Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
325 PM CST Wednesday Dec 4 2013
.Tonight - confidence...Medium
997mb surface low vicinity lse will further intensify. Per impressive
rise/fall couplet low will head right across Western Lake Superior.
As this deepening occurs a strong cold front will whip through southern
WI bringing an end to the mild/foggy and moist conditions and
transition to a much colder and blustery regime. Will have to watch
for some icing up of the wet roadways and sidewalks as temperatures drop
below freezing. Will mention in the severe weather potential statement. With the frontal passage visibilities will
dramatically improve so current end times of dense fog advisory look
OK. 925 temperatures starting out at 6-10c on the plus side this evening
will plummet to 6-10c on the negative side by 12z...with gusty Post-
frontal west winds.
Thursday - confidence...medium
Gradual 850 millibar cold air advection. Temperatures will not move upwards
too much...with consensus to keep temperatures in the 20s. Wind chills
considerably lower. GFS much quicker on decreasing low level relative humidity than
the NAM. This reflected to some extent in the MOS sky cover
forecast. Without a deep upper trough across the area believe some
breaks in low clouds are likely though strong southwest upper flow
may maintain some middle and high level cloud cover at times. Main
story will be the cold.
Thursday night through Saturday...forecast confidence...medium to
A very cold period is still expected with unseasonably cold air
settling into the western Great Lakes. 850 mb temperatures fall around 15c
from today into Thursday...falling to around -10c by 00z/06. The
cold air gets reinforced by a persistent northwest wind flow in the
low to middle levels Thursday night and Friday. High temperatures both Friday
and Sat will be well below normal...most likely below the normal low
temperature for the 6th and 7th. The low to middle level flow remains
northwest through 00z/08 so no lake effect snow is expected.
Several very weak channeled vorticity areas associated with middle-
level short waves pass across southern WI Friday into Friday night.
Although middle-level moisture will be sparse...there may be just
enough forcing to shake out some flurries with lingering low level
moisture caught below inversion.
Saturday night through Wednesday...forecast confidence...medium.
Slight moderation in temperature begins on Sunday as strong west to
northwest flow succumbs to upstream amplifying long wave troffing
over western Continental U.S.. low to middle-level warm air advection shifts into
the southern WI region on sun tightening baroclinic zone. 12z GFS
and latest European model (ecmwf) carries 850 mb circulation northeast along this
baroclinic zone across southern WI/northern Illinois area Sun night into Monday
as middle-level short wave trough moves from the Central Plains into
the central gtlakes.
GFS and European model (ecmwf) in reasonable agreement on carrying around one quarter
inch of quantitative precipitation forecast across much of southern WI during this period. Critical
thicknesses favor all snow. Drier snow-liquid ratios of 15 to 20 to
one and a period of stronger Omega tapping into a deeper dendritic
growth zone should result in snowfalls of 3 to 6 inches across most
of the region...possibly a little more due to some lake enhancement.
Will be watching this system evolve over the next several days but
looking more likely that the first widespread winter weather event
will affect southern WI late in the weekend...possibly having an impact on
The cold air gets reinforced early next week. With potential for
widespread snowcover...temperatures likely to fall into the single digits
and potentially lower.
Aviation/00z tafs/...widespread LIFR/vlifr continues with moist low
level flow ahead of strengthening low pressure in western and northern WI.
Cold front arrives late afternoon/Erly evening. Widespread dense fog will be
replaced by strong cold advection and gusty west winds. This will
allow for improvement in ceilings and visibilities though still expect decent
coverage of MVFR ceilings during the night. Any rain or light mix this
evening will be on the light side. Forecasts show no measurable after
6z. Soundings show decreasing moisture in the low levels Thursday
morning though VFR broken conditions may linger with strong SW upper
Marine...dense fog advisory through 9 PM with passing cold front
about that time to scour out the fog. Gusty west winds ensue and
Small Craft Advisory will kick in at 9 PM. Windy conditions through
at least Thursday night.
WI...dense fog advisory until 6 PM CST this evening for wiz046-047-
Dense fog advisory until 8 PM CST this evening for wiz051-052-
Lm...dense fog advisory until 9 PM CST this evening for lmz643>646.
Small Craft Advisory from 9 PM this evening to 6 am CST Friday
Tonight/Thursday and aviation/marine...pc
Thursday night through Wednesday...mbk