Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 240433
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1033 PM CST Thu Feb 23 2017

Updated aviation portion for 06Z TAF issuance

.SYNOPSIS...
Issued at 226 PM CST Thu Feb 23 2017

A major winter storm will affect the area tonight through Friday
night, with snow across much of the area and a mix of wintry
precipitation in east-central Wisconsin.

The large scale pattern at the onset of the period will have a
trough over the Rockies and ridges over the eastern Pacific and
near the East Coast. The main change during the period will be for
the trough to broaden eastward. The pattern will support a major
cyclone in the central United States the next couple days, then
several more opportunities for precipitation beyond that. The
result should be above normal precipitation for the period.
Temperatures will vary some from day to day, but will probably not
depart too far from seasonal normals.
&&

.SHORT TERM...Tonight and Friday
Issued at 226 PM CST Thu Feb 23 2017

Main forecast challenge remains on the impending storm starting
late this evening and continuing into Saturday. Trying to
determine the snow/mixed precipitation line remains troublesome,
as well as snow accumulations and ice accumulations.

The 20z MSAS surface analysis showed the main area of low pressure
just exiting the central Rockies into the central Plains. A
quasi-stationary front extended east-northeast from the low toward
the Mid-MS Valley. The radar mosaic already picking up on
increasing echoes headed toward WI, an indication of the
increasing lift/forcing headed our way.

After much collaboration with surrounding offices, have placed
northern and central WI into a winter storm warning, with eastern
WI in a winter weather advisory for late tonight through Friday
(and continuing beyond). The low pressure is expected to move
across the central CONUS and reach roughly the Quad Cities by 12z
Friday. A surge of WAA and frontogenetical forcing will overspread
WI late this evening/overnight and bring a swath of snow,
primarily to central and east-central WI. As temperatures warm
aloft late tonight, some freezing rain may mix with the snow
roughly from Wautoma to Manitowoc. Accumulations by daybreak to
range from one- half inch or less near the MI border, 2 to 4
inches for central /east-central WI. Another concern will be the
increased winds which are forecast to reach 15 to 25 mph after
midnight. Gusts could reach 30 mph and create at least drifting
snow, if not some blowing snow. Travel for the morning rush hour
will be impacted with low visibilities and snow-covered roads.

As the surface low tracks northeast across Chicago into central
Lower MI on Friday, snow will continue to fly across north-central
and parts of central WI. The push of warmer air ahead of the storm
is expected to mix freezing rain with the snow across eastern and
the rest of central WI. The big problem remains the potential of a
dry slot that could change the precipitation into drizzle or
freezing drizzle and limit snow amounts. Enough accumulation of
either snow or ice justifies continuing the headline through the
day. Latest thinking is for an additional 2 to 4 inches of snow in
the warning area and perhaps a tenth or two of ice in the advisory
area. Blustery winds to also persist on Friday with north to
northeast winds at 15 to 25 mph and gusts again pushing 30 to 35
mph. Would not be surprised if there is a clap of thunder Friday
morning somewhere over WI on Friday due to steep mid-level lapse
rates.

.LONG TERM...Friday Night Through Thursday
Issued at 226 PM CST Thu Feb 23 2017

Large dry slot is likely to be across at least the southeast half
of the forecast area at the onset of the long-term part of the
forecast. Precipitation is likely to be primarily in the form of
drizzle or freezing drizzle within the dry slot. But mid-level
lapse rates within the dry slot are fairly steep, so any
convective elements embedded within the dry slot will probably
produce snow. Built forecast with the freezing drizzle gradually
shifting out of the area to the east overnight, being replaced by
all snow. Winds over central Wisconsin may be strong enough to
result in some blowing and drifting snow during the evening.
Winds will be stronger in the east, but the drizzle/freezing
drizzle in the dry slot will probably cut down on the
blowing/drifting potential. Opted to extend the advisory for the
east through to 12Z Saturday due to the mixed precipitation during
the evening, and the change back to snow later in the night.

The active pattern will continue for the rest of the forecast, but
models differed on the timing and locations of additional snow
changes. That`s pretty typical of the southwest flow regime such
as the one setting up across the area. Given that, did not make
any significant changes to the extended forecast initialization
grids based on a broad blend of model products.
&&

.AVIATION...for 06Z TAF Issuance
Issued at 1033 PM CST Thu Feb 23 2017

Snow and IFR ceilings and visibilities will arrive from southwest
to northeast tonight. The precipitation may change to sleet or
freezing rain at osh/mtw/grb/atw/sue/ocq/ezs between 12 and 18z
Friday. Northeast winds will gust to 30 knots at sue/mtw/atw/osh/grb.
Snow and drifting snow is likely across the entire region Friday
night.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 AM CST Saturday for WIZ031-
037>040-045-048>050.

Winter Storm Warning until 6 AM CST Saturday for WIZ030-035-036.

Winter Storm Warning from 3 AM Friday to 6 AM CST Saturday for
WIZ005-010>012-018-019.

Winter Weather Advisory from 3 AM Friday to 6 AM CST Saturday
for WIZ013-020>022-073-074.
&&

$$

SYNOPSIS.......Skowronski
SHORT TERM.....AK
LONG TERM......Skowronski
AVIATION.......RDM



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