Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
907 PM CST Wednesday Dec 17 2014
Aviation/06z tafs/...a large stratus deck over northern and
central WI and over all of Michigan will continue to move within the
cyclonic flow over the western Great Lakes and bring another period of
MVFR ceilings to the region through Thursday. High pressure will move into
southern WI Thursday nt but with only weak subsidence and weak boundary
layer flow...the stratus may remain.
Previous discussion... /issued 327 PM CST Wednesday Dec 17 2014/
Tonight and Tuesday - confidence medium
Any remaining flurries should end by dark as we are in a region of
subsidence behind a departing shortwave. Cloud over S WI is thinning
and clouds may become more scattered this evening. However cyclonic
flow will bring more clouds from the north into S WI overnight.
There is uncertainty with the extent and timing of the cloud cover
which may have an impact on low temperatures.
High pressure pushes into the state by Thursday morning bringing a much
drier airmass. A weak middle level ridge axis passes over the region
in the morning before a weak shortwave moves across central
Illinois. Models indicate some scattered clouds so could finally
see some sun.
Thursday night through Saturday night...forecast confidence high.
Cold surface high pressure will remain over the Great Lakes region
through Saturday. Upper level flow will generally be zonal over the
center of the country through Saturday night. Weak 500mb shortwave
troughs will swing across the upper Midwest. These shortwaves will
bring periods of clouds to southern Wisconsin... but overall
moisture and forcing will be minimal so this will be a dry period of
Temperatures will slowly warm from the upper 20s/lower 30s to the
middle 30s through Saturday afternoon. Lows will be around 20.
BUFKIT model soundings show the top of the cloud layer reaching the
snow growth region at times which can lead to occasional flurries.
Sunday through Tuesday... forecast confidence medium.
The GFS and Canadian models are showing a shortwave tracking through
the area that could bring rain or snow into southern WI Sunday
afternoon/evening. The European model (ecmwf) keeps the forcing with the shortwave
weaker and to our northwest.
A strong upper jet from the Pacific northwest will dive into the
center of the country. This will allow for a closed upper low to set
up over the Midwest from sun through Tuesday. The amplified trough
will help to draw warmer air and also moisture into southern WI by
early next week.
The associated surface low pressure system will spread precipitation
northeast through southern WI Monday or Monday night. The GFS is
quicker than the European model (ecmwf). Precipitation type will end up being a mix. It will
likely begin as snow early Monday morning and change over to rain
especially near the Illinois border with daytime heating through Tuesday.
Colder air will wrap into southern WI Tuesday night and any
lingering light precipitation will change over to snow.
Christmas evening and Christmas day... forecast confidence medium.
It is looking more and more likely that we will have a white
Christmas across southern Wisconsin. The large upper low over the
upper Midwest will tap into energy from an upper jet over the
southeast U.S. This will lead to a strong surface low pressure
system developing over the Appalachians and tracking northwest
toward Lake Huron. This will result in persistent light snow and
strong northwesterly winds across Wisconsin...especially Christmas
The European model (ecmwf) and GFS are in remarkable agreement for day 7-8. Of
course... the forecast timing and location of this large system will
waver over the course of the week so keep up with the latest details
as the system approaches.
Clouds are thinning and msn may see some reduction in cloud cover
this evening. Eastern taf sites may see a reduction in cloud cover
later in the evening but confidence is not as high. More clouds are
moving in from the north and will spread across taf sites overnight.
Some uncertainty about whether ceilings will be MVFR or VFR this
evening and whether it would be scattered or broken coverage.
Overnight clouds are more likely to impact eastern taf sites than
msn. More confidence with ceilings rising to VFR after midnight.
tonight/Thursday and aviation/marine...sm
Thursday night through Wednesday...mrc