Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 102340

Area Forecast Discussion
National Weather Service Green Bay WI
540 PM CST Sat Mar 10 2018

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Sunday
Issued at 304 PM CST Sat Mar 10 2018

Quiet weather to hold through the end of the weekend as high
pressure slowly pushes off to the east. There will be an increase
in clouds as a weakening system approaches the region. Exactly how
much cloud cover we see will determine both how cold temperatures
fall tonight and then how far can they recover on Sunday.

The 20z MSAS surface analysis indicated an area of high pressure
that stretched from northern Ontario southward into Lake MI. An
inverted surface trough extended from the cntrl Plains northward
through the eastern Dakotas. Visible satellite imagery showed two
bands of clouds in our vicinity. The first band of low clouds to
the north and east was associated with lake effect. The second
band of clouds to the west was associated with the surface trough.
It will be these clouds that will try and push east, but meet with
resistance from the dry air currently over eastern WI (dew points
in the single digits and teens).

The high pressure is expected to hold from northern sections of
the Great Lakes to PA tonight and help to hold off on any snow
trying to move east ahead of the inverted surface trough. The
battle between dry air over eastern WI and incoming moisture over
western WI will lead to a gradually increase in clouds, but no
precipitation. Min temperatures tonight will be tricky depending
on how fast the clouds advances over the forecast area. Look for
readings to range from 5 to 10 above north, upper teens to lower
20s south. Models pretty much wipe-out the surface trough before
it reaches WI, leaving the retreating high pressure to keep a dry
forecast intact across northeast WI. A cold front is forecast to
approach the region late in the day, but have little impact on our
weather (yet). Partly to mostly cloudy skies can be expected with
max temperatures to range from the middle to upper 30s north-
central WI/near Lake MI, upper 30s to lower 40s elsewhere.

.LONG TERM...Sunday Night Through Saturday
Issued at 304 PM CST Sat Mar 10 2018

The mean flow at the start of the forecast to consist of a closed
upper low over the northeast CONUS, a closed upper low well off
the West Coast and an upper ridge over the Rockies. The flow is
progressive which translates to the western upper low
transitioning to a positive-tilted upper trough over the western
CONUS and the upper ridge reaching east-central NOAM by late week.
For northeast WI, the dry northwest flow aloft will change to a
more unsettled southwest flow late next week. Temperatures will be
near normal through Tuesday, then warm to above normal levels by
late week.

Northeast WI to reside on the western side of a shortwave trough
that is expected to stretch from western Ontario southeast through
Lower MI Sunday night. In addition, the weak cold front will push
through WI during the evening hours. The prevailing northwest flow
aloft and cooler air aloft (8H temperatures around -10C) could
bring some light lake effect snow showers to north-central WI,
while eastern WI could see a small chance of snow showers from a
shortwave dropping southeast. Min temperatures to range from the
lower 20s degrees north-central, to the middle to upper 20s east-
central WI. We will continue to be on the cyclonic side of this
shortwave trough on Monday and with daytime heating, there may be
enough instability to kick off a few snow showers over the
forecast area. This includes additional lake effect for north-
central WI as trajectories remain favorable. The main questions
for Monday are whether low-level lapse rates can become steep
enough to allow any of these snow showers to briefly turn
convective with a short burst of snow and whether temperatures
would warm enough to introduce a mix or changeover to all rain in
the afternoon. Right now, the answer to question 1 is no, but
something to watch. The answer to question 2 is yes as any
precipitation that does fall could be a mix or change to rain
(except the far north where cooler temperatures would keep
precipitation type as all snow). Max temperatures for Monday to
range from the lower to middle 30s north-central, to around 40
degrees eastern WI (away from Lake MI).

Not much changes headed into Monday night as another nor`easter
lifts northeast over the western Atlantic and backs up everything
to the west. The loss of daytime heating will end any snow shower
activity for inland locations, however lake effect snow shower
chances will linger over north-central WI through the night with
persistent north-northwest winds. The shortwave trough finally
begins to shift east on Tuesday, allowing upper heights to rise
across WI and cyclonic flow to weaken. In addition, an area of
high pressure is forecast to build into the Upper MS Valley. This
should gradually end the lake effect threat for north-central WI
and keep the rest of northeast WI dry. Max temperatures to briefly
cool with readings around 30 degrees north, lower to middle 30s

The center of the surface high moves to our south Tuesday night
into Wednesday with a ridge axis extended north into the Great
Lakes. After a cool Tuesday night with min temperatures mainly in
the teens, look for temperatures to begin warming on Wednesday as
winds back to the west. Under mostly sunny to partly cloudy skies,
max temperatures to reach around 40 degrees north, lower 40s south
(except upper 30s over Door County). Quiet weather continues
through Thursday as the surface ridge slides to our east and the
upper ridge axis pushes across the Plains. Despite a northwest
flow aloft, southwest winds at the surface will allow for WAA to
develop (continue) over WI during this time. Temperatures will
rebound accordingly with mins Wednesday night only in the 20s and
max temperatures on Thursday to range from the lower to middle 40s
near Lake MI, middle 40s north and upper 40s to lower 50s south.

The majority of the models bring the upper ridge into the western
Great Lakes late Thursday night/early Friday morning. The
exception bring the GFS which sends a weak shortwave trough
southward to squash the ridge. While one cannot totally dismiss
the GFS, it seems hard to believe that this little trough will be
able to flatten the ridge, thus favor the majority solution which
brings a warm day to northeast WI on Friday. Max temperatures to
range from the lower to middle 40s near Lake MI (south winds),
middle 40s north and upper 40s to lower 50s south. The next chance
of seeing precipitation would arrive Friday night as a piece of
mid-level energy ejects northeast out of the western CONUS upper
trough. Precipitation type will become an issue as temperatures
cool with rain changing over to a mix or all snow later at night.
Depending on how fast this piece of energy moves and the speed of
an approaching cold front sets up, precipitation chances would
likely continue into Saturday. Timing issues this far out places
confidence on the lower end of the scale. Furthermore,
precipitation type would also transition from a mix to all rain as
temperatures warm. Temperatures on Saturday would cool a bit with
readings from the lower to middle 40s north/lakeside, middle to
upper 40s south.

.AVIATION...for 00Z TAF Issuance
Issued at 540 PM CST Sat Mar 10 2018

Good flying weather expected tonight through at least midday Sunday
as high pressure moves off to the east. There is a small chance that
some lake effect clouds could form over Lake Michigan late tonight
or Sunday and drift inland to MTW or SUE, but probably not. MVFR
ceilings could move into central and northcentral Wisconsin Sunday
afternoon along with a few sprinkles or flurries.



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