Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 121145
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
545 AM CST Sun Nov 12 2017

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Monday
Issued at 345 AM CST Sun Nov 12 2017

Cyclonic upper flow and time of year favor cloudiness
today and tonight, but low level cold advection and a surface high
building in from the west suggest clearing. So am thinking it will
be mostly cloudy today and tonight with a better chance of sunshine
tomorrow when there will be flat upper ridging and a surface high
right across Wisconsin.

Temperatures today and tomorrow will be 5 to 7 degrees below
normal, but should seem much milder than the last couple of days
as winds will be light.

.LONG TERM...Monday Night Through Saturday
Issued at 345 AM CST Sun Nov 12 2017

Model guidance continues to show a persistent pattern with an
eastern Pacific upper ridge and a western Canada/NW CONUS upper
trough through the end of the week. This pattern would provide a
progressive low amplitude flow across the rest of the CONUS. Two
systems will be worth watching this week. The first one arrives
late Tuesday into Wednesday morning and the second arrives
Thursday night into Friday. Temperature profiles will be critical
as to precipitation type, especially at night. These periods will
be the main forecast focus.

Weak WAA will continue into WI Monday night, however the
atmosphere is too dry yet to support any precipitation. Look for
clouds to gradually thicken overnight and when combined with
southwest winds in the boundary layer, temperatures should not
fall too far. Readings should range from the middle to upper 20s
north, upper 20s to lower 30s south. The WAA is forecast to
increase over the region on Tuesday and latest timing of
saturation will be reached over central WI by late morning or
early afternoon. Temperatures are expected to be hovering close to
freezing when the precipitation would begin, thus a rain or mixed
rain/freezing rain scenario is possible initially before
temperatures warm enough for all rain to fall in the afternoon.
Farther to the east, it still appears that there will be enough
lingering dry air near the surface to hold off on any
precipitation until evening, thus dry conditions to hold under
cloudy skies. Max temperatures to range from the upper 30s to
lower 40s north, lower to middle 40s south.

Precipitation is forecast to become more widespread Tuesday night
across northeast WI as a cold front pushes into central WI by 12z
Wednesday. The persistent southwest winds/WAA into WI should keep
enough warm air in place to keep the precipitation as all rain
through the night. As such, temperatures will only drop a few
degrees Tuesday night with readings in the middle 30s north,
middle 30s to around 40 degrees south. Rain chances will continue
over eastern WI Wednesday morning until the cold front completes
its passage through the state. By the time the cooler air arrives
behind the cold front, the precipitation will have ended, so no
snow to worry about. Otherwise, subsidence and influx of drier air
will allow for a decreasing cloud trend from west to east on
Wednesday. The exception could be north-central WI where west-
northwest winds would bring some lake effect clouds and perhaps a
small chance of snow showers or mixed snow showers/freezing rain
to that part of the forecast area. Max temperatures will range
from the upper 30s to around 40 degrees north-central, to the
middle to upper 40s eastern WI.

The slight chance of lake effect snow showers will continue into
Wednesday night, before tapering off as high pressure builds into
the western Great Lakes and trajectories become unfavorable. The
rest of northeast WI to see mostly clear to partly cloudy skies
and dry conditions Wednesday night. This high pressure rolls
across WI Thursday morning and be to our east Thursday afternoon.
WAA will already be underway to our west across the Upper MS
Valley, so we could see an increase in mid/high clouds later in
the day. Max temperatures Thursday will cool a bit compared to
Wednesday with readings only in the middle 30s north, middle 30s
to around 40 degrees south.

Our next precipitation maker pushes into WI Thursday night and
continues through Friday as one area of low pressure moves across
southern Canada and a second low pressure moves from the central
High Plains northeast toward the Great Lakes. There continues to
be timing issues among the models headed into Friday (the GEM has
slowed down/the ECMWF has sped up/the GFS remains the fastest), so
confidence is still rather low at this point. This timing is
important as to how fast/much warmer air would get pulled into WI,
especially for Thursday night and precipitation type issues. A
slower solution would keep the atmosphere cold enough to support
more snow than mixed precipitation Thursday night with
accumulations likely. By Friday, expect precipitation to
transition more to either a mix or all rain as the warmer air will
have arrived. Stay tuned!

As a trialing strong shortwave trough sweeps across the Great
Lakes Friday night, the southern surface low is progged to
strengthen rapidly and reside roughly in/around Lake Huron
vicinity by 12z Saturday. Initial variety of precipitation types
going into Friday evening should transition to snow as colder air
overspreads WI by late Friday evening, then taper off after
midnight as CAA/subsidence takes over. This will also be a
blustery system Friday night into Saturday as the deep low
pressure gradually pulls away from WI. The main focus for Saturday
will be possible lake effect snow showers over north-central WI as
8H temperatures over western Lake Superior drops to around -13C.
Max temperatures by Saturday will fall below normal with readings
only in the upper 20s to around 30 degrees north, to the middle
30s over east-central WI.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 544 AM CST Sun Nov 12 2017

Patchy ground fog should dissipate by 14z this morning. Scattered
to broken clouds at 2500 to 3500 feet are expected today and part
of tonight, with some mid and high clouds above. Clearing is
expected across the whole area late tonight with VFR conditions
Monday.
&&

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

$$

SHORT TERM.....RDM
LONG TERM......Kallas
AVIATION.......RDM



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