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Area forecast discussion
National Weather Service Green Bay WI
600 am CDT sun may 28 2017

Updated aviation portion for 12z taf issuance

Short Monday
issued at 310 am CDT sun may 28 2017

Showers are quickly developing ahead of an approaching shortwave
currently making its way through the western Great Lakes. This
activity will continue to develop during the morning hours, as
hi-res models bring a swath of light to moderate rain to most of
the cwa as this feature tracks through the area. Therefore will
continue with fairly high pops given the confidence in the models
and adjust the previous forecast for timing per the new
observational data available.

Another round of rain is expected this afternoon as a cold front
sweeps through the western Great Lakes region. This activity will
have a better chance for thunder as it encounters 500 to 1000 j/kg
of MUCAPE ahead of the cold front. Rain chances will continue
tonight, mainly across the north, as yet another shortwave tracks
through the western Great Lakes. The active pattern continues into
Memorial Day as yet another shortwave spins off a low pressure
system situated to the north with yet another surface boundary
tracking through the region.

Temperatures during this period will slowly fall, with highs today
ahead of the cold front expected to get into the upper 60s to
lower 70s. Temperatures tonight will be a bit cooler, ranging from
the middle 40s across the north, with lows around 50 across the
south. By Memorial Day highs will range from the upper 50s across
the north, with lower to middle 60s across the south.

Long term...Monday night through Saturday
issued at 310 am CDT sun may 28 2017

Models are consistent in their depiction of the closed upper low/
shortwave trough moving east into southeast Canada/northeast
conus, a new upper trough to push into the western Continental U.S. Late week
and a weakening upper ridge from The Rockies into western Canada.
Northeast WI to remain in a northwest flow aloft through at least
Thursday, leaving the region susceptible to any weak systems
diving southeast. Timing of precipitation chances continues to be
the main forecast challenge for the upcoming week. Temperatures
will initially be cool as the initial shortwave exits the area,
however readings are forecast to gradually warm closer to normal
by late week.

Loss of daytime heating will allow for stabilization of the
atmosphere to occur, thus expect most of any shower activity/
early evening storm to dissipate prior to midnight. May leave a
small overnight pop over the north closer to the closed upper low
that will be moving across northern sections of the Great Lakes. A
decent amount of cloud cover will be left over the region with min
temperatures in the lower to middle 40s north-central WI, upper
40s to around 50 degrees east-central WI.

Cool, cyclonic flow remains over the western Great Lakes on
Tuesday and with the models showing another shortwave trough
digging southeast through the main trough, expect to see clouds
build with another day of precipitation chances. Lapse rates are
not as steep as Monday, thus while a stray thunderstorm cannot be
ruled out, coverage would be too minimal to carry in the forecast.
Tuesday still looks like the coolest day of the week with Max
temperatures only in the lower to middle 50s north-central, to the
lower 60s across eastern WI.

Any shower activity will once again end Tuesday evening as daytime
heating wanes, leaving another mostly cloudy night across
northeast WI. Min temperatures are expected to be about a degree
or two cooler than Monday night with lower 40s north-central, to
the upper 40s east-central WI. Northeast WI to reside on the
western fringe of the cyclonic flow on Wednesday with high
pressure trying to push east, but mainly pass to our south.
Anticipate some cumulus development, but with the cooler air mass
shifted east, lapse rates are not as steep. Only a spotty shower
is expected at this time with the vast majority of the area
staying dry under partly sunny skies. At least mixed sunshine and
weak warm air advection will allow Max temperatures to reach the lower 60s
north-central/near lake mi, middle to upper 60s elsewhere.

After a quiet Wednesday night where the weak surface ridge axis to
extend into WI, precipitation concerns return later on Thursday as
another shortwave trough digs southeastward in the northwest flow
aloft. There are some model differences as the European model (ecmwf) takes this
trough far enough to our west and south to keep any showers away
from northeast WI. However, the GFS and CMC sweep this trough into
WI with the CMC faster than the GFS. Difficult to judge how much
this shortwave trough will dig, so will generally keep a small pop
in the forecast for Thursday. Warmer air aloft will continue to
overspread WI on Thursday with Max temperatures in the middle to
upper 60s near lake mi, around 70 degrees north and lower 70s

The model uncertainty continues into Thursday night as the CMC
pushes this system away from WI, whereas the GFS still has showers
going through the night into Friday. The European model (ecmwf) has changes its
tune and now brings a secondary system into WI Thursday night
with enough instability for possible thunderstorms which may
linger across southern sections of the forecast area into Friday.
In addition to the precipitation disparity, the European model (ecmwf) is much
quicker in raising upper heights into WI on Friday, while the
GFS/CMC still have a semblance of a northwest flow aloft with
cooler air over WI. Have followed model consensus for now which
keeps a small chance pop over the forecast area Thursday night
into Friday. Depending on whether any precipitation actually
develops on Friday, temperatures could have a wide range of
possibilities. Have compromised between the cooler GFS and warmer
European model (ecmwf) with readings close to values seen on Thursday.

Unfortunately, we cannot even rule out a few showers headed into
next weekend if the European model (ecmwf) is correct with another weak shortwave
trough crossing the area. The GFS/CMC favor more of an area of
high pressure over the Great Lakes and have no precipitation with
seasonal temperatures. Bottom line is that confidence GOES
downhill starting on Thursday and hope that the models get into
better sync in the coming days.

Aviation...for 12z taf issuance
issued at 600 am CDT sun may 28 2017

Despite showers developing over the taf sites early this morning,
VFR conditions prevailed across the forecast area to start the taf
period. The activity should remain showers for much of the area
this morning, however a ribbon of instability could bring some
thunderstorms to mtw before heading east. Additional activity is
expected later this afternoon and this evening as a cold front
sweeps through the western Great Lakes. Conditions could drop to
MVFR with the showers and storms later today. There is enough
potential for thunder across the western taf sites with the cold
front to put in the current set of tafs. However there is some
question as to whether thunder will make it all the way to the
eastern taf sites late this afternoon and into this evening.
Activity should simmer down later this evening into tonight as the
front tracks to the east and with the loss of daytime heating.

issued at 310 am CDT sun may 28 2017

Light winds are expected today, with a gradual and steady increase
tonight. Westerly winds will increase, occasionally gusting to
near 25 knots, Memorial Day through Tuesday as low pressure
meanders across Ontario. Small craft advisories or small craft
should exercise caution headlines may be needed in subsequent
issuances for Memorial Day and Tuesday.

Grb watches/warnings/advisories...


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