Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 220341

Area Forecast Discussion
National Weather Service Green Bay WI
1041 PM CDT Sat Apr 21 2018

Updated aviation portion for 06Z TAF issuance

.SHORT TERM...Tonight and Sunday
Issued at 304 PM CDT Sat Apr 21 2018

The latest RAP analysis and satellite/radar imagery show a large
high pressure system stretching from the mid-Atlantic states to
the western Great Lakes early this afternoon. Clouds continue to
stream into the southern half of Wisconsin ahead of a closed low
pressure system centered over the southern Great Plains. Radar
returns have been confined to far southwest Wisconsin, due to the
very dry air in place over the northern half of the state.
Speaking of dry air, humidities have fallen into the middle teens
over northern WI where sunshine and good mixing have supported
falling dewpoints through the day. As mid-level ridging moves into
the region, forecast concerns mainly revolve around temps and

Tonight...High clouds will continue to sink to the south tonight
with the arrival of strong subsidence ahead of the mid-level
ridge. With high pressure remaining at the surface and little
change of the airmass in place, mostly clear skies and light winds
should result in another good radiational cooling night. Used a
blend of best performing guidance combined with last night lows
for temps tonight, which results in temps ranging from the upper
teens over northern Vilas county to the near 30 over the southern
Fox Valley.

Sunday...The storm system over the southern Plains will move east
into the mid-Mississippi Valley. The mid-level ridge axis will
hold firm over the northern Great Lakes, which will keep clouds
and any hint of precip well south of the region. Despite the full
sunshine, temps should not be appreciably different than today, as
the airmass changes little. The only difference will be the light
east wind developing which the sunshine should mitigate except
right near the Lake. So kept temps very similar to today`s
readings, except near Lake Michigan. Also dropped dewpoints
considerably during the afternoon, with min humidities falling
into the mid 20s across the northwoods.

.LONG TERM...Sunday Night Through Saturday
Issued at 304 PM CDT Sat Apr 21 2018

The warmer temperatures will continue to melt the snowpack
through the rest of the weekend and into next week. Some rivers
in the Wolf River basin currently at bankfull are forecast to
exceed minor flood stage as early as Monday morning. The next
chance for precipitation will move into the area on Tuesday
afternoon due to a shortwave trough and its associated cold front.
Another shortwave trough builds in behind it on Thursday.

For the start of the long term, a split flow pattern remains
dominate across the CONUS with an aggressive low pressure system
sweeping across the Middle Mississippi Valley, while the Upper
Mississippi Valley remains within a ridge of high pressure. By
Monday morning, the Upper Mississippi Valley will be on the
back side of the ridge as it begins to evolve into a zonal flow.
This pattern will allow temperatures to rise slightly above normal
for Monday to the lower 60s with the exception of the lakeshore
counties due to an onshore wind. These mild temperatures will
continue to chip away at the snowpack.

The flow evolves on Tuesday with the approaching shortwave
trough/cold front from the northwest. There is more uncertainty
amongst the models than yesterday with the evolution of this
pattern by Tuesday afternoon. The models are in agreement having
split flow occur with a closed low pressure system staying well
to the southwest of Wisconsin and a shortwave trough over the
Great Lakes. The disagreement comes in with the GFS. The GFS shows
a more aggressive development of these two waves of energy which
results in more lift and precipitation over the area. The ECMWF
and Canadian have the two waves of energy staying far enough
apart to have little influence over the area for Tuesday
afternoon and Tuesday night. As a result, a blend of the models
were used leaving some chance pops for rain showers over far
northeast and east-central Wisconsin Tuesday evening. With the
consistency of the split flow amongst the Canadian and ECMWF, it
is possible for the pops to lower again.

After the waves of energy exit the region, another shortwave
trough begins to enter the region on Thursday. Model guidance is
not in agreement with the timing of this shortwave trough causing
problems with p-type. The GFS and Canadian are in agreement with
the arrival for precipitation on Thursday morning and exiting by
Friday morning. The ECMWF has the shortwave trough entering the
area Thursday evening and exiting by Friday evening. Kept the
blended models chance pops across all periods due to the
uncertainty and will narrow down the exact time period later.

.AVIATION...for 06Z TAF Issuance
Issued at 1041 PM CDT Sat Apr 21 2018

Some SCT-BKN cirrus clouds will track through the southern TAF
sites during the overnight hours. Further north clear to mostly
clear skies will prevail. VFR conditions will prevail over all of
the TAF sites through the period with light winds. The very dry
airmass in place has thus far prevented fog development during the
overnight hours, therefore there are no plans to put it in with
this set of TAFs.

Issued at 304 PM CDT Sat Apr 21 2018

Warming temperatures will continue to produce 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 afternoon 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



LONG TERM......Hykin
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