Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
849 PM CST Friday Dec 6 2013
It/S a quiet night out there. Temperatures are cold...but they are
behaving themselves as a light wind keeps things stirred up.
Without that wind we/d likely be dropping below zero overnight.
No major changes to the forecast this evening.
VFR conditions through the period.
Previous discussion... /issued 340 PM CST Friday Dec 6 2013/
Tonight and Saturday...forecast confidence is medium.
Middle to high clouds over the southeast portions of the area will
gradually move east of the area early this evening. There may be an
area of lower to middle level clouds that clip western portions of
the area later this evening...though not sure how widespread the
clouds may be. Otherwise...mostly clear skies are expected.
West northwest winds will continue tonight...within modestly tight
pressure gradient. 925 mb temperature trends with the winds suggest
lows should be in the zero to 5 above zero range in most of the
area...with zero to 5 below zero in the northwest counties. Some
concern that lows will fall lower than forecast despite the winds
remaining up tonight...especially northwest of Madison.
This will bring wind chills of 10 to 15 below zero across most of
the area later tonight and early Saturday...locally colder in spots.
Will mention this in the hazardous weather outlook.
As the high moves into the area Saturday...winds will gradually
weaken. Middle to high clouds should slowly work into the area from
the west southwest. Highs should only reach the 10 to 15 above range
across the area.
Saturday night through Monday...forecast confidence medium to high
Arctic high pressure to prevail Sat nt but high clouds will
increase throughout the night thereby limiting the radiational
cooling. European model (ecmwf)...GFS...and Canadian all very similar with a weak
but developing low pressure that will approach from the plains.
The surface low will become closed off at 1015 mb over Southern Lake Michigan by
06z Monday and continue to deepen as it moves through the northern
Great Lakes and into Canada on Monday. A large scale and fairly
impressive low to middle level warm advection pattern with
accompanied frontogenesis will overspread the region on sun with
snow intensifying later in the afternoon and evening when the
shortwave trough approaches from the SW.
A consensus of the aforementioned models on quantitative precipitation forecast yields quantitative precipitation forecast totals
of 0.15-0.20. Did not include the NAM which is an outlier with its
stronger deepening and higher quantitative precipitation forecast. The Omega is better placed
within the dendrite zone from yesterday and the dendrite growth
zone averages about 200 mb deep which is impressive. Will go with
snow ratios close to 20 to 1 given the deep dendrite zone and a
1000-500 mb thickness around 522 dm. This yields 3 to 4 inches
with the exception of 2-3 near the Illinois border. There has been a
slight northward trend with the shortwave trough and quantitative precipitation forecast so lesser
amounts near the Illinois border is reasonable. Lake enhanced snows then
need to be added in for Ozaukee and Sheboygan counties given the
1000-850 mb southeasterly flow and lake to 850 mb temperature differences of
15-17c. The lake effect snow charts suggests adding at least 3
inches so forecasting 5-7 inches of snow in Sheboygan and northern
Ozaukee counties...with locally more always possible. A Winter Weather
Advisory will be considered for much of the County Warning Area in later
forecasts. Brisk westerly winds and cold advection to prevail on
Monday with the Arctic airmass returning after highs in the 20s on
Long term...Tuesday through Friday...forecast confidence medium
An Alberta clipper will move across the northern Great Lakes on Tuesday
with southern WI on the southern periphery of just slight chances of light
snow. The associated Arctic front will reinforce bitterly cold
temperatures through Wednesday nt. The Arctic high will shift east for Thursday and
Friday with southerly flow and warm advection returning. Temperatures do modify
during this time.
VFR conditions are expected tonight and Saturday across taf sites.
West northwest winds will remain gusty at times tonight...mainly at
the eastern sites. Middle to high clouds over the eastern sites will
gradually move to the east by early this evening. There may be some
low to middle level clouds that clip Madison later this evening.
Otherwise...high pressure moving eastward into the region into
Saturday will bring quiet weather...with gradually weakening winds
on Saturday. Middle to high clouds will begin moving into the region
A relatively tight pressure gradient over the area into tonight will
bring west northwest winds of 15 to 20 knots. A few gusts to 25
knots are expected...mainly north of Port Washington. However...most
of the gusts should be below 22 knots...so will not issue a Small
Craft Advisory for this period. High waves will remain over the open
waters of Lake Michigan.
Winds and waves may reach Small Craft Advisory levels for a time
Sunday...as low pressure moves toward the area.
There is a better chance for Small Craft Advisory level winds and
waves Monday into Wednesday...as a tight pressure gradient develops
with an Arctic airmass moving into the region. Offshore flow should
keep high waves toward the open waters. Freezing spray may occur as
tonight/Saturday and aviation/marine...wood
Saturday night through Friday...gehring