Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 152358

Area Forecast Discussion
National Weather Service Green Bay WI
558 PM CST Mon Jan 15 2018

Updated aviation portion for 00Z TAF issuance
and new information added to update section

Issued at 543 PM CST Mon Jan 15 2018

Much of the WW.Y is scheduled to expire at 00Z, and will allow
that portion to do so. Steadier snows will still be exiting
Wood/Portage/Waushara, but it doesn`t look like there will be
enough left to justify an extension.

Although some scattered snow showers and flurries will linger
across the area, this is transitioning to primarily a lake-
effect event. Winds over Lake Michigan in the 925-850 mb layer
have been backing NE, and will back a little further before
settling in for the rest of the night. Although some ice coverage
exists on the northern portion of the lake, it`s probably not
enough to affect lake snow development much. The transition from a
more onshore flow to a more shore-parallel flow will favor the
current transverse bands/scattered snow showers re-organizing into
a more substantial shore-parallel band. There is also some
indication that a double land-breeze circulation will form over
the lake, which would also favor this evolution. This may lead to
another period of heavier snows along the lakeshore, especially
from Kewaunee southward. But the band is unlikely to become as
strong or as organized as the one last night, and will probably
shift down the lakeshore and not linger in any given area too
long. Will maintain the mention of another couple inches of snow
along the lakeshore based on this scenario.

Winds will also gradually become more favorable for lake-effect in
the Lake Superior snowbelt.

Updated product suite will be out ASAP.

.SHORT TERM...Tonight and Tuesday
Issued at 230 PM CST Mon Jan 15 2018

The main forecast focus to be on lake effect potential from both
Lakes Michigan and Superior as winds gradually back from east to
north tonight, then north to west on Tuesday.

The 19z MSAS surface analysis showed a weak area of low pressure
situated over southeast WI. There were two troughs that extended
from this surface low. The first trough extended from the low
pressure west-northwest into west-central WI. The second trough
extended northward into north-central WI. The highest
concentration of snow was associated with each of these troughs
with visibilities typically in the 1-2SM range.

Winds are expected to gradually back from east-northeast this
evening to north-northeast by daybreak as the low pressure tracks
toward southern Lake Huron. Expect light snow to continue across
most of northeast WI this evening until the north-south surface
trough rotates southward into southern WI. The most concentrated
area of snow will remain near Lake MI as lake effect brings snow
bands from a northeast to southwest direction. Additional
accumulations well inland should be relatively minor (one-half
inch or less), however areas closer to Lake MI could see an
additional 1-3" with the higher amounts along the lake. Even
though light snow may still be falling over north-central/central
WI into early evening, will allow the winter weather advisory for
these locations to expire at 6 pm CST. On a different note, as
winds shift to the north later tonight over Lake Superior, this
will bring lake effect snow showers into north-central WI with
possible accumulations of 1-2" over parts of Vilas County.

The lake effect from Lake MI will linger into Tuesday morning
before winds turn northwest and keep snow bands offshore. The lake
effect into north-central WI will also linger into Tuesday morning
before winds turn to the west and keep snow bands in Upper MI. Any
additional accumulations will be relatively minor (around one-half
inch near Lake MI/one-half to one inch northwest corner of Vilas
County). For the rest of northeast WI, clouds are expected to
linger for most of the day, before some breaks occur later in the
afternoon as drier air arrives from high pressure situated over
the central CONUS. Max temperatures for Tuesday to range from 15
to 20 degrees north, 20-25 degrees south.

.LONG TERM...Tuesday Night Through Monday
Issued at 230 PM CST Mon Jan 15 2018

Dry conditions with moderating temperatures expected for the
remainder of the week. A couple of precipitation chances return
late in the week and more likely late next weekend.

A cold night is in storm for Tuesday night with Sub zero overnight
low temperatures expected. Winds at this time appear too light to
place wind chill values in advisory levels. Moderating temperatures
are then expected for the remainder of the week as the western
upper ridge nudges into the mid section of the country. Increasing
pressure gradient with the warm air return Wednesday into
Wednesday night producing 15 to 25 mph winds. These winds may
combine with the new fluffy type of lake effect snows to produce
pockets of drifting snow over rural open areas along the
lakeshore counties.

Medium range progs are in good agreement with a northern stream
short wave passing over the Northern Great Lakes region later
Thursday into Thursday night. Anticipate precip chances will be on
the increase across the north later this week if progs remain
consistent with this feature.

Upper heights continue to fall and turn more zonal Friday into
Saturday as the western upper trough develops.  While another
piece of energy in the northern stream glances by to the north
around Saturday, attention turns to the southern stream as progs
deepen a trough toward the latter half of the weekend. This
system appears to have a jet couplet structure for large scale
lift. Morning GFS track this system over central Illinois Sunday
into Monday, focusing the heavy precipitation over the southeast
third of the state. Snow would be the precipitation type for east
central areas. Meanwhile, The ECMWF track is more northwest of the
GFS but has trended southward from the previous run. The Canadian
was a compromise of the GFS and ECMWF.

.AVIATION...for 00Z TAF Issuance
Issued at 543 PM CST Mon Jan 15 2018

Ceilings will remain mixed, primarily between 1500 and 3500 ft
this evening. Though scattered snow showers will occasionally
restrict visibilities, most areas should see visibilities recover
to 5-6SM. The exception will be the east, where lake-effect snow
showers will continue into early Tuesday.

Winter Weather Advisory until midnight CST tonight for WIZ038-

Winter Storm Warning until 6 AM CST Tuesday for WIZ022-040-050.


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