Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 130311

Area Forecast Discussion
National Weather Service Green Bay WI
911 PM CST Tue Dec 12 2017

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

Issued at 856 PM CST Tue Dec 12 2017

No headline changes planned this evening. The lead band of snow
has pushed into the western part of the forecast area. It may tend
to weaken as it shifts east, but will snows will probably continue
to develop southeast into the area as well. So anticipate
intermittent snowfall early tonight. Precipitation should increase
in intensity and become more widespread after midnight.
Considered pulling the advisory start time back to begin it
earlier. But although snow will cause some slippery roads
overnight, the main impact will be later tonight and tomorrow so
plan to leave the advisory start time as is for now.

Still some rather substantial differences in the models in the
placement of the maximum precipitation. At this point, favor the
ECMWF which seems to focus the max on east-central Wisconsin. Many
of the other models were considerably overdone with the previous 2
rounds of snow, while the ECMWF did better in forecasting lighter
amounts. Although it`s not exactly the same thing, will defer to
it for placement of the snows as well. If radar indicates more
persistent snows across the north late tonight into tomorrow
morning, the north may eventually need to be added to the advisory
as well.

Updated product suite will be out ASAP.

.SHORT TERM...Tonight and Wednesday
Issued at 218 PM CST Tue Dec 12 2017

The main concern in the short term is snowfall with the upcoming
system that is expected to bring accumulating snow to the area,
mainly during the day on Wednesday.

Radar imagery indicated some light snow and flurries in Minnesota
and far western Wisconsin at 20Z. Models showed this weaker band
of light snow/flurries moving into central Wisconsin this evening
ahead of the main snow even. There could be a few tenths of an
inch of snow with this before midnight, but heavier snow is
forecast to move into the area later in the night as a surface low
and mid level short wave reach southern Minnesota by 12Z
Wednesday. The 12Z NAM, GFS & ECMWF all brought quite a bit of
precipitation into central and eastern Wisconsin between 12Z and
18Z as the short wave and surface low move to northern Illinois
and another mid level short wave approaches from the northwest.

Based on the 12Z models there should be enough snowfall during the
day on Wednesday to justify at least an advisory across a
majority of the forecast area. 850 mb winds and temperatures
become favorable for lake effect in eastern Wisconsin later in
the event. Highest model QPF suggests that there could be enough
snow in parts of east central Wisconsin to justify a warning, but
confidence is not currently high enough to go this route. As a
result, we have gone the advisory route in hopes that 00Z model
trends make the outcome of the event look clearer.

.LONG TERM...Wednesday Night Through Tuesday
Issued at 218 PM CST Tue Dec 12 2017

Fairly busy long term pattern for the remainder of the week and
into this weekend.

Snow is expected to linger into Wednesday evening over eastern
Wisconsin as a clipper low pressure system tracks southeast of the
area. Lake enhanced snow may also contribute to additional
accumulation along Lake Michigan wednesday evening before ending.

Wind fields late Wednesday night into Thursday appear more
westerly across the area ahead of a short wave trough approaching
from the north. This would suggest a lesser lake effect snow
potential across the far north central Wisconsin until Friday.

The strong upper ridge over the western Conus breaks down late
this week into the weekend resulting in a more zonal flow and a
moderating temperature trend. A series of westerly low pressure
systems will slide over the area with one system producing a
decent warm air advection pattern and precipitation for Saturday.
A few more systems in the northern stream brush over the northern
Great Lakes region late in the weekend and into early next week.

.AVIATION...for 06Z TAF Issuance
Issued at 856 PM CST Tue Dec 12 2017

Flight conditions will vary considerably early tonight as the lead
snow band shifts east across the area and weakens. Conditions are
still expected to deteriorate rapidly late tonight as snow
increases across the area. Anticipate IFR and occasional LIFR
conditions Wednesday as the main snow band moves through.

Winter Weather Advisory from 4 AM to 6 PM CST Wednesday for

Winter Weather Advisory from 4 AM to 9 PM CST Wednesday for


AVIATION.......Skowronski is the U.S. government's official web portal to all federal, state and local government web resources and services.