Area Forecast Discussion
Issued by NWS Green Bay, WI

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000
FXUS63 KGRB 222342
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
542 PM CST Thu Feb 22 2018

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Friday
Issued at 231 PM CST Thu Feb 22 2018

A PV anomaly, currently visible over Colorado on water vapor
imagery, will lift northeast towards the western Great Lakes
region during the overnight hours. Although RAOB and METAR
observations currently show abundant dry air in low levels,
moisture advection from the south will moisten the low levels
through the afternoon and evening hours, allowing for
precipitation to eventually develop across the area this evening.
Once the low levels become saturated, there will be abundant
isentropic lift across the region tonight ahead of the shortwave,
with strong winds perpendicular to the isentropic surface in a
deep 280K to 290K layer. The PV anomaly will exit the region to
the northeast Friday morning, ending much of the precipitation
across the area. Due to the strong lift and moisture influx, this
system looks to squeeze about 0.20 to 0.30 inches of qpf across
northeast Wisconsin.

An influx of warm air in the mid and low levels will cause
precipitation type concerns as snow transitions to a wintry mix
from south to north across central and east-central Wisconsin this
evening, then across north-central Friday morning. Precipitation
is expected to transition to mainly rain late tonight into early
Friday morning across east-central Wisconsin as surface
temperatures warm to above freezing. Snowfall totals will range
from 3 to 5 inches across north-central Wisconsin where the
precipitation type is expected to mainly be snow, with 1 to 3
inches across much of central Wisconsin, and an inch or less
across east-central Wisconsin. There could also be a few
hundredths of ice accumulation across central and east- central
Wisconsin with sleet and freezing rain. Given the different
precipitation types across the east and the snow across the north
will continue the Winter Weather Advisory for the entire area
tonight into Friday morning as travel could become hazardous
during the morning commute on Friday morning.

.LONG TERM...Friday Night Through Thursday
Issued at 231 PM CST Thu Feb 22 2018

In general, the mean flow initially to consist of a Pacific upper
ridge, a western CONUS upper trough and a Gulf of Mexico to
western Atlantic upper high. The main forecast issue will be
timing of individual pieces of energy ejecting northeast out of
the upper trough. One of the bigger pieces of energy to impact
northeast WI this weekend with snow/wintry mix. Toward the middle
of next week, most of the models close off an upper low in the
vicinity of CA with a split flow developing across the CONUS. A
weak system could bring a chance of snow or a rain/snow mix
Tuesday night and a rain/snow mix or rain on Wednesday.
Temperatures under this pattern should remain above normal through
next Thursday.

High pressure is expected to move across the Great Lakes Friday
night, bringing mostly clear to partly cloudy skies, light winds
and temperatures close to seasonal levels. Look for min
temperatures to range from 10 to 15 degrees north-central WI, to
the middle 20s along Lake MI. Attention then turns to the Plains
where a large piece of energy is forecast to eject northeast from
the western CONUS upper trough. As this occurs, a surface low is
progged to strengthen and move into the Midwest by 00z Sunday.
Look for clouds to steadily increase over northeast WI with the
leading edge of precipitation approaching central WI by late
Saturday afternoon. Depending on temperatures, any precipitation
would be either be either all rain or a rain/snow mix. Max
temperatures to range from the middle 30s north, middle 30s to
around 40 degrees south.

There continues to be some model issues with both the track and
intensity of this surface low headed into Saturday night. The NAM
and GFS both favor a track across western WI which would keep the
heavier snows to our west and bring milder air into eastern WI.
This milder air will create all kinds of precipitation type
problems, including freezing rain potential. In addition to type
issues, a dry slot could rotate through parts of central/east-
central WI after midnight and turn the precipitation to drizzle or
freezing drizzle. The ECMWF takes the surface low through central
WI, while the GEM favors eastern WI. Obviously, a farther east
track would bring more snow/less mixed precipitation into the
forecast. Too much uncertainty for any headlines, but will
continue to hit the potential in the HWO. Min temperatures to
range from the middle 20s north-central, to the lower 30s east-
central WI. The surface low continues to quickly track northeast
into eastern Ontario on Sunday, however will need to carry a
chance pop through Sunday morning for most of the forecast area as
the system to still be in the process of exiting the region.
Precipitation type by this time should be all snow as CAA
overspreads WI. There could be a minor accumulation across
northern WI (less than 1"), but for the most part, improving
conditions are expected by Sunday afternoon with even some
sunshine over central/east-central WI. Max temperatures to range
from the lower 30s north-central, middle to upper 30s elsewhere.

A weak surface ridge slides into the western Great Lakes Sunday
night with mostly clear to partly cloudy skies, west winds
diminishing to 5 to 10 mph and cool conditions. This ridge moves
to our east on Monday with winds backing to the southwest and WAA
starting up. The air mass to be too drop to support precipitation
and have removed the slight chance pops from the previous
forecast. Have raised temperatures a bit as a result with readings
in the middle 30s north-central, upper 30s to around 40 degrees
for the rest of the forecast area. A weak system passing north of
Lake Superior will drag a cold front across most of northeast WI
later Monday night. The atmosphere still looks too dry to support
precipitation, so prefer to keep Monday night dry. Some of the
models do eventually saturate the atmosphere enough to bring a
small chance of precipitation to parts of northeast WI by Tuesday
afternoon. For now, look for clouds to increase on Tuesday and
will mention a small pop in the forecast. Max temperatures for
Tuesday to range from the middle to upper 30s north-central,
lower to middle 40s for parts of central WI.

The aforementioned cold front is now forecast to not sag as far
south as was advertised 24 hours ago, now stalling somewhere near
the WI/IL border Tuesday night. Cannot rule out a chance of mainly
light snow (perhaps a mix early in the evening) generally along/
south of Highway 29. Models go their separate ways by Wednesday as
the GFS pretty much pulls the upper trough eastward into the
Plains and spins up a strong surface low over the central Plains.
The GEM keeps a closed upper low near AZ with no surface low over
the Plains, while the ECMWF keeps a broad upper low over the
southwest CONUS and a weak surface low reflection over the central
Plains. Such a variance in solutions makes for a low confidence
forecast and will use the consensus solution that a slight chance
of a rain/snow mix north, slight chance of rain south.

Model inconsistencies are very apparent Wednesday night into
Thursday, especially with the GFS advertising a major system
impacting northeast WI versus nothing comparable from the other
models. Way too many unknowns at this time to justify going with
big pops. Will simply go with low chance pops for Wednesday night
into Thursday and allow the models to get into better agreement.
&&

.AVIATION...for 00Z TAF Issuance
Issued at 540 PM CST Thu Feb 22 2018

VFR conditions will probably hold for a short time early this
evening, then a rapid deterioration to low-end MVFR or IFR
conditions is expected as a band of mixed precipitation moves into
the area from the southwest. LLWS will also develop. Precipitation
will diminsh and end Friday morning, but low ceilings will
probably linger well into the afternoon.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 9 PM this evening to 9 AM CST
Friday for WIZ005-010>013-018>022-038>040-048>050-073-074.

Winter Weather Advisory until 9 AM CST Friday for WIZ030-031-
035>037-045.
&&

$$

SHORT TERM.....Kurimski
LONG TERM......Kallas
AVIATION.......Skowronski



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