Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 200910
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
410 AM CDT Fri Apr 20 2018

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Saturday
Issued at 314 AM CDT Fri Apr 20 2018

Forecast temperatures will be the challenge today into Saturday.

Abundant April sunshine Thursday showed no respect to the
widespread and gradually diminishing snow cover, with highs in the
upper 40s to lower 50s. With a colder start this morning and some
cirrus approaching from the west, expect similar afternoon max
temperatures today. The air mass is very dry under the surface
high with model blend guidance dewpoint numbers initially too
high. Moisture from melting snow likely not be felt yet due to
snowmelt collapsing into snowpack. Observations across the area
not reporting fog so far early this morning, but have a few more
hours to develop.

A few more mid to high level clouds may drift over southern
wisconsin tonight into Saturday as a weather system slides into
lower to mid Mississippi Valley region. Minimum temperatures
tonight will likely be a bit warmer due to more clouds. Otherwise
the high pressure ridge will dominate into Saturday with sunshine
returning to most areas.  Quite a difference from last weekend.

.LONG TERM...Saturday Night Through Thursday
Issued at 314 AM CDT Fri Apr 20 2018

The great snow melt of April 2018 will continue through this
weekend into early next week as surface high pressure and upper
ridging to provide for plenty of sunshine and at or above normal
temperatures. Below normal precipitation is expected as one nearly
closed upper low passes well to our south this weekend and a cold
front/shortwave trough move through WI Tuesday afternoon/night
with little gulf moisture available. The main concern for next
week to be potential river flooding due to snow melt.

Northeast WI to remain on the western fringe of high pressure,
while an upper ridge moves into the Upper MS Valley Saturday
night. Any high clouds from a system passing well to our south
(Gulf Coast) should remain to the south, thus mostly clear skies
are expected with nearly calm winds. This set-up could lead to
some patchy fog with melting snow adding moisture to the lower
levels of the atmosphere. Min temperatures to range from the
middle to upper 20s north, upper 20s to lower 30s south. We remain
on the western side of the high pressure on Sunday, as the upper
ridge reaches the Great Lakes region. The Gulf Coast system
continues to slowly track east and will not have any impact on our
weather. Plenty of sunshine is expected with a light south-
southeast wind. Max temperatures to be mainly in the upper 50s to
around 60 degrees, except lower 50s near Lake MI.

Quiet weather to continue with little movement in the surface high
and the upper ridge sliding into the eastern Great Lakes. Perhaps
some more high clouds arrive as warmer air aloft pushes into WI
through the night. Light south-southeast winds with more snow melt
may again bring some patchy fog to parts of the forecast area. Min
temperatures to mainly be in the 30-35 degree range. Another
mostly sunny day is expected on Monday, despite the surface high
starting to lose its influence on WI. Any gulf moisture to remain
non-existent, thus a lack of moisture makes it hard for clouds to
develop. 8H temperatures continue to slowly warm, therefore max
temperatures should also edge upward. Look for readings to top out
around 60 degrees over most locations, but again cooler near the
Lake with lower to middle 50s.

Attention turns to an approaching cold front which most models
bring into northwest WI by 12Z Tuesday. The vast majority of any
precipitation however is forecast to be behind the front in the
vicinity of a trailing shortwave trough that moves into the
northern Plains Monday night. Therefore, no precipitation will
reach northeast WI Monday night. The forecast becomes more
uncertain on Tuesday as the cold front slows down and essentially
bisects the state from southwest to northeast by 00Z Wednesday.
The main difference among the models is the handling of the
shortwave trough where the GFS and GEM push the trough into the
Upper MS Valley, while the ECMWF holds the trough over the
northern/central Plains. The former models would bring more
clouds/chance of rain showers into northeast WI, while the latter
model keeps just about all the precipitation away from the area.
The best solution here would appear to bring a chance of light
rain showers into central WI Tuesday afternoon and keep eastern WI
dry. Max temperatures on Tuesday are another problem depending on
cloud cover/rain chances. For now, have readings in the upper 50s
to lower 60s inland, lower to middle 50s near Lake MI.

The model disparity increases Tuesday night as the GFS/GEM deepen
the shortwave trough as it moves into the Great Lakes, while the
ECMWF remains weaker. This makes all the difference between a
decent rain event (0.25-0.50") and hardly any precipitation at
all. This is also important with river levels running high from
the earlier snow melt. Have followed the model consensus which
does bring precipitation into northeast WI with rain mixing with
or changing to all light snow north. This uncertainty would then
carry over into Wednesday between rain gradually coming to an end
or no precipitation to start with. Hopefully, models will be able
to resolve this in the coming days. Max temperatures for Wednesday
will be cooler behind the cold front with readings in the upper
40s near Lake MI, around 50 degrees north and lower 50s south.

Improving conditions are forecast for Wednesday night/Thursday as
the shortwave trough pulls away and skies become partly cloudy.
Temperatures should be able to rebound by Thursday with max
temperatures back into the middle to upper 50s (cooler lakeside).
&&

.AVIATION...for 12Z TAF Issuance
Issued at 409 AM CDT Fri Apr 20 2018

Some mid to high level clouds may pass over today
into Saturday morning, otherwise VFR conditions to prevail through
Saturday.
&&

.HYDROLOGY...
Issued at 314 AM CDT Fri Apr 20 2018

Warming temperatures are expected to produce significant melting
of the current snowpack over northeast WI. This will lead to
runoff and a gradual rise of area rivers and streams. Several
rivers are expected to surpass bankfull this weekend and possibly
flood stage early to middle part of next week. People living near
rivers and streams should keep a close eye on the anticipated
river rises and keep up to date on the latest river forecasts.
&&

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

$$

SHORT TERM.....TDH
LONG TERM......AK
AVIATION.......TDH
HYDROLOGY......AK



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