Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
347 am CST Friday Dec 6 2013
Today and tonight...forecast confidence high.
The main 250 mb trough over the western U.S. With a 170 knot jet
maximum moves across Wisconsin today...reaching the eastern Great
Lakes this afternoon as the trough axis begins to flatten and
lifts northeast. Most of the upper divergence and 700 mb upward
motion stays south of the forecast area today and tonight.
700 mb layers are very dry...with some increase in relative humidity along the
Illinois border today and early this evening...with drying as the
upper jet maximum lifts northeast. 850 mb relative humidity is also dry. Therefore
the clouds will be jet related and be thicker to the southeast.
Main concern will be the cold temperatures with just enough wind to
make wind chills come in to play. Wind chills should be in the
single digits today and around 10 below tonight...with some 15
below readings over south central and northern parts of
Saturday through Saturday night...forecast confidence high
A broad upper trough will keep the cold air stationed over the upper
Midwest. The surface high will move overhead on Saturday so west to
northwest winds will finally diminish. As the high slides east
Saturday night...the middle to upper level clouds will increase as warm
advection begins ahead of an approaching longwave trough. High temperatures
will be in the teens once again Saturday. Then they will fall
quickly Sat night but level off as the clouds move in...so lows
should bottom out in the upper single digits rather than colder.
Sunday...forecast confidence medium to high.
Light snow will spread into southern WI from the west on Sunday as
an upper level trough moves into the Great Lakes region. The system
will be saturating from the top down...so a little uncertain about
how quickly the snow will begin to reach the ground. The best chance
for snow will be after 18z inland.
Near the lake...ongoing southeast to easterly 1000-850mb flow will
lead to low levels saturating earlier. However there is debate about
where the snow will begin first...near Kenosha/Milwaukee or
Sheboygan. Expecting lake enhancement to play a role in snow totals.
Highest snowfall rates expected Sunday evening as the low undergoes
cyclogenesis over lower Michigan and southern WI lands roughly on
the southern edge of the deformation zone with 850-700mb
frontogenesis and upward motion in the vicinity.
BUFKIT model soundings and time-height series show a deep dendrite
growth zone /dgz/...nearly 200mb or 8000ft. Omega values are not too
impressive...maximum around -6 at the top of the dgz. Went with
consensus of models for quantitative precipitation forecast but added a little near the lake... and
a blend of the NAM and GFS for snowfall ratios which is around 18 to
19:1. Storm total snowfall amounts seem to be around 3 to 4 inches
inland with around 5 near Sheboygan. The 06z NAM came in with higher
quantitative precipitation forecast/snow amounts than the previous 00z NAM and GFS runs and will
have to wait to see if this is an outlier or a new trend. Will
continue to mention 3 to 6 inches with highest amounts near
Monday through Thursday...forecast confidence medium
Dry air will quickly move in behind the system so expect precipitation to
taper off quickly after midnight Sun night. Cut probability of precipitation out almost
completely after 12z Monday. Arctic air will funnel in on brisk
northwest winds and we will get into the coldest temperatures of the season
At this time...Wednesday seems like it will be the coldest but that has
been wavering over the past several model runs.
Few clouds around 2 thousand feet mainly in the Wisconsin River Valley
area. Otherwise VFR through the period with mainly jet related
cirrus today and this evening.
Small Craft Advisory conditions expected to subside by sunrise
and will let the Small Craft Advisory expire.
Lm...Small Craft Advisory until 6 am CST early this morning for
Today/tonight and aviation/marine...hentz
Saturday through Thursday...mrc