Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 210804

Area Forecast Discussion
National Weather Service Green Bay WI
304 AM CDT Sat Apr 21 2018

Forecast discussion for routine morning forecast issuance

.SHORT TERM...Today...Tonight...and Sunday
Issued at 303 AM CDT Sat Apr 21 2018

Quiet conditions will continue this weekend under a surface high
pressure system anchored over the Great Lakes.

Despite the departing melting snow cover, max temperatures have
had little issue reaching near normal levels with the April
sunshine. Surface dewpoints have also been much lower than most
guidance readings, especially for the afternoon hours. Air mass
has been so dry that melting snow has not produced overnight fog,
while some locations have observed large diurnal ranges with
overnight lows dropping well into the 20s.

A weather system tracking over lower to mid mississippi valley
region this morning, while a weak short wave passes northeast of
the area in the northern stream, will produce some high level
clouds in the morning. Clouds may lower to a mid level deck over
far southern areas. Otherwise eventually plenty of sunshine
expected later today and for the rest of the weekend.

.LONG TERM...Sunday Night Through Friday
Issued at 303 AM CDT Sat Apr 21 2018

The slightly amplified flow this weekend across the CONUS is
forecast to gradually become more amplified by the end of next
week as an upper low develops over the eastern Pacific, an upper
ridge builds over the western CONUS and a broad upper trough
develops over east-central NOAM. Precipitation chances to be
focused on Tuesday afternoon/Wednesday with the passage of a cold
front/ shortwave trough and perhaps Thursday afternoon/Thursday
night as the upper trough sets up over the Great Lakes.
Temperatures are expected to be above normal to start the week,
which will lead to some rivers surpassing flood stage.
Temperatures do trend down below normal from mid-week onward as
the mean flow turns northwest.

A surface ridge is still forecast to stretch from the western
Great Lakes eastward to the New England states Sunday night. A
nearly vertically-stacked system will move across the Gulf Coast
states toward the TN Valley, but not impact our weather.
Therefore, look for generally partly cloudy skies, light winds and
temperatures slightly below normal. This would bring mins mainly
into the lower to middle 30s. Not much changes overall to start
the work week with the high pressure holding on over the western
Great Lakes and the Gulf Coast system slowly trudging east. One
change that does occur will be the eastward movement of a cold
front into the Upper MS Valley by 00Z Tuesday, but this feature is
too far away to affect the forecast area. Anticipate a good amount
of sunshine on Monday and as 8H temperatures slowly warm, we
should see max temperatures inland around 60 degrees on average
with locations with less snow perhaps as warm as the middle 60s.
Areas near the Bay and Lake MI to be cooler thanks to an onshore

This cold front moves into western Upper MI/northwest WI Monday
night, however models continue to show all the associated
precipitation to be post-frontal across the Upper MS Valley/
northern Plains in the vicinity of the trailing shortwave trough.
Thus, another dry night across northeast WI with min temperatures
to be in the middle to upper 30s. Models are still having some
issues with regards to the movement of the shortwave trough headed
into Tuesday. While the cold front does sweep into the Great Lakes
region, possible solutions of the trough range from closing into
an upper low over northeast Nebraska (GFS), to a closed upper low
over west-central Nebraska (GEM), to an open wave on the ECMWF.
Therefore, there is uncertainty on how hard to hit the pops on
Tuesday. The trend is to carry higher pops closer to the trough/
upper low where better forcing to be situated. In the end, have
kept pops over northeast WI on the low side for Tuesday afternoon.
More clouds and the threat for rain showers should knock
temperatures down a touch compared to Monday. Look for max
temperatures Tuesday to be in the lower 50s near Lake MI, middle
to upper 50s north-central WI and around 60 degrees elsewhere.

The chance for light rain showers would carry over into Tuesday
evening, more due to shortwave energy moving through the mean flow
aloft. As temperatures cool Tuesday night, expect to see any rain
to mix with or change over to all light snow over northern and
parts of central WI. By Wednesday, most of the precipitation would
be focused near the shortwave trough/closed upper low located over
the Mid-MS Valley. May keep a minimal pop over parts of central
and east-central WI, however it is looking like Wednesday may be
mostly dry now for most of northeast WI. Max temperatures will be
cooler with readings in the middle to upper 40s near Lake MI,
lower to middle 50s inland.

After a quiet Wednesday night, attention turns to the next surface
cold front/mid-level shortwave trough that quickly drops southeast
into the western Great Lakes on Thursday. Anticipate an increase
in precipitation chances, especially by Thursday afternoon.
Temperatures will be warm enough such that precipitation would
fall as all rain, with readings in the upper 40s to lower 50s near
Lake MI, middle to upper 50s inland.

This latest shortwave trough is progged to strengthen over the
east-central CONUS Thursday night into Friday. Precipitation
chances would continue Thursday night and as the air mass cools,
we will again be looking at a mix of precipitation types. There
are timing issues among the models as to how fast this trough will
slide to the east. A faster exit may allow any precipitation to
end before daybreak, while a slower movement would linger
precipitation chances through at least Friday morning. Have
followed the model consensus for now which favors the faster exit,
so Friday is looking dry at the moment. Temperatures will remain
below normal through the end of the week.

.AVIATION...for 06Z TAF Issuance
Issued at 1029 PM CDT Fri Apr 20 2018

High level clouds will pass over the region through Saturday
morning, with skies clearing out later in the morning into the
afternoon hours. Otherwise VFR conditions should prevail at most
locations through the TAF period. Given the dry air and low dew
points currently in place will continue to leave fog out of the
TAFs during the overnight hours.

Issued at 303 AM CDT Sat Apr 21 2018

Warming temperatures will continue 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 as early as Monday evening for the Wolf River basin.
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.



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