Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 110251

Area Forecast Discussion
National Weather Service Green Bay WI
851 PM CST Sun Dec 10 2017

Updated aviation portion for 06Z TAF issuance
and new information added to update section

Issued at 845 PM CST Sun Dec 10 2017

The latest runs of the models that would have ingested the 00Z
RAOBs suggest snow will be a little slower to overspread the area
than initially expected. That seems reasonable give that surface
obs and radar data suggest snow is first crossing into northwest
Minnesota. The change does not appear major--perhaps just an hour
or two delay in onset.

It looks like an initial band of snow in an area of strong
isentropic lift will quickly sweep east across the area very late
tonight and early Monday. Snow behind that band will probably be
more in the form of scattered/custers of snow showers driven by
forcing from the mid-level shortwave and enhanced by the fact the
lift will be acting on an air mass with steep mid-level lapse

Still anticipate a widespread 1-3 inch snowfall across the area.
A few locations could get higher totals, especially given the
possible convective nature of the snow. But discerning where that
would occur will be difficult until the evolution of the snowbands
becomes more apparent on radar. Light precipitation in the wake of
the main snow band and outside of any areas of convection could
fall as freezing drizzle as mid-level moisture gets stripped out
by the incoming dry slot.

The main change to the forecast this evening will be to slow the
arrival of the snow a bit. Otherwise, no significant forecast changes
appear necessary. Updated product suite including SPS will be out

.SHORT TERM...Tonight and Monday
Issued at 310 PM CST Sun Dec 10 2017

Precipitation chances and amounts associated with an approaching
clipper system are the main concerns in the short term.

Lake effect snow showers and flurries should come to an end this
evening as winds back and direction becomes unfavorable for lake
effect in the forecast area.

A surface low pressure system and mid level trough in the
Canadian Plains will approach Wisconsin tonight and move through
the state during the day on Monday. Warm advection ahead of the
low will bring snow into the area late tonight and during the day
on Monday, with the majority of the snow falling between midnight
and noon. Looks like most locations will pick up between 1 and 3
inches of snow from this system. Green Bay and the Fox Valley are
likely to see the heaviest snowfall as many head to work or
school. Will forgo an advisory for now, but the evening shift may
want to issue pending analysis of data from 00Z model runs. Snow
should wind down Monday afternoon, but lake effect is expected to
continue in the Lake Superior snowbelt.

Lows tonight will range from around 10 degrees to near 20. Highs
on Monday should be in the lower 20s to lower 30s.

.LONG TERM...Monday Night Through Sunday
Issued at 310 PM CST Sun Dec 10 2017

High amplitude flow with a strong ridge over the western Conus
and deep cold trough over the eastern states will keep the forecast
area in the cold northwest flow regime this week, resulting in a
series of clipper type systems passing over. Track and timing of
these clippers will be the primary challenge.

An early week clipper system will be departing Monday night,
leaving the area in a cold air advection pattern, and increasing lake
effect snow potential across the far north. Northwest trajectory
indicates lake effect snow showers will persist into much of
Tuesday before a ridge of high pressure approaches from the west
toward Tuesday night.

What clearing occurs Tuesday night with surface ridge may be
brief as another clipper system drops in toward mid week. The GFS
prog is looking similar to the early week system, but the ECMWF is
a bit further southwest. More cold air pours into the area behind
this system for the lake effect snows to start up again across
the far north.

Upper pattern begins to change late in the week and into the
weekend toward a more zonal flow with moderating temperatures.
Another northern stream system or clipper type system is progged
to drop into the area next weekend. The warmer boundary layer temps
suggests the potential of a mix variety.

.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.



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