Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
1102 am CST Monday Dec 9 2013
Aviation/18z tafs/...cold advection continues this afternoon with
low level thermal trough dominating into middle evening. Axis of vorticity
maximum extends into southern WI this evening. Cumulus field expected to be more
extensive this afternoon north of taf routes. Next more vigorous
wave arrives later tonight into Tuesday morning. Higher probability of precipitation will be in northern
County Warning Area. Best window for any precipitation will be 12z-18z. NAM and European model (ecmwf)
keep measurable precipitation north of routes. GFS a bit more robust into
all of southern WI. Will increase clouds later tonight into Tuesday but
keep snow out of tafs for now. Ceilings may approach MVFR levels. West
winds will ramp up once again Tuesday as trough shifts east.
Previous discussion... /issued 350 am CST Monday Dec 9 2013/
Today and tonight...forecast confidence is
Last bands of very light snow...with forcing from vorticity maximum and ahead
of cold front...will be moving through mainly eastern half of County Warning Area.
Some 2 mile visibilities still being reported as far west as Mineral Point
so will leave headline as is for now...though little to no
additional accumulation expected. May be able to drop west early but
early morning commuters will still be dealing with snow covered and
slippery roads until crews can get at them along with the briefly
reduced visibilities so Don/T want to get cute and drop headline.
Clearing comes quickly on heels of back edge of light snow.In spite
of some sunshine...cold advection drops temperatures after frontal
passage early this morning...then only rise a degree or two during
the midday hours before dropping off late afternoon and overnight.
West winds behind the front will be gusting up to between 20 and 25
miles per hour from middle morning to middle afternoon...with some blowing and
drifting of the light snow cover.
Next clipper-type short wave approaching from northwest. Potent
layer q-vector convergence maxes move across County Warning Area with the lower
levels responding to isentropic lift on the order of 5 to 6
mubars/sec. 30-50mb condensation pressure deficits move into the northwest
County Warning Area between 09z and 12z so will keep the slight chance probability of precipitation there.
Increasing cloud cover and surface winds not decoupling will prevent
lows from dropping below zero tonight...but will still see single
Tuesday...forecast confidence high.
The snow over northern Wisconsin that will be possibly clipping the
northern mkx forecast area will end by middle to late morning. The GFS
is slower with the upper level ridge moving in than the GFS.
At the surface...this clipper will pack a punch with regard to wind
on the back side. There will be a tight surface pressure gradient
between the clipper and high pressure over the plains. West winds
will ramp up to 20-25 miles per hour with gusts to 30 miles per hour in the afternoon.
Temperatures will fall quickly in the afternoon and evening with lows in
the single digits.
Tuesday night and Wednesday...forecast confidence medium to high.
Another shortwave trough will swing through the Midwest Tuesday night
into Wednesday. The vorticity advection will not be as strong as the Monday
night/Tuesday system...but this one will have strong 850mb warm air
advection...frontogenesis and upward motion.
We are looking at a fairly narrow band of high-intensity snow for a
2 to 3 hour period...dropping around 2 inches of snow.
There is still discrepancy between models about where the intense
snow band will line up. The NAM and GFS bring it primarily through
northern Illinois and the European model (ecmwf) is more broad but focuses quantitative precipitation forecast more on the
Wisconsin side of the border. Timing of this snow band could have
higher impact if the system slows down. Right now...it is forecast
to spread through southern WI from 09 to 12z Wednesday morning. Any later
and it will impact the morning rush hour.
Wednesday night through Thursday...forecast confidence medium to
Wednesday highs will occur right away during the morning. Then temperatures
will plummet through the day. Lows Wednesday night in the single digits
below zero are expected to occur around midnight when the surface
high pressure ridge is overhead.
Winds will then turn to the southwest Thursday morning and temperatures will
begin rising once again. Snow pack should keep temperatures a little cooler
than guidance...so expecting highs in the middle to upper teens. East
central WI could see a few flurries with a passing shortwave.
Friday through Sunday...forecast confidence medium.
Temperatures will moderate into the middle/upper 20s for the weekend. A series
of shortwaves moving through the upper Midwest could bring light
snow here and there.
MVFR visibilities and MVFR/IFR ceilings with last bands of light snow crossing
County Warning Area should be east of kmsn before 12z and the eastern taf sites by
13z. Will see clouds lift to VFR and become broken to scattered by
late morning. West winds will be gusting to 25 knots from middle-
morning to middle-afternoon then ease. Could cause some blowing and
drifting of new snow. VFR conditions for the remainder of the
No change to going Small Craft Advisory as tight pressure gradient
behind departing and deepening low will bring west wind gusts up to
criteria level by or shortly after 12z. Winds will ease for a time
tonight with weak ridging...but winds will come back up as the next
in the series of clipper-type systems cross the region. Have issued
a gale watch beginning at 8 am Tuesday until 9 PM Tuesday night with
forecast soundings indicating mixing up to 35 to 40 knot winds
between 1000 feet and 1500 feet above the surface. Winds remain at or
near small craft criteria through Wednesday evening.
Lm...gale watch from Tuesday morning through Tuesday evening for
Small Craft Advisory until 10 PM CST this evening for lmz643>646.
Today/tonight and aviation/marine...rem
Tuesday through Sunday...mrc