Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
1056 am CST Wednesday Jan 28 2015
Aviation/18z tafs/...impressive warm air advection with koma at 17c and kgrb at
-2c. Cirrus riding atop the stratus deck in SC WI. Stratus blob
lifting north from SC WI though cirrus continues to spill across
and is thickening up somewhat. Still looking for a renewed surge
of IFR/LIFR ceilings and some IFR visibilities later tonight into early Thursday
as low pressure tracks south of WI. Models settling on about 0.04
to 0.06 in liquid precipitation. Little saturation in dendridic growth
zone so surface temperatures will be key for any freezing precipitation. Does
appear to be a window for that potential especially since temperatures
are struggling today with pronounced low level inversion and cloud
cover. Cold advection does commence later tonight so warm layer
will be slowly eroding with time. Warm layer most pronounced prior
to precipitation onset. Expect improvement in ceilings as day GOES on with
some drying after precipitation...though low level relative humidity forecasts keep cloud cover
in place for most of the day.
Marine...postponed Small Craft Advisory several hours as new
short term forecast soundings delay start of south to southeast
gusty winds reaching Small Craft Advisory levels. Never the
less...still expect rapidly tightening pressure gradient to cause
wind gusts to 25 kts later this afternoon through the evening. As low pressure
trough moves across Lake Michigan late tonight and early Thursday...a lull in
the winds is expected before veering to the northwest and
increasing later in the morning. Gusty south to southeast winds will
cause high waves this afternoon but will subside a bit on Thursday as wind
speeds decrease and become offshore.
Previous discussion... /issued 321 am CST Wednesday Jan 28 2015/
Today and tonight...forecast confidence is medium.
A weak upper level ridge moves across southern Wisconsin today
ahead of a shortwave over the plains that reaches southern
Wisconsin by sunrise Thursday. Weak upper divergence occurs this
afternoon and for much of the night before weakening late. A
rather weak middle level shortwave moves in late tonight. Weak 700 mb
upward motion begins over south central Wisconsin this morning and
across the area this afternoon and evening. The upward motion
strengthens by evening before weakening again late.
700 mb warm air advection pushes temperatures above zero celsius
by evening before cooling toward sunrise Thursday as the 700 mb
trough axis pushes into far southeast Wisconsin. Warm air
advection brings 850 mb temperatures to 4 celsius over Sheboygan
and 10 celsius just southwest of Madison this evening. 850 mb
temperatures cool below zero celsius across the forecast area by 6
am CST Thursday morning due to cold air advection behind the 850
mb trough and evaporative cooling.
After an initial middle level band of clouds push east early this
morning the 700 mb layer dries and never saturates completely
tonight...but does come close over the far east. 925/850 mb layers
do saturate rather quickly late in the evening and remain moist
A weak surface trough moves across southern Wisconsin late
tonight reaching Lake Michigan around 6 am CST Thursday.
Soundings indicate a freezing rain potential most of the night.
Even during the early morning hours the NAM has temperatures just
above zero celsius from 6 to 8 thousand feet over all but the extreme
north. During the time of best lift the soundings are still in
the process of becoming saturated...and by the time it does the
lift is weakening. Therefore precipitation may have a hard time of
measuring with only a few hundredths expected. Surface
temperatures will be close to freezing over much of the forecast
area...and will likely be just above in the far southwest and
extreme south central.
Due to the marginal nature...and later occurrence tonight will
opt to not issue any Freezing Rain Advisory at this time.
Short term...Thursday through Friday night...forecast confidence
Synoptic models are in decent agreement with showing cold air
advection kicking in across the area Thursday into Thursday night.
There should be a brief period Thursday morning for ice crystals
to form...which should combine with passing 500 mb shortwave
trough to result in more of a light snow or light sleet chance
across most of the area. The far southeast may be warm enough
still in the morning for a light rain/light freezing rain/light
sleet mix. Kept chance probability of precipitation going Thursday morning.
Dry air should bring dry conditions by Thursday
afternoon...lingering into Thursday night. Tight pressure gradient
will result in gusty northwest winds Thursday...bringing colder
temperatures for Thursday night.
Weak high pressure Friday will continue to bring dry conditions
with colder temperatures. Models then show warm air advection
developing Friday night...with the NAM showing some light quantitative precipitation forecast.
Other models dry...so kept forecast dry Friday night.
Long term...Saturday through Tuesday...forecast confidence is
European model (ecmwf)/GFS models appear to be in better agreement with system for
Saturday night into Sunday night across the region. They have
trended northward with the surface low and 500 mb shortwave
tracks...though still have differences with the tracks. Both
models do bring northern portions of 500 mb shortwave trough
through the area Saturday night and Sunday...with the GFS bringing
in an area of low level frontogenesis.
This results in a period of light to moderate snow across the
area Saturday night and Sunday...tapering off Sunday night. Quantitative precipitation forecast
amounts have increased compared to yesterday. Using the expected
snow to liquid ratios...thinking a 4 to 6 inch snowfall is
possible. This certainly could be an advisory type of event at
this point...so stay tuned to later forecasts.
After a period of near normal temperatures Saturday into
Sunday...cold air advection Sunday night into early Monday should
bring below normal temperatures back into the region. Winds with
these temperatures may bring wind chills into the teens below zero
Sunday night and Monday night. Models then become more different
with features into Tuesday. Used consensus model temperatures into
Tuesday for now.
MVFR ceilings will remain over mainly south central Wisconsin but may
scatter out during the morning. Expect MVFR then IFR ceilings to
develop tonight ahead of an approaching trough of low pressure.
Some very light freezing rain will be likely...although surface
temperatures may rise above freezing in areas mainly southwest and
south of Madison.
Winds still on track to rise to Small Craft Advisory levels this
afternoon through late tonight ahead of low pressure crossing the
region...with south to southeast winds building high waves. There
will be a lull to the winds...and waves will subside as the
low/surface trough crosses the western Great Lakes Thursday
morning. Northwest winds rise to small craft levels...with near
gale force gusts behind the low Thursday afternoon and night.
Lm...Small Craft Advisory from 3 PM this afternoon to 6 am CST Friday
Today/tonight and aviation/marine...hentz
Thursday through Tuesday...wood