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Area forecast discussion
National Weather Service Green Bay WI
239 PM CST Fri Dec 13 2019

Forecast discussion for routine afternoon forecast issuance

Short term...tonight and Saturday
issued at 239 PM CST Fri Dec 13 2019

Main forecast concerns revolve around period of snow showers late
this afternoon then the freezing drizzle threat later this
evening and overnight.

Well defined band of snow showers continues its trek across Minnesota
and western WI this afternoon, ahead of an approaching shortwave
trough. Expect this band to push across the area, slowly weakening
as it reaches eastern WI. Quick half inch to an inch of snow is
expected, mainly across central and northern WI, where a brief
burst of moderate snow is likely. Lower amounts expected as you
head east. Some indication this band could mix with a little
freezing drizzle as mid-level level dry air could limit ice
crystals in the cloud. But thinking the dominate precip type will
be snow. That changes as the evening progresses, as models
soundings showing a pretty strong freezing drizzle signal.
Soundings showing 2000+ meter saturated layer depth with rhs over
90% and temps in this layer between -5c and -10c, which is very
favorable for freezing drizzle. Observations in our area and
upstream already showing reduced visibilities and low cloud
decks, so don't think low level moisture will be an issue. One
piece that is missing for a slam dunk case would be a east to
southeast upsloping flow. Temps will be flirting with -10c as you
work toward the Michigan border, so more of a light snow / freezing
drizzle mix is expected over northern WI. Since confidence is
increasing that much of the area will see some freezing drizzle
tonight, beefed up the chances and wording in the severe weather potential statement and Special Weather Statement.
Clouds and precip will hold up temps tonight, with most spots
dropping into the teens (central and northern wi) to Lower/Middle
20s across the east.

For Saturday, lingering snow shower, flurry, and freezing drizzle
activity is expected, as another weak shortwave and cold front will
provide weak forcing. Snow shower activity will be most widespread
across far north central WI, especially Vilas co., Where lake
effect activity will begin as winds turn northwest in the
afternoon. Additional accumulations of an inch or two are likely.
Cold air advection will be taking place through the day, with
temps struggling to get out of the 20s.

Long term...Saturday night through Friday
issued at 239 PM CST Fri Dec 13 2019

A near zonal flow across the Continental U.S. This weekend will gradually
become more amplified by the middle of next week as a closed upper
low hits the California coast and a downstream upper ridge pushes across
the central Continental U.S.. it still appears that a system will pass to our
south on Monday, leaving warm air advection ahead of a clipper system around
Wednesday to bring northeast WI any chance of precipitation. Thus,
precipitation trends look below normal with temperatures below
normal through mid-week, then readings edge closer to normal late
week.

Lake effect snow showers are expected to continue across north-
central WI into Saturday evening before gradually diminishing
overnight as high pressure builds into the western Great Lakes
region. Parts of Vilas County could pick up another inch of
accumulation with lesser amounts farther south and east.
Otherwise, look for skies to become partly cloudy after midnight
across central/east-central WI as drier air advects into WI. Min
temperatures to drop to around 5 above zero over central WI, 10 to
15 degrees eastern WI. The high pressure to spread across WI on
Sunday, bringing mostly to partly sunny skies to the forecast
area. Max temperatures to range from 10 to 15 above central WI,
upper teens to lower 20s eastern WI.

This high pressure is forecast to stretch from the northern plains
through the Great Lakes on Sunday night and help to suppress a
system to our south (movement from the Southern Plains to the Ohio
valley). The northern edge of the main precipitation shield to
only reach the WI/Illinois border, thus no impacts from this system
other than some increase in clouds. The amount of cloud cover to
be key to min temperatures as any prolonged clear skies would
allow temperatures to tank. For now, have readings from zero to 5
below north-central WI, to the lower teens near Lake Michigan. Monday is
shaping up to be a mostly cloudy day as the system continues to
track up the Ohio Valley and weak high pressure remains over WI.
Max temperatures to range from the middle teens north-central WI,
to the lower 20s east-central WI.

Clouds could briefly break Monday evening, however the approach of
a cold front and associated shortwave trough should begin to
increase the clouds once again later Monday night. This cold
front/shortwave trough sweep southeast into the Great Lakes on
Tuesday, however moisture appears to be very limited. Thus, we may
only see some flurries and a wind shift to mark the passage of
the cold front. The GFS is more impressive with the cold air advection/gusty
winds by Tuesday afternoon compared to the ECMWF/CMC. Either way,
Tuesday will be a cold day with Max temperatures only around 10
above zero north-central WI, to the middle teens eastern WI.

A modest surface ridge axis is forecast to move into WI Tuesday
night and if clouds can stay long enough, along with projected
light winds and a cold air mass aloft, temperatures could again
easily tank into the double digits below zero across northern/
central WI. However, models hint that mid/high clouds could arrive
as warm air advection begins to return to the area, so have kept those areas in
the single digits below zero. This warm air advection will combine with a clipper
system well to our north, to bring a chance for light snow to most
of northeast WI on Wednesday. Moisture again appears limited, thus
do not anticipate much in the way of accumulations. Max
temperatures to range from 10 to 15 above north, middle teens to
around 20 degrees south. The clipper is expected to dive southeast
to roughly Eastern Lake Superior Wednesday night and reach the
eastern Great Lakes on Thursday. At this point, just about all of
the associated precipitation would occur to our north and east, so
have kept a dry forecast intact. Max temperatures on Thursday to
range from the lower 20s north, middle to upper 20s south.

Models begin to diverge by the latter part of next week, mainly
with the handling of a shortwave trough to move across the
southern Continental U.S.. the GFS is much faster and opens up the Gulf to
pull moisture north all the way into WI by next Friday. Other
models are slower with this trough, allowing a surface ridge axis
to keep northeast WI dry through Friday. Knowing the propensity
for the GFS to be too quick moving troughs across the conus,
prefer to keep Friday dry with Max temperatures in the middle 20s
north, upper 20s to lower 30s south.
&&

Aviation...for 18z taf issuance
issued at 1200 PM CST Fri Dec 13 2019

Low clouds, a period of snow showers, and periods of freezing
drizzle will bring overall poor flying conditions to the area,
with mainly MVFR and IFR (possibly lifr) conditions expected this
afternoon into Saturday. A brief period of VFR conditions will be
possible through early afternoon, but any breaks will fill back
in with low clouds as an upper level disturbance approaches from
the west. This disturbance will bring a round of snow showers,
along with some patchy freezing drizzle, through early evening.
Brief periods of lower ceilings/vsbys are expected, especially at
kauw/kcwa/krhi. As forcing weakens and moisture decreases in the
mid-levels this evening, plenty of lingering low level moisture
will lead to areas of freezing drizzle through early Saturday
morning. Some questions remain on just how widespread the freezing
drizzle will be, but confidence increasing a period of freezing
drizzle will occur.
&&

Grb watches/warnings/advisories...
none.
&&

$$

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