Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 030006
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
606 PM CST Fri Dec 2 2016

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Saturday
Issued at 235 PM CST Fri Dec 2 2016

Little change from previous forecast as low-level cyclonic flow
hangs on for at least the overnight hours with persist cloud
cover allowing for little diurnal temperature variation. As
yesterday, once again bumped up minimum temperatures a couple
degrees over the model blend with readings generally in the 25 to
30 degree range. Any minimal lake effect flurry potential pretty
much over as low-level fetch backs to westerly this evening.

Weak surface high pressure ridge will finally build into the
western Great Lakes during the day on Saturday. However not too
optimistic on seeing much in the way of sunshine once again...except
perhaps for central Wisconin later in the afternoon. Highs on
Saturday will be close to what we saw today, mainly in the 30 to
35 degree range.

.LONG TERM...Saturday Night Through Friday
Issued at 235 PM CST Fri Dec 2 2016

Split mean flow regime to persist this weekend with the southern
stream dipping around a closed upper low over northern Mexico
before lifting across the Gulf Coast states and a northern stream
racing across the northern tier of states. This northern stream
will send a mid-level trough across the Great Lakes on Sunday and
bring light snow or a rain/snow mix to northeast WI. By early next
week, a strong upper jet is forecast to plow into the SW CONUS and
kick the Mexican upper low northeast into the Ohio Valley. This
system will interact with a closed upper low near Lake Winnipeg
and an inverted surface trough to bring another wintry mix to
northeast WI Tuesday/Tuesday night. There continues to be model
inconsistencies with the handling of a potentially bigger system
for Wednesday into Thursday. One thing the models do seem to agree
on is the return of much colder air for later next week.
Temperatures will likely fall below normal and bring more typical
early winter conditions to the region.

A mid-level trough/surface cold front is forecast to move across
the northern/central Plains Saturday night with the leading edge
of light precipitation approaching central WI toward daybreak.
Expect to see clouds thicken through the night, but the bulk of
the precipitation will remain to our west. Min temperatures to be
in the lower 20s north, middle 20s south except for upper 20s
right along Lake MI. The mid-level trough quickly sweeps into the
Great Lakes region on Sunday, while the trailing cold front just
reaches far western WI by 00z Monday. All the focus for
precipitation will be associated with the trough as models
indicate pretty good mid-level forcing with the help of embedded
shortwaves within the trough. Despite a lack of deep moisture
(gulf is cut-off by the southern stream system), models do now
bring around one-tenth of an inch of QPF to northeast WI on
Sunday. This bring the question of precipitation type into play
with the low-level thermal profiles determining who receives all
snow and who receives a mix of rain and snow. Temperatures will be
cool enough to have precipitation fall as all snow through the
morning hours with central WI receiving around an inch of
accumulation by midday. As temperatures slowly warm into the
middle to upper 30s over southern sections of the forecast area in
the afternoon, expect to see some rain mix with the snow which
would obviously impact accumulation numbers. In the end,
anticipate most locations to see around an inch of new snow with
lower values closer to Lake MI and parts of central WI more in the
1-2" range where the precipitation remains all snow.

Chances for light snow will continue into Sunday evening as the
mid-level trough begins to pull away and the trailing cold front
moves through the remainder of WI. Any additional accumulations
appear to be negligible as better forcing shifts to the east.
Behind this system, modest subsidence and the approach of a weak
surface ridge may provide for some breaks in the cloud cover later
Sunday night. Min temperatures to range from the middle 20s
north-central, to around 30 degrees near Lake MI. Any breaks in
the clouds into Monday appear to be short-lived as northern stream
jet energy moves across the northern/central Rockies and helps to
spin up an area of low pressure over the northern Plains. A return
to a southerly wind over WI will allow for weak WAA to return to
the area, thus aiding in cloud development. Max temperatures to
remain above normal on Monday with middle to upper 30s north,
upper 30s to lower 40s south.

Northeast WI to remain between systems through Monday night as the
northern stream system moves across the northern/central Plains
into the Upper MS Valley, and the southern stream closed upper low
to lift northeast toward the lower reaches of the Ohio Valley/Mid-
MS Valley. For the most part, the 12z model output is committed to
not phasing these streams headed into Tuesday with the northern
stream taking its energy into the western Great Lakes, while the
southern stream takes its energy into the eastern Great Lakes. An
inverted surface trough is progged to cross WI on Tuesday and when
coupled with enough mid-level forcing between these two systems,
this should be able to bring at least a chance of precipitation to
northeast Wisconsin on Tuesday. Precipitation type will again come
down to low-level thermal profiles which shows light snow at the
onset before transitioning to a mix or perhaps all rain over
eastern WI by late Tuesday morning. Max temperatures to range
from the middle 30s north- central, to around 40 degrees east-
central WI.

Individual deterministic models continue to struggle with the
digging/no-digging of a progressive broad upper trough across the
central/eastern CONUS during the Wednesday-Friday time frame. The
GFS and CMC pretty much do not deepen the upper trough until it
hits the eastern CONUS late week with a surge of arctic air and
scattered snow showers for most of northeast WI through Friday. If
trajectories are favorable, north-central WI may have to deal with
potentially significant lake effect snows as 8h temperatures crash
to around -20C by Thursday. If this upper trough deepens earlier
over the central CONUS (which some of the models have hinted at),
this would allow for a secondary system to develop over the
southern Plains and move northeast toward the Great Lakes
Wednesday night into Thursday. The latest trends have now favored
the GFS/CMC solution, which translates to less of a snow threat
during this time. However, would prefer to see a little better
model consistency from run-to-run before completely jumping off
the snow bandwagon. For now, will continue to run chc pops for
Wednesday and Thursday focused more on snow shower activity as
arctic air surges into the region. Expect max temperatures to be
5-10 degrees below normal by Thursday and with west-northwest
winds of 10 to 20 mph, wind chill values could be as low as 5
above zero at times.
&&

.AVIATION...for 00Z TAF Issuance
Issued at 602 PM CST Fri Dec 2 2016

The trough over the western Great Lakes and low-level moisture
will continue to keep mainly MVFR cigs in place across the
forecast area through much of the TAF period. Across far northern
Wisconsin, occasional IFR cigs can be expected into Saturday AM.
Some improvement to VFR cigs possible Saturday afternoon,
especially across east-central Wisconsin.

&&

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

$$

SHORT TERM.....ESB
LONG TERM......Kallas
AVIATION.......



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