Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
322 am CST Thursday Dec 25 2014
Today and tonight...forecast confidence...high
An upper level shortwave will exit the eastern Great Lakes with a
larger trough developing over the northern and central rockies.
The zonal flow across southern Wisconsin becomes more southwest
tonight. Weak upper divergence this afternoon and tonight along with
weak 700 mb upward motion. The 700 mb relative humidity decreases early this
morning...but increases again today...especially toward central
Wisconsin before drying again tonight. Main story is the low
clouds. Drying from above with an inversion around 1 thousand feet this
afternoon. Low clouds may scatter out briefly later this afternoon
and evening...but stratus potential increases later tonight as south
winds increase and the moisture below the 1 thousand foot inversion
increases. This could cause some drizzle...especially in areas
northwest of Madison. Temperatures there may drop just below
freezing with minimal freezing drizzle impacts given the
recent rather warm surface temperatures.
Friday through Saturday... forecast confidence medium.
Friday weather will remain quiet and mild over southern WI. Highs
are expected in the lower to middle 40s with a southerly breeze. There
will likely be low clouds around due to the low level moisture
spreading back into the region. Drizzle should hold off until late
A leading shortwave trough ahead of the longwave trough within
the left exit region of a strengthening upper jet... a
tightening 850mb baroclinic zone... and q-vector convergence
will lead to light precipitation over central/northern WI Friday evening.
Many models also spread light quantitative precipitation forecast into southeast WI Friday night along
the surface cold front... especially after midnight. The ecwmf is
drier in southeast WI. Kept chance probability of precipitation for now... but expect probability of precipitation
to increase as models come into better agreement.
The 00z European model (ecmwf) and also the 06z NAM are slower with bringing the
strong 500mb shortwave trough through WI Saturday morning than the
GFS. Wpc favors the slower solution which means higher quantitative precipitation forecast for
southeast WI. I went with a compromise using lighter quantitative precipitation forecast than the
European model (ecmwf) suggests. The models do not linger the precipitation into the evening
like the previous runs... so removed probability of precipitation for Sat evening.
Temperature profiles show that most of the precipitation in southeast WI
will fall as rain and then gradually change over to snow overnight
into Saturday morning before ending. In the northern forecast
area... the temperature profile is colder and we are looking at a rain/snow
mix changing over to snow by midnight. Precipitation amounts will be
light... less than 0.15 inch. Any snow accumulation would be an inch
or less northwest of Madison.
The ecwmf hangs onto the light precipitation longer in southeast WI than
the GFS on Saturday. Cold air advection and clouds will lead to
steady temperatures around 30 through the day.
Sunday through Wednesday... forecast confidence high.
Surface high pressure and dry weather will dominate for Sunday and
maybe Monday... but temperatures will cooler which means right
around normal for late December.
Another broad trough will cross the country early next week and may
bring some light snow across Wisconsin on a shortwave Monday
and Monday night. This feature is looking drier now.
The Arctic air now looks like it will stay bottled up to the north
of the Canadian border through most of next week. Plan on fairly
quiet period of weather with temperatures around normal.
Mainly MVFR ceilings this morning with some IFR ceilings possible near and
after sunrise as the lower level inversion lowers. Main story is
the low clouds. Drying from above with the inversion around 1 thousand
feet this afternoon. Low clouds may scatter out briefly later this
afternoon and evening...but stratus potential increases later
tonight as south winds increase and the moisture below the 1 thousand
foot inversion increases. This would cause some IFR ceilings and
possible drizzle...especially in areas northwest of Madison.
Temperatures there may drop just below freezing with minimal
freezing drizzle impacts given the recent rather warm surface
West winds will gust to around 25 knots through the morning so will
leave the Small Craft Advisory in effect.
Lm...Small Craft Advisory until noon CST today for lmz643>646.
Today/tonight and aviation/marine...hentz
Friday through Wednesday...mrc