Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 111017
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
417 AM CST Mon Dec 11 2017

Forecast discussion for routine morning forecast issuance

.SHORT TERM...Today...Tonight...and Tuesday
Issued at 416 AM CST Mon Dec 11 2017

Forecast concerns include precipitation trends, type and snowfall
amounts with the clipper system, and developing strong winds and
lake effect snow in the wake of the system.

A potent short-wave trof was moving through southeast ND early
this morning, while an associated clipper low was moving through
northwest MN. A band of WAA snow was moving into GRB CWA, and
causing visibilities to drop to 1-2 miles in parts of central and
north central WI. Regional radar showed a area of showery
precipitation occuring in central MN, in association with the
strong short-wave. Surface observations indicated a mixed of bag
of precipitation types with this area of precipitation.

Steady light to occasionally moderate snow will continue across
most of the area through mid to late morning, with most places
receiving 1 to 2 inches of accumulation. Precipitation type
may get interesting across parts of central WI and the southern
Fox Valley as a weak dry slot arrives this morning, and
precipitation become more convective in nature. Have gone with
a mix of snow and freezing rain in this area. Don`t expect a huge
impact from the freezing rain, as it will be falling on top of
snow rather than directly glazing road surfaces. Regardless,
travel conditions will be hazardous for the morning commute.

The main deformation band of snow with the short-wave should stay
to our south this afternoon, so precipitation is expected to
gradually taper off in most places. The exception will be in
far north central WI, where lake effect is expected to increase
as winds turn NNW and lake-H8 delta-t`s increase to 20-22 C.
The lake effect may become enhanced as another short-wave moves
through in the late afternoon and early evening, so Vilas county
may see another inch or two of snow. Although lake-effect is
expected to continue through tonight and Tuesday, it should become
lighter and more scattered as much drier arctic air feeds into
the region.

Winds will crank up as strong CAA develops in the wake of the
clipper system later this afternoon into tonight. Gusts to
25 to 35 mph can be expected in most places, though gale force
gusts are likely on Lake Michigan, and gusts may top 45 mph at
times in northern Door county. Will be issuing a Gale Warning on
Lake Michigan and a Wind Advisory for Door County from this
evening through late Tuesday morning.

Much colder air will arrive tonight into Tuesday, with lows
in the single digits and lower teens, and highs in the teens.

.LONG TERM...Tuesday Night Through Sunday
Issued at 416 AM CST Mon Dec 11 2017

Models agree with holding the amplified flow consisting of a
western NOAM upper ridge/eastern NOAM upper trough through
Thursday. This pattern is then forecast to de-amplify thereafter
as a series of Pacific systems act to flatten the western ridge.
The prevailing northwest flow into WI will continue to send
clipper systems into the region, although timing/track of each
clipper continues to be problematic. Once the pattern changes,
timing of Pacific systems becomes an issue by next weekend.
Temperatures will be below normal through Friday, then transition
to above normal by Saturday.

A modest ridge of high pressure is expected to move across the
western Great Lakes region Tuesday night. Skies will start the
night partly cloudy, however as the next clipper system drops
southeast into northern MN overnight, the onset of WAA should
bring an increase in mid/high clouds to northeast WI. The timing
of these clouds will be critical as to how far temperatures fall
since winds will be light. For now, have min temperatures falling
to around zero north-central WI, to around 10 above zero near Lake
MI. Models continue to struggle with the eventual track of this
clipper, either dropping it south-southeast toward the Mid-MS
Valley (ECMWF/CMC) or take it through the Great Lakes (NAM/GFS).
Models also differ on the strength of the surface low and
placement of mid-level shortwaves moving through the northwest
flow aloft. Due to these differences, prefer to keep pops in the
chance category with the caveat being light snow should be seen by
most locations at some point on Wednesday. Max temperatures to
remain well-below normal with readings only in the middle to upper
teens north, upper teens to around 20 degrees south.

A cold, cyclonic flow sets up over WI Wednesday night with a
chance for additional snow showers, especially over north-central
WI as lake effect turns on with northwest winds and 8H
temperatures over western Lake Superior residing around -16C.
Depending on how the bands of snow line up, will have to watch
Vilas County closely over the next couple of days. Despite the
CAA, cloud cover should minimize temperature drop. Look for
readings to dip into the 5 to 10 above zero range north, 10 to 15
range central/east-central and around 15 above near Lake MI. The
passage of a mid-level shortwave trough on Thursday may be enough
to kick off a few snow showers or flurries across northeast WI.
However, the persistent northwest flow should continue the lake
effect threat for north-central WI through the day. The brunt of
the coldest air is forecast to shift east on Thursday and this
should allow max temperatures to reach around 20 degrees north-
central, middle 20s eastern WI.

The last clipper within this northwest flow passes to our north on
Thursday night with the models generally only bringing clouds to
the region. The exception (again) will be north-central WI where
west-northwest winds to prevail. This lake effect is expected to
end on Friday as a ridge of high pressure moves into the Great
Lakes and winds back to the west. Max temperatures on Friday
should be similar to Thursday (perhaps a degree or two warmer if
we can see any sunshine).

By Friday night, the mean flow will have already turned more west
than northwest over the eastern half of the CONUS and the models
bring the first Pacific system into the Upper MS Valley by 12z
Saturday. A surge of WAA into WI ahead of this system is forecast
to bring an increasing chance for light snow into WI Friday night.
The exact location of this snow may depend on the location of a
warm front in which the models disagree on its location. Have
followed the consensus solution which focuses higher pops over
northern WI. This system is then progged to move across northern
sections of the Great Lakes on Saturday with a chance for light
snow mainly focused over the northern half of WI. If any
precipitation can get into central or east-central WI Saturday
afternoon, it may fall as a rain/snow mix as temperatures are
forecast to reach the middle 30s.

A trailing piece of energy and weak surface trough drop into the
region later Saturday night into Sunday. This little system could
bring a small chance of light snow to northeast WI, but any
accumulations look very minor at this time. Max temperatures on
Sunday to be above normal with upper 20s to around 30 degrees
north, lower to middle 30s south.
&&

.AVIATION...for 06Z TAF Issuance
Issued at 845 PM CST Sun Dec 10 2017

Another fast moving storm system dropping southeast from Canada
will sweep through the area late tonight and Monday. Snow from
the incoming storm system should begin to spread across the area
around 09-10Z, reaching the lakeshore by 13-14Z. Anticipate IFR
conditions due to reduced visibilities as the snow moves through.
Low ceilings in the wake of the main snow band will maintain IFR
or low-end VFR conditions.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 8 PM this evening to 10 AM CST Tuesday for
WIZ022.
&&

$$

SHORT TERM.....Kieckbusch
LONG TERM......Kallas
AVIATION.......Skowronski



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