Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 221726
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1226 PM CDT Wed Mar 22 2017

Updated aviation portion for 18Z TAF issuance

.SYNOPSIS...
Issued at 348 AM CDT Wed Mar 22 2017

Cool today, then several rounds of precipitation during the
latter part of the work week and the upcoming weekend.

The persistent eastern North American upper trough will finally
shift off to the east the next couple days, giving way to a split
flow regime. The forecast area will primarily be influenced by the
southern branch of the developing split, where the mean trough
position will be over the southwestern United States. The trough
position back to the west suggests cyclones be slow-moving and
weakening by the time they reach the area.

Temperatures will be well below normal today as a strong
anticyclone from northwest Canada crosses the area. Then readings
should return closer to seasonal normals, though they may be held
back some during days with widespread precipitation. Total
precipitation for the period is likely to be above normal,
primarily due to a slow moving southern stream cyclone affecting
the area late this week into the upcoming weekend.
&&

.SHORT TERM...Today...Tonight...and Thursday
Issued at 348 AM CDT Wed Mar 22 2017

Clouds off Lake Michigan will affect the east for a time this
morning, and radar returns suggest flurries are possible as well.
The westward extent of the clouds and how long they stick around
are the main questions, with models offering varying ideas. Opted
to keep them into mid-morning, then mix them out as this seemed
to be what the majority of the models indicated. But it will need
to be watched closely and may need refinement. Went with a blend
of the top performing guidance products for max temps, which was a
little colder than the broad-based blend.

Considerable uncertainty exists on how quickly light precipitation
will break out on the back side of the retreating anticyclone. The
models offered varying ideas on this. Based the forecast primarily
on the ECMWF, which has had some precipitation into the northwest
part of the forecast area by 12Z Thursday on its last 4 runs.
Isentropic lift and a weak mid-level shortwave riding through the
ridge position will generate some lift, so brought low PoPs across
the northwest part of the forecast area after midnight, and
lingered them into early Thursday. Temperatures aloft will still
be cold enough for snow during this period.

After a short lull, precipitation chances will increase again
Thursday afternoon. Forcing will be stronger during this period,
and additional moisture will be arriving as well. Kept PoPs in the
chance category, as the main surge of precipitation probably
won`t occur until Thursday night. Precipitation type will become a
forecast issue as well. Temperatures aloft will continue to warm,
and surface temperatures should be above freezing during the
afternoon. So limited precipitation types to rain and snow, and
left out any freezing precipitation during the Thursday period.
Steep mid-level lapse rates will be approaching from the west, but
still look a little too far away to warrant an inclusion of
thunder in the forecast. Later shifts can reassess and add thunder
if necessary. Strengthening S/SE flow and thickening clouds
suggested edging temps back a bit from the previous forecast.

.LONG TERM...Thursday Night Through Tuesday
Issued at 348 AM CDT Wed Mar 22 2017

Looks like a messy forecast with plenty of chances for
precipitation as a couple of surface systems and upper level
troughs impact the area.

A warm front from a surface system over the Plains, and a cold
front associated with a low making its way across Canada will
bring rain Thursday night, with the rain changing to freezing rain
across much of central and northern Wisconsin as temperatures
drop overnight. Any freezing rain should change to rain by late
Friday morning as temperatures warm up again.

Precipitation type is an issue again Friday night into Saturday
morning as winds around a Canadian surface high bring colder, and
drier air into the area. Some type of wintry mix looks possible in
northern Wisconsin during this period, but the far north may see
little or no precipitation. As temperatures warm on Saturday any
precipitation should be in the form as rain by midday.

Rain chances increase from south to north Saturday night as the
surface low gets closer to Wisconsin. Surface temperatures will be
cold enough for another shot of freezing rain or a wintry mix,
generally north of Highway 29 into Sunday morning.

There were enough differences in QPF fields among the 00Z models that
the initial blended forecast is probably as good a solution as
any at this point. This has a generally drying out of the forecast
area by Tuesday afternoon.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1226 PM CDT Wed Mar 22 2017

High pressure over the Great Lakes is forecast to shift to the
east tonight and allow for a southerly wind to return to WI. An
increase in clouds tonight will mark the start of warmer air, but
also introduce a small chance of light snow late tonight into
Thursday morning. After a brief break in the precipitation
chances later Thursday morning, potential for light rain arrives
later Thursday afternoon ahead of a system moving across the
Rockies. This will begin a stretch of unsettled weather that is
likely to continue through the upcoming weekend. Other than the
MTW TAF site that is dealing with MVFR lake clouds this afternoon,
VFR conditions are expected through Thursday before flying
conditions deteriorate Thursday night.
&&

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

$$

SYNOPSIS.......Skowronski
SHORT TERM.....Skowronski
LONG TERM......MG
AVIATION.......AK


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