Area Forecast Discussion
Issued by NWS Green Bay, WI

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000
FXUS63 KGRB 060928
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
328 AM CST Tue Dec 6 2016

Forecast discussion for routine morning forecast issuance

.SYNOPSIS...
Issued at 324 AM CST Tue Dec 6 2016

Some rain and snow showers today, then blustery and colder
tonight.

Strong ridging near the Dateline and high upper heights over the
northeast Pacific were blocking the flow of the westerlies into
North America at high latitudes. That was allowing cold air to
build up over Canada, and resulting in a strong fairly low-
amplitude flow across the CONUS. Little change in this overall
pattern is expected during the forecast period.

The upper pattern favors temperatures dropping back much closer to
seasonal normals that we`ve had for some time. Unlike a cool
down in mid-November, the cold weather will stick around longer
this time. At this point it appears the core of the cold air
building over Canada will not get driven down into the forecast
area during the forecast period, though there are indications
that could happen shortly thereafter.

Disturbances riding through fast westerly flow aloft will provide
several opportunities for precipitation, most of which is likely
to fall as snow. But pinpointing the timing of the snow chances
beyond the first couple forecast period will be difficult. Some of
the disturbances could turn out to be efficient snow producers
given the strong baroclinic zone near the region, but significant
snow amounts are likely to occur in narrow zones that will also be
difficult to pinpoint. Normal precipitation for a week at this
time of year is a little less than a half an inch, and the best
guess is that most areas will end up near or a little below that
for the period. The colder air will also make lake-effect snows
more likely, so amounts in the snowbelt will be heavier.
&&

.SHORT TERM...Today...Tonight...and Wednesday
Issued at 324 AM CST Tue Dec 6 2016

A compact cyclone over the northern Plains this morning will shift
into eastern Ontario by Wednesday evening. A milder air mass,
warmed further by southeast winds off Lake Michigan, was flowing
into the area in advance of this system. Most guidance is off to
a rough start for temperatures with numerous products, included
many of the blends, having max temp forecasts for today that are
below current readings.

Precipitation with the incoming weather system was increasing to
the west this morning. But that band of precipitation will tend to
wrap northward around the cyclone, so eastward progress into the
forecast area will be slow. It is also possible saturation will
not initially extend to a deep/cold enough level for ice crystals.
So will continue trend of previous forecast of carrying some
DZ/FZDZ this morning (based on surface temperatures), but have
main precipitation type as SHSN as forecast soundings suggest
moisture will deepen as the strongest lift shifts through. Will
also mix the SHSH with SHRA where boundary layer temps appear warm
enough to result in some melting of the snow before it reaches the
ground.

The colder air will have to wrap all the way around the stacked
cyclone as it lifts into southwest Ontario tonight. So raised mins
a bit--especially over the southeast part of the forecast area.
Some snow showers or flurries are possible over the northwest
portion of the forecast area--closer to the low. Winds will also
increase from the WSW during the evening, and are likely to remain
gusty even at night as CAA steepens low-level lapse rates.

Snow showers and flurries are expected Wednesday, especially over
the northwest part of the forecast area, as the colder air becomes
established across the region.

.LONG TERM...Wednesday Night Through Monday
Issued at 324 AM CST Tue Dec 6 2016

Main forecast issues that need to be addressed will include: the
onset of colder air that will send temperatures below normal for
an extended period of time, the potential for a couple of days of
lake effect snows for north-central WI late this week and perhaps
another accumulating snow this weekend.

While a broad upper trough to engulf the central CONUS Wednesday
night, it will be the approach of a secondary cold front that is
expected to drop southeast into the western Great Lakes region
that will bring the next chance of snow showers. Latest timing has
this front just reaching northern WI toward daybreak, therefore
best chance of seeing light snow showers would be northern WI
where a minor accumulation (< 1") is possible. Plenty of clouds
and a steady west wind should prevent temperatures from falling
too much with readings primarily in the upper teens to lower 20s.

This cold front will sweep through the rest of WI on Thursday,
veer the winds to the west-northwest and usher in only slightly
cooler air. 8H temperatures are not as cold as what had been
advertised several days ago, thus max temperatures should still be
able to reach the lower to middle 20s north, middle 20s to around
30 degrees south. The main story though for Thursday will be the
wind shift which is expected to allow for lake effect snow showers
to develop across north-central WI. Even though trajectories are
not perfect and 8H temperatures are not that cold, delta-T values
still reach the middle to upper teens and there is a lack of any
significant low-level inversions. Thus, it is possible for Vilas
county to see 1-3" of snow by 00z Friday.

Not much changes headed into Thursday night with a weak cyclonic
flow, a northwest wind, 8H temperatures over western Lake Superior
around -16C and delta-T values in the upper teens to lower 20s.
This could add another 1-3" of snow to the snowbelt region of
Vilas county, so will need to watch accumulations for a possible
advisory. The rest of northeast WI should remain cloudy, but dry
Thursday night with min temperatures generally in the middle to
upper teens (perhaps around 20 degrees over Door county). The lake
effect snow showers will linger into Friday across north-central
WI, however the activity should gradually wane through the day as
a ridge of high pressure slides across the Upper MS Valley and
winds begin to slowly back toward the west. Southern sections of
the forecast area may see some sunshine, but temperatures will
remain below normal. Look for max temperatures to range from 15-20
degrees north-central/central WI, lower to middle 20s eastern WI.

This surface ridge moves across WI Friday night, but is fairly
weak and model forecast soundings still show plenty of mid/upper-
level moisture around the area. Any substantial break in these
clouds could tank temperatures, especially where any snow is still
on the ground. For now, have min temperatures from 5-10 above zero
north-central, 10-15 above elsewhere except middle teens near Lake
MI. A return of southerly winds will usher in a burst of WAA into
WI on Saturday with models showing light snow trying to push into
northeast WI during the afternoon. Temperatures may warm slightly,
but with thickening cloud cover and precipitation approaching,
much of the warming will be muted. Look for max temperatures to be
in the upper teens to around 20 north, lower to middle 20s south.

Forecast becomes more interesting, but uncertain for the latter
half of the weekend as the models indicate a shortwave trough will
lift northeast from the central Plains into the Great Lakes. The
problem is that the models have timing issues, as well as the
track of the associated surface low. Until the models get a better
handle on the overall track of this system, snow accumulations
could range anywhere from under an inch to several inches. Have
followed the consensus solution for now with high chance/likely
pops and mention potential in the HWO. Max temperatures on Sunday
will range from the middle 20s north-central, to around 30 degrees
east-central WI.

It looks like the chance for light snow would continue into Sunday
night as the system pulls away from the region. Any additional
accumulations appear to be minor at this point. By Monday, a weak
ridge of high pressure is progged to move into WI and end any
precipitation threat (even lake effect over Vilas county).
Temperatures will remain a bit below normal through Monday with
readings in the upper teens to lower 20s north-central/central,
middle to upper 20s eastern WI.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 324 AM CST Tue Dec 6 2016

Ceilings have settled into the IFR category over most of the area,
though low clouds still remain scattered over the north. Expect
the low clouds to shift into the north by daybreak. Once in place,
the low clouds are likely to linger until the cold front crosses
the area. Ceilings should improve in the wake of the front.
&&

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

$$

SYNOPSIS.......Skowronski
SHORT TERM.....Skowronski
LONG TERM......Kallas
AVIATION.......Skowronski



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