS and to
some extend the ECMWF are a little faster bringing in some rain
with the Canadian slower. Will keep some POPs across central and
north central WI on Saturday to account for the slower solution
and keep chance POPs going through Saturday night into Sunday. As
flow turns more out of the SE, will get a little moisture return,
with dewpoints climbing into the mid-upper 50s, so rain/storm
chances higher with this system.

High temperatures on Wednesday will be the coolest of the week,
with highs ranging from the lower 60s across north central
Wisconsin to the low 70s farther south. Temperatures moderate only
slightly by the end of the week, with highs in the upper 60s to
middle 70s. Wednesday and Thursday nights look to be pretty
chilly, especially across northern WI where some of the typical
cool spots could drop into the 30s. The cool nights and mostly
clear skies should allow for some fog to develop as well.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1244 PM CDT Mon Aug 21 2017

Cu will increase this afternoon as the convective temperature is
reached. But primarily VFR conditions are expected. The main area
of showers and storms is expected to move through between 23Z and
06Z. Ceilings and visibilities will be briefly lower with the
stronger storms. Some Sc with MVFR ceilings may develop late
tonight in the wake of the main rain band, but the clouds should
exit tomorrow morning.
&&

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

$$

SYNOPSIS.......Skowronski
SHORT TERM.....Skowronski
LONG TERM......Bersch
AVIATION.......Skowronski


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