Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
946 PM CST Tuesday Dec 10 2013
Update...southern WI on northern fringe of passing layer q-vector
convergence area and weak short wave trough just to the south
overnight and early Wednesday. Lower visibilities and higher dbz in Iowa lines up
well with stronger isentropic Omega on 275 Theta surface which
remains mostly south of the County Warning Area. Stronger forcing just clips
southwest and south central County Warning Area after 06z-10z. May lower amounts
slightly but will stick with the 1-2 inch range in the south due to
high snow-liquid ratios expected. -Sn will end quickly from west to
east later tonight and early Wednesday.
Thickening cloud cover halted early evening temperature fall...so
adjusted a few areas upward for minimum temperature.
Aviation/06z tafs/...still expect -sn to affect taf sites between
06z and 12z...with a period of MVFR ceilings and possibly IFR visibilities.
Marine...will let Small Craft Advisory expire for southern 3 zones as weak
high pressure ridge passes by the area. Recent nearby vessels
also reporting lowering prevailing winds. Will keep advisory going for
northern zone as pressure gradient remains a little tighter in
this area. Blustery northwest winds return Wednesday morning
behind passing low pressure trough. Borderline returning to small
craft conditions in southern three zones but will hold off on
expanding advisory for now.
Previous discussion... /issued 409 PM CST Tuesday Dec 10 2013/
Tonight...forecast confidence high
The 500mb trough and surface low will quickly exit to the east this
evening. Another 500mb trough will dig into the upper Midwest
overnight. There are two waves of vorticity advection with it...one
across northern WI and one across Iowa and Illinois. The southern one will
have more moisture with it...precipitable water values of about 0.2
in southern WI.
850mb warm advection...Omega and frontogenesis will be prevalent in
the dendrite growth zone /dgz/ tonight. Expect a swath of snow to
spread from west to east across southern WI from around 04z through
13z. If current timing holds...it should begin snowing in Madison at
midnight and Milwaukee at 2 am. The models are still in good
agreement in the heavier quantitative precipitation forecast and stronger Omega within the dgz to be
centered over dvn and lot forecast areas. Therefore did not go with
Consensus has 0.04 to 0.08" of quantitative precipitation forecast brushing the south so less than
an inch accumulation north of Milwaukee and Madison and 1-2" south
of there. Expecting there to be a sharp cutoff with the heavy snow
band...but expecting a broad area of light snow that extends north
of Milwaukee/Madison due to the general vorticity advection and a
tilted fgen zone with 700mb frontogenesis northwest of the 850mb
High clouds are already beginning to stream into southern WI...so
temperatures are very tricky tonight. There will only be an hour or
two to radiate out before the clouds put a lid on the cooling and
then we start to warm with the warm air advection. Went with early evening lows in
the single digits.
Wednesday...forecast confidence high.
Snow will end from west to east early Wednesday morning...exiting
southeast WI by 15z. Snow is expected to be falling in southeast WI
during rush hour...although the intensity should be getting lighter.
High temperatures will occur right away in the morning and then they
will be falling all day long. Winds will be breezy out of the
northwest so expect wind chills in the teens below zero during the
Wednesday night through Thursday night...forecast confidence
A surface ridge will slide eastward across southern WI Wednesday nt. Skies will initially
be clear in the evening so expect a rapid drop below zero for temperatures
and wind chills. Will likely need a Wind Chill Advisory for wind chills
of 20 below to 25 below zero. Clouds to increase late as low to middle
level warm advection develops on the back side of a broad upper trough
that passes through. Forecast soundings indicate a period of near
saturation with the warm air advection...although lift is very weak. Went with
slight chances of light snow late night and into Wednesday am.
Southwesterly winds will then prevail on Thursday with 1000-500 mb thicknesses
rising to 528-534 dm and 850 mb temperatures to -6 c. Surface temperatures to
rebound into the middle to upper teens. Light and variable winds for
Thursday nt although MO cloudy skies are expected with weak warm
advection continuing aloft.
Friday through Tuesday...forecast confidence medium
From Friday into Saturday night...a west-east confluent zone will set
up in the vicinity of southern WI and surrounding areas as a shortwave
trough within the southern stream lifts northeastward into the Ohio River valley
and a shortwave trough in the northern stream moves into the upper MS
River Valley. Middle level frontogenesis and positive vorticity advection will bring light snow
to the region. Higher intensity snow may occur if the southern system
moves farther north. There will be some higher intensity snow in
eastern WI due to lake enhancement on Ely boundary layer flow and lake
to 850 mb temperature differences of 14-17 c. The cold temperatures through the
depth of the troposphere will continue to support high snow
ratios. Thus several inches of snow is expected over eastern WI with a
few inches over south central WI. Brisk northwesterly winds and cold
advection to develop Sat nt into sun with light snow possibly
lingering into sun. High pressure to then prevail sun nt with
Arctic air in place. Snow chances to return Monday-Tuesday as an
Alberta clipper moves through the region.
Aviation/00z tafs/...coming soon.
VFR and diminishing winds this evening. The next round of snow will
spread into Madison around midnight and Milwaukee around 2 am. Up to
one inch is expected by the end time of 6 am at msn and 8 am at mke
if the current forecast holds up. The highest snowfall amounts and
snowfall rates are expected to remain south of the WI/Illinois border.
IFR and below alternate minimum level visibility is expected during
the peak snow early Wednesday morning. Fuel alt ceilings expected to
persist a little longer after the snow falls...then scattering out
during the afternoon.
Winds will gradually diminish this evening as a high pressure ridge
briefly moves through. Downgraded the Gale Warning to Small Craft
Advisory through the evening. Extended the Sheboygan Small Craft Advisory
until Wednesday evening to account for higher wind gusts overnight tonight
and gusty northwest winds Wednesday behind the next low pressure system.
With cold temperatures some freezing spray is expected especially
toward open waters.
Lm...Small Craft Advisory until 10 PM CST Wednesday for lmz643.