Area Forecast Discussion
Issued by NWS Green Bay, WI

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362
FXUS63 KGRB 181746
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1146 AM CST Sun Feb 18 2018

Updated aviation portion for 18Z TAF issuance

.SHORT TERM...Today...Tonight...and Monday
Issued at 406 AM CST Sun Feb 18 2018

There are many forecast concerns, including a northward shift in
precipitation and snowfall potential today, precipitation type
and snowfall/ice accumulations on Monday, and winter weather
headlines.

Dry air was advecting into the forecast area early this morning,
with dew points in the single digits over north central WI.
Given this fact, it appears that the drier ECMWF/NAM solutions of
the past couple days had the right idea, and the latest models
have all trended drier and farther north. Have backed off on pops,
QPF and snowfall amounts over northern WI, and now only expect an
inch or two of snow over Vilas county, and little accumulation
farther south. This is expected despite the fact that a 50-60 knot
LLJ and strong WAA will be occurring. Have increase winds, and
expect most locations to see gusts in the 30-35 mph range during
the afternoon. Temperatures should range from around 30 far north
to 40 in the typical central WI warm spots.

A cold front will work its way southeast through the forecast
area overnight, and should be near GRB by daybreak. Post-frontal
precipitation is only expected over our far northwest counties,
with perhaps another dusting to an inch of snow accumulation and
spotty light freezing rain/drizzle. Lows will range from the
teens northwest to the lower 30s southeast.

On Monday, the cold front will continue to shove south of the
region, with gusty northeast winds drawing colder air into the
the forecast area. Widespread precipitation will form as 850 mb
WAA and mid-level frontogenetic forcing increase. The strongest
forcing is expected to move through during the middle of the day.
Models disagree on the depth of saturation that will occur, which
will make a huge difference between snow vs freezing rain in parts
of the area. Right now, think that freezing rain will be most
likely over the entire area early and late in the day, with a
period of moderate snow over the northwest half of the forecast
area during the middle of the day, as the stronger forcing and
deeper saturation moves through. The strengthening warm layer over
our southeast counties should keep precipitation as a mix of
freezing rain, sleet and rain there. Expect roughly 2-4 inches of
snow over northern WI, with a tenth to two-tenths of an inch of
ice south.

After coordination with the long-term forecaster and surrounding
offices, we have decided to issue a winter storm watch for
ice accumulations of a quarter to half inch over central,
northeast and east central WI from Monday through Tuesday.
Advisories will eventually be needed farther north, but will hold
off on those and just issue a Special Weather Statement for that
area.

.LONG TERM...Monday Night Through Saturday
Issued at 406 AM CST Sun Feb 18 2018

Focus in this part of the forecast continues to revolve around the
impacts from wintry weather expected on Monday night into Tuesday
night.  Looking at the latest model solutions, the ecmwf remains a
little further north than the gfs in regards to frontal positions
and low level thermal fields during this period.  Have a slight
preference for the ecmwf over the gfs.

Monday night through Tuesday night...A cold front will remain nearly
stationary on Monday night and Tuesday, stretching from Lake Huron
to northwest Illinois.  As one wave of low pressure travels from the
IL/WI border to the northern Lower Peninsula of Michigan, mid-level
drying is anticipated to occur early Monday evening from southwest
to northeast.  Progged soundings indicate that even though
saturation will extend from the ground to above 700mb, saturation
will struggle to reach above -10C for most locations, which would
indicate more of a freezing drizzle threat than anything else. The
latest model trend is to bring a stronger low pressure system along
the front on Tuesday.  Though models have some timing differences
with this secondary low, it will likely bring deeper saturation back
into parts of central and most of northeast WI.  It may also help
temps warm above freezing along the lakeshore, and possibly the Fox
Valley too, for a time during the afternoon. But with most of the
area northwest of the Fox Valley below freezing, combined with a
warm nose remaining between 850-700mb, increased ice accumulation
amounts for both morning and afternoon. It does not appear that this
more widespread precip will reach north-central WI, where a colder
thermal profile would be more conducive for snow accumulations.
Following this secondary low, models finally accelerate the front
eastward, so precip should end from west to east on Tuesday night.
Adding up amounts during this time, now getting ice amounts of up to
a quarter inch over central to east-central WI during the Monday
night and Tuesday period.

Rest of the forecast...Arctic high pressure will build into the area
for Wednesday and Thursday.  The ecmwf tries to bring the back edge
of a precip shield to northeast WI on late Wednesday night, but does
not have support from other guidance, so will ignore it for now.
Models then have timing differences for the next system to impact
the area on next Friday-Saturday.  This system may also bring some
warm air with it, but will see how the track/timing shapes out.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1139 AM CST Sun Feb 18 2018

Complex situation developing later tonight into Monday with
respect to mixed precipitation types. For the far north, it
should be a mix of snow and sleet, for locations just north and
west of the Fox Valley, the precipitation will start as a mix of
freezing rain, sleet and snow and then transitions over mainly
freezing rain later Monday afternoon into Tuesday. The situation
is even more complex across the Fox Valley east to the lake due to
questions with respect to boundary layer temperatures. Frontal
boundary is expected to drop southward across the area later
tonight. Increasing northeast winds expected later tomorrow
morning. In these types of situations, colder air funnels down the
bay quicker than expected. This could cause temperatures at or
slightly above freezing tomorrow morning at KGRB could fall below
freezing during the afternoon. Farther to the south at
KATW/KMTW/KOSH, temperatures could be several degrees above
freezing by late morning, then fall later in the afternoon. Better
icing potential in this area will be Monday night into Tuesday.
Otherwise, IFR cigs arrive across the north later tonight and push
southward into central and northeast Wisconsin on Monday.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Monday morning through Tuesday afternoon
for WIZ020>022-030-031-035>040-045-048>050-073-074.
&&

$$

SHORT TERM.....Kieckbusch
LONG TERM......MPC
AVIATION.......Eckberg



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