Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
343 PM CDT Wednesday Mar 12 2014
Tonight and Thursday...forecast confidence is medium.
Area of pesky light to moderate snow showers are moving southward
across southern Dane and eastern green counties...associated with
convergence from inverted trough. These were being helped along by
the moist and unstable low levels within the dendrite snow growth
zone per area forecast soundings. Could see a few more of these snow
showers or flurries redevelop late this afternoon along this
boundary...until drier airmass from the north pushes into this area.
For now...left forecast dry as any redevelopment will be localized
and short lived.
Drier air moving into the area from the north will mix out the cloud
cover in south central Wisconsin by the evening. Middle level clouds
will then gradually shift eastward into the area later tonight into
Thursday. High pressure will slide southeast across the area
tonight. Light/calm winds and clear skies for a time this evening
should allow temperatures to fall quickly...before some clouds later
tonight slow the drop. 925mb temperature fields and radiational
cooling conditions will bring lows well below seasonal normals. Lows
mainly in the single digits above zero east to 10 to 15 far
South southwest winds become gusty on Thursday...as pressure
gradient increases with focused warm air advection. A period of
light snow showers are possible...as these situations tend to bring
light precipitation. Moisture is not deep...but enough to keep low
probability of precipitation going. Probability of precipitation may need to be raised in later forecasts if better
column moisture is seen across the area. Also a concern for light
freezing drizzle...as ice Crystal layer is somewhat dry.
Thursday night through Friday...forecast confidence medium.
Warm air advection will be ongoing overnight Thursday night. There could
be some high clouds over southern WI...but the better moisture will
be over northern WI. Winds will remain breezy out of the south which
will allow just enough mixing to keep temperatures from dropping much.
Expecting at or above freezing min temperatures and leaned toward the
warmer side of guidance...toward European model (ecmwf) and Canadian model values.
There will be a window of opportunity Friday morning for temperatures to
rise into the middle/upper 40s before a potent middle level shortwave
drops down through Minnesota and WI Friday afternoon. Winds will become gusty
out of the west behind this surface trough/cold front...then
gradually diminish through the night. Expecting clear skies
overnight as dry air settles in...with min temperatures in the middle 20s.
Saturday and Sunday...forecast confidence medium.
Winds will turn to the north on Saturday as surface high pressure
moves into western Ontario. This will cause a back-door cold front
to slip Down Lake Michigan late Saturday afternoon through evening.
A few models try to generate precipitation along the lake with this
front...but Delta-ts look too marginal. Nonetheless...there could be
some lake effect clouds. Meanwhile...mid-level clouds will be
spreading into southwest WI as an 850mb baroclinic zone begins to
set up and a weak shortwave moves through that area.
Temperatures toward Sheboygan will struggle to reach freezing...while weak
warm air advection will allow for temperatures to rise to around 40 toward
the Illinois and Iowa borders.
The ecwmf tries to spread light snow into southern WI on Sunday with
a weak shortwave...while the GFS keeps US dry with the surface high
overhead. Either way...the temperatures will be below average in the
Monday through Tuesday...forecast confidence low.
There are differences in how strong an upper level ridge will become
over the southwest Continental U.S. Early next week. The European model (ecmwf) keeps US on the
cold side of the 850mb baroclinic zone while the GFS puts US in the
warm sector. Therefore...low confidence in the consensus forecast
Also of note...the European model (ecmwf) swings a potent 500mb shortwave across the
Midwest. This would suggest accumulating snow for southern WI and
northern Illinois. The GFS keeps southern WI dry.
VFR ceilings will mix out at Madison by evening. VFR conditions are
then expected tonight and Thursday. Gusty north winds will linger
until early evening...then weaken quickly as high pressure moves
into the area.
High pressure will slide south across the area tonight...bringing
light/calm winds. Mostly clear skies this evening will gradually
increase with middle level clouds from west to east later tonight into
Thursday. Winds will become south southwest and become gusty during
May see light snow showers through the day...with warmer air moving
into the region. For now...will not mention in tafs due to low
confidence in timing and areal coverage. Light freezing drizzle is
also possible...but low confidence at this time so left out of tafs
Gale Warning was cancelled...and replaced with a Small Craft
Advisory until 03z Thursday across the nearshore waters. Current
marine observation sites are only gusting to around 30 knots...and
should continue for another few hours before weakening by middle
evening. High waves in ice free areas will gradually subside during
this time as well.
Southwest to west wind gusts may reach Small Craft Advisory levels
Thursday night into Friday...as a tight pressure gradient develops
over the region.
Lm...Small Craft Advisory until 10 PM CDT this evening for lmz643>646.
Tonight/Thursday and aviation/marine...wood
Thursday night through Wednesday...mrc