Area Forecast Discussion
Issued by NWS Green Bay, WI

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000
FXUS63 KGRB 060846
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
246 AM CST Wed Dec 6 2017

Forecast discussion for routine morning forecast issuance

.SHORT TERM...Today...Tonight...and Thursday
Issued at 245 AM CST Wed Dec 6 2017

The prevalent northwest flow aloft will continue over the region
through Thursday as the broad upper trough has set up shop over
east-central NOAM. The main forecast challenge will be trying to
determine extent of lake effect snow showers over north-central WI
and any synoptic light snow showers/flurries across the rest of
the forecast area.

The 07z MSAS surface analysis showed an intense cyclone near
Hudson Bay with several troughs rotating south, then southeast
through the Great Lakes region. Each of these troughs have either
snow showers or flurries accompany them and this pattern is
expected to continue today into tonight. The pressure gradient is
not as tight as 24 hours ago, thus winds are not as gusty.

The broad upper trough to remain parked over the Great Lakes
region, keeping a northwest flow into WI. Within this upper
trough, models show a mid-level shortwave trough/embedded sheared
shortwave dropping southeast into the region today. The passage of
this feature will veer winds from west to west-northwest and
increase the potential for lake effect snow showers over north-
central WI, especially during the afternoon hours. Meanwhile, this
same feature will bring scattered snow showers and/or flurries to
the rest of northeast WI today. While most locations to only see a
dusting of snow, parts of Vilas County could see an inch or two of
snow with the added lake enhancement. Blustery conditions are also
expected with sustained winds of 10 to 20 mph and gusts
approaching 30 mph at times. Max temperatures to range from around
20 degrees north-central, to the middle to upper 20s eastern WI.

Lake effect snow shower chances will continue over north-central
WI tonight with a persistent west-northwest wind in place. Delta-T
values remain in the middle to upper teens, thus up to an inch of
additional accumulation is possible over the snowbelt region of
Vilas County. The rest of northeast WI should only see more
flurries mainly in the evening as the aforementioned shortwave
trough to still be in the process of exiting the area. Min
temperatures to be around 10 above zero over central WI, upper
teens near Lake MI. Wind speeds of 10 to 15 mph will create wind
chills primarily in the single digits above zero.

Northeast WI to essentially reside between systems on Thursday
with a very weak reflection of a surface ridge sliding into WI.
Winds are forecast to back a bit on Thursday which would end the
lake effect threat for Vilas County and allow for some sun to
return to the forecast area. Despite the anticipated mixed
sunshine, temperatures will remain below normal with readings in
the upper teens to lower 20s central WI, middle 20s eastern WI.

.LONG TERM...Thursday Night Through Tuesday
Issued at 245 AM CST Wed Dec 6 2017

Large scale pattern pretty well locked into place for the long
term period, with a strong ridge over the western U.S. and a deep
trough across the Great Lakes, with relatively weak disturbances
riding southeast across the area. This pattern will favor a few
chances for snow shower and flurries and bouts of lake effect snow
for north central Wisconsin. Temperatues will remain (slightly)
below normal, with near or slight above normal temps in the
Sunday/Monday timeframe as the ridge flattens out slightly.
Another surge of colder air is expected mid next week as the upper
ridge re-amplifies.

Lake effect activity should be limited Thursday night across far
north central WI as winds remain westerly. Trajectories become
more favorable on Friday, ahead of the first of a parade of
shortwaves. The timing of this shortwave has slowed slightly,
focusing the best chance for area-wide snow showers Friday
afternoon into Friday night. Moisture will be limited with this
system, however, models have trended up with precipitable waters
values, now between 0.20-0.30" and QPF values have increased as
well, mainly between 0.05 and 0.10" (GFS). Confidence is
increasing the area will see snow, especially Friday evening, so
increased POPs, but do to timing issues, kept POPs in the chance
category. They will likely need to be increased again once the
minor timing issues are resolved. Increased QPF to 0.04-0.08" for
most spots Friday night. With snow ratios running between 20:1
and 24:1, snow totals of an inch or two are expected.

The lake effect snow machine will be in full effect Friday night
into Saturday as winds veer to the NW. Deep moisture, inversion
heights up to 8000 ft and delta T`s increasing to over 20C will
provide very favorable conditions. The heaviest accumulations will
of course stay in Upper Michigan, but far northern WI will
probably see more accumulation. Farther south, a few snow showers
and flurries are expected as lake effect activity diminishes away
from Lake Superior. Models may be underdoing the lake effect
activity, will POPs/QPF might need to be increased.

The lake effect snow shuts of by Sunday morning as flow turns more
west/southwest. This brief pattern change will allow 850mb temps
to climb closer to 0C across central WI and near -5C over northern
WI. High temps should be able to climb into the middle/upper 20s
north and to around 30 farther south. The next shortwave will
arrive somewhere in the Sunday night or Monday timeframe, bringing
additional chances for snow showers, with another round of lake
effect snow over north central WI. Exact timing of this system
will determine if Monday will be another day near or slightly
above normal or a return to colder readings.
&&

.AVIATION...for 06Z TAF Issuance
Issued at 1046 PM CST Tue Dec 5 2017

West winds will continue to gust to 20 to 30 knots for much of the
TAF period. Shortwaves in the cyclonic flow over the western Great
Lakes will bring periods of light snow showers and flurries to the
TAF sites through the period. The best chance will be across
northern Wisconsin given better moisture. Conditions will
generally bounce between MVFR and VFR with snow showers, with IFR
possible at times across the far north.
&&

.MARINE...
Issued at 245 AM CST Wed Dec 6 2017

A Gale Warning remains in effect through 6 am for the nearshore
waters of Lake Michigan. Gusts to 30 kts will likely continue
today and 25 kts for tonight, thus a small craft advisory will be
in effect until 12z Thursday.
&&

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

$$

SHORT TERM.....Kallas
LONG TERM......Bersch
AVIATION.......Kurimski
MARINE.........Kallas



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