Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Milwaukee/Sullivan WI
852 PM CST Thursday Nov 26 2015
Area of moderate rain...with pockets of heavy rain...continue to
shift northeastward through the forecast area. This is being
helped along by strong low level frontogenesis and upper
divergence from the right rear quadrant of a jet streak. Mesoscale
models continue to show the rain affecting the area late this
evening...then gradually ending from west to east across the area
later tonight into early Friday morning.
As the precipitation winds down...cold air advection will push
into the area and may bring a mix of light snow and light sleet
for an hour or two before ending. Any accumulations would be very
light and on grassy areas...as pavement temperatures are above
freezing. Not expecting much of an impact from this...as air
column is drying out as it cools down.
Rainfall amounts so far are generally from 0.50 to 1.25
inches...the highest from Southeast Rock into southeast Waukesha
and southern Milwaukee counties. Should see some rivers
rise to bankfull and perhaps minor flood stage...especially the
Fox and Turtle Creek basins.
Expect moderate rain...with ceilings and visibilities fluctuating
between IFR and MVFR categories...for most of the rest of the
night. There may be a mix of light snow and light sleet between
09z and 12z Friday at Madison...but not expecting any
accumulations on the warm runways. Rain should end at the eastern
sites by 15z Friday.
Winds will veer to the north and become gusty later tonight into
Friday evening...especially at the eastern taf sites. Ceilings and
visibilities will improve to VFR category by the middle to late
morning hours from west to east across taf sites...with clouds
mixing out gradually Friday night.
Small Craft Advisory continues into Friday night across the
nearshore waters of Lake Michigan. Gusty south to southwest winds
will quickly veer north over the next few hours. These winds will
quickly become gusty with a tightening pressure gradient. Gusts to
30 knots are expected later tonight and Friday...before gradually
weakening Friday night.
Gale force gusts are not out of the question during this
time...but think most of these will remain over the open waters.
High waves are expected into Friday evening...then subsiding from
north to south by later in the night.
Previous discussion... /issued 330 PM CST Thursday Nov 26 2015/
Tonight and Friday...forecast confidence...high
Widespread rainfall will continue into this evening...with precipitation
winding down northwest to southeast later tonight into Friday
morning. Still looks like there will be a brief changeover to snow
for a time...possibly mixed with sleet...as colder/drier air works
into the area. Little to no accumulation is expected.
Should see precipitation exit the southeast by Friday afternoon...with some
decrease in clouds possible in the northwest half of the forecast
area. Temperatures will be below normal tomorrow behind the departing cold
Friday night and Saturday...forecast confidence high.
The entrance region of the 250 mb upper jet over the Great Lakes
Friday evening weakens as the jet sags south with the approach of
another jet associated with an upper low over the western U.S.
The upper divergence is mainly to the south of the forecast area
Friday night. Mainly neutral or very weak 700 mb upward motion.
The 850 to 700 mb relative humidity is low. There is weak northeast winds at 850 mb
Friday evening that turn north northwest after midnight.
A favorable northeast wind will allow for a long fetch over Lake
Michigan and lead to some lake effect clouds by early Saturday
morning in far southeast WI. Lake temperatures are around 45f/7c
near southeast WI... and 850mb temperatures are expected to drop to
around -3c with even colder 925 mb temperatures. Delta-T values look a
little low for lake effect but not by much. At a minimum... there
will be some lake clouds over the area. Both the NAM and GFS are
dry. Forecast soundings show a strong low level inversion near 3
thousand feet with a strong dry level above the inversion. Therefore
only a few flurries would be possible with any lake effect that
Saturday night and Sunday...forecast confidence high.
Brief upper level ridging occurs over southern Wisconsin. Weak
upper level convergence occurs with mainly 700 mb downward
motion...except for a few bands of weak upward motion pushing
north through the forecast area. 700 mb and especially 850 mb
layers are dry.
Surface high pressure is centered over the upper Mississippi
Valley and Great Lakes.
Sunday night and Monday...forecast confidence high.
The upper ridge pushes east to the Great Lakes Monday as a
closed upper low over the western states drifts into the Central
Plains Monday. Upper level divergence begins late Sunday night and
continues Monday. 700 mb upward motion is weak until Monday
afternoon when it increases...especially far south. 700 mb layers
are dry until rapidly saturating Monday afternoon. 850 mb layers
remain dry but saturate right up to the Wisconsin border across
The GFS does bring precipitation into much of the forecast area
late Monday afternoon. Thickness and thermal profiles would
Monday night and Tuesday...forecast confidence medium.
The upper low should cross Wisconsin Tuesday and bring another
round of precipitation Monday night...then diminishing from the
south Tuesday. Colder air will wrap into southern WI from west to
east Monday night. The 12z European model (ecmwf) is slower with the cooling.
Therefore the transition to a mix of snow is questionable Monday
night...but by Tuesday the upper levels are cold enough for
snow...but the boundary layer is still rather warm...even on the
GFS. So southern WI could see some light deformation area snow or
rain snow mix Tuesday afternoon and evening... but the majority of
the snow will fall in central and northern Wisconsin.
Wednesday and Thursday... forecast confidence medium.
The GFS is quicker in moving off the upper low...while the 12z
European model (ecmwf) and 06z dgex still has it over lower Michigan Wednesday
Ridging Thursday will lead to dry weather and seasonal
Widespread rainfall will continue into tonight...with precipitation winding
down later tonight into Friday morning from northwest to southeast.
Will likely see a brief transition to a wintry mix as the precipitation
winds down and colder air moves in behind the departing cold front.
This mix should have little or no impact given how warm it will be
into this evening.
Ceilings continue to slowly lower this afternoon...with low ceilings
expected overnight. Will probably see slow improvement Friday as
colder/drier air works into the area...with a possible return to VFR
by later in the afternoon.
Visibilities will probably go down within heavier rainfall.
Otherwise...not expecting much reduced visibility...as threat for
fog looks like less than previously thought given how fast snow
Extended the Small Craft Advisory for the southern zones through
much of Friday night...as winds/waves will pick back up behind the
cold front. Issued one for the north beginning this evening and
going through Friday evening...as waves will wind down first in the
north under the northerly flow.
Lm...Small Craft Advisory until 4 am CST Saturday for lmz645-646.
Small Craft Advisory until midnight CST Friday night for lmz643-
tonight/Friday and aviation/marine...ddv
Friday night through Thursday...hentz