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Area forecast discussion
National Weather Service Green Bay WI
505 am CST Fri Jan 17 2020

Updated aviation portion for 12z taf issuance

Short term...today...tonight...and Saturday
issued at 340 am CST Fri Jan 17 2020

Mostly sunny skies and very cold temperatures will greet everyone
this morning with locations across central and north-central
Wisconsin well below zero with temperatures around zero across
east- central Wisconsin. High pressure directly overhead will
push east towards the eastern Great Lakes region through the
afternoon hours. This will allow a low pressure system to emerge
over the plains and head towards the upper Mississippi Valley
later this afternoon. Clouds will increase throughout the day as
the low approaches, as afternoon temperatures warm into the 20s.
The low will also bring snow to the southwest portion of the
forecast area later this afternoon with warm air advection and
isentropic lift. Given the earlier arrival of the snow across the
southwest adjusted headlines to reflect an earlier start time than
the afternoon forecast.

The bulk of the snow is expected to fall tonight as the low moves
over the upper Mississippi Valley region. In addition an
increased pressure gradient will cause southeast winds to increase
tonight, which will cause some blowing snow. However there are
some concerns regarding snowfall totals tonight as the dendritic
growth zone (dgz) is fairly high, between 10 and 15k ft, with the
best lift just below this layer. In addition, there is dry air
just above the dgz which at times dips down into this layer. This
dry air could introduce some freezing rain into the column
tonight, especially across central and east-central Wisconsin
which would act to keep snowfall totals subdued. Therefore despite
some model output approaching warning criteria the concerns lean
towards keeping the current advisory in effect and forgo an
upgrade.

The snow continues on Saturday as the low tracks through the
western Great Lakes on its way towards the central Great Lakes
region. Some of the models are actually showing a burst of snow on
Saturday morning as the dgz sinks to 5 to 10k ft and coincident
with fairly substantial lift. If any period in the forecast is a
concern of pushing the area into warning criteria this would be
the period given the better intersection of lift and moisture with
the dgz, especially across far northeast Wisconsin where storm
totals have the best chance of getting close to warning criteria.
However some models clear out the moisture quicker than others so
the scenario of this burst of snow isn't a slam dunk either.
These aforementioned concerns lend credence to keeping the
advisory instead of upgrading to a warning as there are still a
lot of questions regarding moisture, lift, and where the lift is
occurring. By Saturday afternoon the snow will be coming to an end
from southwest to northeast as the low lifts northeast away from
the region. Overall this event looks to produce 4 to 7 inches of
snow across the region tonight and Saturday, with locally higher
amounts possible.

Long term...Saturday night through Thursday
issued at 340 am CST Fri Jan 17 2020

The main highlights for the extended forecast are the continuation
of snowfall into Saturday night, followed by below normal
temperatures through midweek, and then the potential for more
snow from Wednesday through next weekend.

Saturday night through Sunday...with the snow producing low
pressure system positioned over Lake Huron Saturday night,
Wisconsin will see a continuation of snow and gusty west to
northwest winds. Gusts between 25 to 35 mph are possible, with
gusts up to 40 mph along the Lakeshore. Therefore, the potential
for blowing and drifting snow across central and east-central
Wisconsin will continue into early Sunday morning. As the system
continues eastward, snow will taper off from southwest to
northeast Saturday Night. Lake effect snow will remain possible
through Sunday afternoon across Vilas County due to the northwest
winds. Additional snowfall accumulations will be minor with up to
an inch possible in far northern Vilas County. Much colder air is
expected to fill in behind the low pressure system as a surface
high pressure system slides into the northern plains. Low
temperatures on Saturday night will drop to the single digits
above zero with lower 10s in far northeast Wisconsin and along the
Lakeshore. High temperatures on Sunday will also be colder with
readings in the teens above zero with warmer readings in the lower
20s in far northeast Wisconsin and along the Lakeshore.

Sunday night through Tuesday...the upper-level northwest flow
will remain over Wisconsin through Tuesday. In addition, the
surface high pressure system will sweep southeast across the
middle Mississippi Valley. This will keep the below normal
temperatures in place through Tuesday. High temperatures will be
in the teens above zero for Monday and Tuesday. Low temperatures
for Sunday night and Monday night will range from the single
digits below zero west to the single digits above zero east. In
addition, little to no clouds are possible for both Monday and
Tuesday.

Rest of the extended...after some upper-level ridging Tuesday
night, model guidance points to the next possible wave of energy
to impact the region on Wednesday. Model guidance indicates an
upper-level cut-off low will sit over the Central Plains Wednesday
night through Thursday night, keeping a mean upper-level southwest
flow over Wisconsin. This flow would maintain chances of snow in
the forecast through Thursday, before the main surface low
pressure system shifts east/northeast towards the upper
Mississippi Valley Thursday night into Friday. However, model
guidance is not in agreement with the track of this system or the
amount of precipitation associated with it. Temperatures for the
second half of the work week are forecast to be above normal with
highs generally in the lower to middle 30s.
&&

Aviation...for 12z taf issuance
issued at 500 am CST Fri Jan 17 2020

Poor flying conditions are expected to develop across the
area later this afternoon or early this evening as low clouds
and snow overspread the region. Snow should arrive at kauw/kcwa
around 22z, krhi/kgrb/kauw between 00z and 03z. Snow will
continue through the night and Saturday morning, before
tapering off Saturday afternoon. Gusty southeast winds
will develop this afternoon and tonight, and included low
level wind shear in the tafs. Gusty northwest winds
on Saturday may create some areas of blowing and drifting
snow.

&&

Marine...
issued at 340 am CST Fri Jan 17 2020

A strong low pressure system will approach the western Great Lakes
today and tonight, passing through the area Saturday morning, then
shifting off to the east Saturday afternoon and evening. Ahead of
the system gale force winds out of the southeast are forecast to
develop across the Lake Michigan nearshore waters tonight and last
into early Saturday morning. A lull in the winds is then expected
later Saturday morning into early Saturday afternoon before
increasing back to gale force late Saturday afternoon into
Saturday night from the west. Will upgrade the gale watch to a
Gale Warning with this forecast issuance for the Lake Michigan
nearshore waters.
&&

Grb watches/warnings/advisories...
Winter Weather Advisory from 6 PM this evening to 6 PM CST
Saturday for wiz005-010>013-019>022-031-038>040-048>050-073-074.

Lakeshore flood advisory from 6 PM this evening to 6 am CST
Saturday for wiz022-040-050.

Winter Weather Advisory from 3 PM this afternoon to 6 PM CST
Saturday for wiz018-030-035>037-045.
&&

$$

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