Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 010425
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1025 PM CST Wed Nov 30 2016

Updated aviation portion for 06Z TAF issuance

.SHORT TERM...Tonight and Thursday
Issued at 240 PM CST Wed Nov 30 2016

No significant changes to previous short term forecast as
weakening mid-level low continues its slow progress eastward
across the forecast area. Leading edge of the light precipitation
shield finally making it into central Wisconsin late this
afternoon. Boundary layer temperatures remain warm enough to keep
the light precipitation mainly as light rain or drizzle. Short
range guidance suggests embedded shortwave impulse over southeast
Minnesota will swing east across the southern half of Wisconsin
overnight. Precipitation shield associated with this impulse seems
to be a bit more organized across southwest Wisconsin. As
suggested by the NAM and HRRR, expect this more contiguous area
of light precipitation to impact the southern portion of the
forecast area and the Fox Valley. As boundary layer temperatures
cool slightly overnight, expect to see a bit more light snow or
snow shower activity. Any notable accumulation will be minimal
however. As system drifts a bit further east, also expect a bit
more favorable environment for LES enhancement over mainly Vilas
County later tonight where perhaps an inch or so of slushy
accumulation possible by daybreak. Temperatures overnight will
not fall appreciably over the Fox Valley and lakeshore areas
remaining just above the freezing mark. North and west of the Fox
Valley, temperatures may fall slightly below freezing, thus could
be some minor accumulations on roads creating some scattered
slippery spots especially north of highway 8.

On Thursday the upper low will continue to weaken as it tracks
east with light precipitation also waning in areal coverage.
Highest pops will remain across Vilas County given continued enhancement
from Lake Superior. Sufficient boundary layer warming on Thursday
should keep snowfall accumulations limited with the the greatest
totals of an inch or two over Vilas County. With lingering
troughiness extending westward across the forecast area on
Thursday, expect to see mainly overcast conditions once again.

.LONG TERM...Thursday Night Through Wednesday
Issued at 240 PM CST Wed Nov 30 2016

Initially, a split flow exists over the CONUS with a more active
southern stream rotating around a closed upper low over old Mexico
through the Gulf states and a weaker northern stream moving across
the northern tier of states. The streams attempt to consolidate
toward the middle of next week as a strong northern stream system
dives southeast from the Pacific Northwest into the central/
southern Rockies. For northeast WI, one weak northern stream
system is expected to arrive on Sunday bringing a chance for
either light snow or a rain/snow mix. The forecast toward the
middle of next week is still uncertain due to questions as to
when and where the stream phasing will occur and whether a surface
low will ride up a cold front to bring snow to the western Great
Lakes. Temperatures will remain at or above normal through the
period with Saturday likely the coolest day.

Slow-moving upper low will continue to linger over southeast
Ontario/southwest Quebec Thursday night, leaving a cool/cyclonic
flow over the western Great Lakes. Even though 8H temperatures are
not that cold (-6C over western Lake Superior), expect to see a
little lake enhancement across north-central WI. Any accumulation
would be very minor. Otherwise, plenty of clouds will persist over
northeast WI with min temperatures mainly in the 25-30 degree
range. Potential for a few lower 20s over the north if any breaks
in the cloud cover develops. Warmest readings to be along Lake
Michigan where lower 30s are expected. Cyclonic flow begins to
lose its grip on the Great Lakes on Friday as the upper low
steadily weakens and moves into the Canadian maritimes. Cannot
rule out a few snow showers lingering over north-central WI as
northwest winds to continue over the region. Cloudy skies will
prevail through the day with max temperatures in the lower 30s
north-central, middle to upper 30s elsewhere.

Any lingering light snow showers/flurries over north-central WI
should come to an end Friday night as an area of high pressure
moves from the Midwest toward the Ohio Valley. Winds would
gradually back more to the west, thus losing any lake influence
from Superior. There may be a few breaks in the cloud cover over
southern parts of the forecast area overnight, but in large,
expect more mostly cloudy skies for Friday night. Min temperatures
should again drop into the 25-30 degree range with cooler
readings north-central/central WI, warmer readings eastern WI. If
there is to be any mixed sunshine in the extended forecast, it may
very well be on Saturday as the high pressure moves across the
Ohio Valley. Winds would gradually back to the west-southwest and
if the low-level inversion can be broken, parts of northeast WI
should see some sunshine. Slightly cooler air aloft filters into
WI during the day, thus max temperatures to only reach around 30
degrees north-central, mainly middle 30s elsewhere.

Two separate systems are progged to impact the central CONUS
during the latter part of the upcoming weekend. There is a closed
upper low over old Mexico with southern stream energy interacting
with a frontal boundary draped over the Gulf Coast. Meanwhile, a
weaker northern stream system consisting of a mid-level shortwave
trough/trailing weak cold front sweeps east from the northern
Plains into the Great Lakes. Look for clouds to return across the
forecast area Saturday night with the leading edge of
precipitation approaching central WI toward daybreak. By Sunday,
sufficient mid-level forcing will exist to have a band of
precipitation work across WI. Precipitation type will be an issue
as temperatures across central/eastern WI rise into the upper 30s.
Anticipate primarily all snow Sunday morning, before rain would
mix with the snow around midday through the afternoon.
Precipitation amounts will be tempered by the fact that gulf
moisture would be cut-off by the southern stream system.
Therefore, where precipitation remains all snow (north-central),
accumulations would be light (mainly an inch or less - as of now).

Light snow or rain/snow mix will continue into Sunday evening
before ending as a ridge of high pressure quickly builds eastward
from the Upper MS Valley/Midwest. Do not anticipate much in the
way of additional accumulations, thus this will be a weak early
winter event. The mean flow is amplified by the start of the work
week with broad upper troughing over the western CONUS and modest
upper ridging over the east-central CONUS. Monday looks to be a
quiet day, although more clouds than sunshine as the surface ridge
to already be to our east and weak WAA returns to WI. Max
temperatures for Monday to be in the middle 30s north-central,
upper 30s to around 40 degrees elsewhere.

Models show a lead impulse ejecting out of the now-Rockies upper
trough into/across the northern Plains/Upper MS Valley Monday
night and into the western Great Lakes on Tuesday. The ECMWF has a
secondary area of low pressure lifting north on Tuesday which
would provide for a little better precipitation chances, but this
is not a guarantee at this point. The air mass over WI is still
not cold (8H temperatures from -2C to zero), thus enough warmer
air could sneak into eastern WI to allow for another rain/snow mix
to develop. Moisture again appears limited with this system, thus
snow accumulations over northern WI would be on the light side.
There continues to be monumental shifts from run-to-run among the
models with the next system that would impact the region during
the middle of next week. Have little to no confidence in any
particular model, thus have used a blend for now which brings
chance pops back to the forecast area by Wednesday afternoon.
&&

.AVIATION...for 06Z TAF Issuance
Issued at 1025 PM CST Wed Nov 30 2016

A Large but weak upper low system will continue to drift over the
Great Lakes region overnight into Thursday. Mainly MVFR cigs
expected but scattered IFR cigs with MVFR vsbys due to light rain
or snow showers and patchy fog will persist through Thursday
morning. Conditions to gradually improve to vfr conditions later
Thursday as the system shifts eastward.
&&

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

$$

SHORT TERM.....ESB
LONG TERM......AK
AVIATION.......TDH


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