Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 071750

Area Forecast Discussion
National Weather Service Green Bay WI
1150 AM CST Thu Dec 7 2017

Updated aviation portion for 18Z TAF issuance

.SHORT TERM...Today...Tonight...and Friday
Issued at 256 AM CST Thu Dec 7 2017

Other than some light lake effect snow showers or flurries this
morning and again tonight across north-central WI, the main focus
to be on an approaching clipper low pressure and mid-level trough
Friday afternoon.

The 07z MSAS surface analysis showed strong low pressure over the
Canadian Maritimes and strong high pressure over the northern/
central Plains. The pressure gradient between these two weather
features remained fairly tight across WI with sustained winds of
10 to 20 mph. Radar mosaic indicated one more band of light snow
showers and flurries moving across parts of central and east-
central WI, while a more persistent band of snow showers impacted
north-central WI due to lake effect. Satellite imagery did now
clearing skies across MN/northwest WI in conjunction with the
arrival of drier air ahead of a weak surface ridge.

The departure of a mid-level shortwave trough, combined with the
approach of the weak surface ridge, should allow for a decrease in
clouds from west to east this morning. The only exception would be
along the MI border where west winds may still bring some lake
clouds inland. Any snow showers/flurries over Vilas County will
end this morning as well as trajectories become unfavorable. The
break in the clouds is expected to be short-lived, as slightly
warmer air aloft moves into WI and a mid-cloud deck arrives. Max
temperatures to range from the upper teens north-central WI, to
the middle 20s eastern WI.

Clouds will continue to thicken over northeast WI tonight as a
modest mid-level shortwave trough and weak surface trough drop
southeast into the region. Winds just above the surface veer a bit
to the west-northwest and this could again bring a chance of light
lake effect snow showers to north-central WI. The rest of the
forecast area may see a few flurries. Min temperatures to be
around 10 above zero north, middle teens near Lake MI and 10 to 15
degree range elsewhere.

Weak lake effect snow showers will continue into Friday morning
across north-central WI, however the main focus to be on a clipper
system/stronger mid-level shortwave trough moving southeast from
the Canadian Prairie. This new system is now progged to reach the
western Great Lakes region by 00z Saturday with an increasing
chance for light snow, especially over northern and central WI.
Have raised pops a bit from the previous forecast, however the
main impact from snow accumulation should hold off until Friday
evening. Nevertheless, it is possible that 1/2 to 1 inch of new
snow may already be on the ground across north-central by late
Friday. Max temperatures to range from the lower 20s north, to the
upper 20s east-central WI.

.LONG TERM...Friday Night Through Wednesday
Issued at 256 AM CST Thu Dec 7 2017

A northwest flow pattern remains pretty locked into place during
the long term, with a strong ridge over the western U.S. and a
trough over the Great Lakes/eastern U.S. This pattern will
continue to bring chances for snow and bouts with lake effect
snow as shortwave/clipper-type systems ride withing the northwest
flow. Temps will remain below normal for most of the period;
however, a period of near normal temps are expected ahead of the
clipper Sunday and Monday.

The first system will dive across the area Friday evening. It will
be a fast mover and lacking deep moisture, but precipitable waters
of 0.20-0.30", strong mid/upper level dynamics and sufficient
lift/frontogenetic forcing, some in the dendritic snow growth
region, quickly cross the area, leading to an area-wide snowfall.
Models continue to slightly differ on timing and amounts, with
the GEM the slowest and the GFS the fastest and the ECMWF now
showing the highest QPF amounts (was the lowest two nights ago).
Will continue to have the highest POPs in the Friday evening
period, with lower POPs overnight as the system drops south of the
area. Using a blend of the model QPF seems reasonable, with most
of the area seeing 0.06-0.13" of QPF. Snow ratios in the lower 20s
across northern WI and the upper teens farther south gives the
area a solid 1-3" snowfall. Locally higher amounts around 4" are
not out of the question if some banding can occur and the higher
QPF amounts verify.

Will keep some higher POPs overnight across north central
WI, as the lake effect machine cranks up with favorable NNW winds
and delta T`s over 20. However, moisture and inversion heights are
looking to be a little less favorable as mid-level drying occurs.
This should help keep the lake effect snow in check with
additional accumulations overnight Friday into Saturday under an
inch or two. Elsewhere, look for snow to end from north to south,
with only lingering snow showers or flurries continuing into
Saturday morning. Any lingering lake effect activity looks to
diminish by Saturday night as winds back to the W/SW.

Quiet conditions are expected on Sunday for most spots. The only
exception could be far north central / northeast WI where lake
effect snow showers could clip the area as winds veer back to the

The next clipper will dive across the western Great Lakes Sunday
night into Monday. Another clipper will dive down across the area
during the middle of next week. A period of lake effect snow is
expected behind each system. Either one of these systems could
bring some accumulating to the area, but models have not good
handle on either, so confidence is low on the details.

.AVIATION...for 18Z TAF Issuance
Issued at 1149 AM CST Thu Dec 7 2017

Mainly VFR conditions this afternoon with high clouds across the
area and low/mid clouds exiting, except over far northern
Wisconsin were MVFR conditions will exist at times. This evening,
mid- clouds gradually return, followed by low clouds overnight
and into Friday morning. Winds veer slightly more NW across Lake
Superior early Friday morning and could bring some light lake
effect snow showers across northern Vilas County. Otherwise, light
snow showers or flurries are possible across central and north-
central Wisconsin ahead of a clipper system Friday morning, but
most of the area will remain dry through the TAF period ending at
Noon Friday. This clipper is expected to bring widespread snow to
the area late Friday afternoon and Friday evening which would
result in extended IFR conditions.



SHORT TERM.....Kallas
LONG TERM......Bersch
AVIATION.......KLB is the U.S. government's official web portal to all federal, state and local government web resources and services.