Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
913 PM CST Monday Jan 26 2015
Will be issuing one more Special Weather Statement to cover the mixed wintry precipitation...mainly
over south central WI. Surface low is weakening as it moves south
with forcing from departing sheared short wave diminishing
rapidly. Area of precipitation upstream diminishing with low
level convergence weakening as surface low fills...trending well
Back edge of precipitation is making steady progress south and
should clear most of S central Wisconsin by 1 am. Left the low
chance/slight chance probability of precipitation in the far east through most of the
overnight...but should not see much in the way of accumulation.
Dual pol radar imagery indicating mainly pockets of light
snow...mainly under highest correlation coefficient values...with
freezing precipitation in between...which is mainly freezing drizzle
though there have been a few reports of sleet. Only expect spotty
additional accumulations of a few tenths of an inch of snow...but
enough to keep untreated roads slick.
With region staying under cyclonic flow and low-level relative humidity fields
keeping high values over southern WI until at least 18z...will keep
IFR ceilings through 12z and at least MVFR ceilings through 15z-18z.
Latest NAM brings some drier air in around middle-morning to the east and
pushes west through the rest of the County Warning Area during the late
morning...a trend supported by the sref ceiling probabilities.
00z NAM 925 mb relative humidity forecasts indicate a potential return of low
clouds with warm air/moisture advection as weak ridge of high
pressure sags south in the later part of the afternoon and
evening. Will await latest GFS to see if it follows this trend.
Previous discussion... /issued 337 PM CST Monday Jan 26 2015/
Today and tonight...forecast confidence is medium
Surface low pressure moving into northwest WI will weaken as
it drops southeastward across the state tonight. Light snow ahead
of the surface trough axis will be slow to exit southeast WI this
evening. It is being forced by weak low and middle level frontogenesis
and warm advection. Flurries may linger through the night in
southeast Wisconsin due to low level convergence along the Lakeshore
with easterly flow associated with an inverted trough over the
region. Observations across south-central and western Wisconsin have
been reporting freezing drizzle. This is supported by soundings from
several models and they indicate freezing drizzle may persist into
the evening in south-central WI.
With warm air advection tonight do not expect temperatures to change much
overnight in southeast WI and they may increase slightly in far
southwest WI. High pressure will build into the region on Tuesday
but expect clouds to continue through at least the morning with
lingering low level moisture.
Wednesday...forecast confidence is high.
High pressure will continue to slide off to the east...while warm
advection arrives ahead of the next approaching low pressure
system...but the column remains too dry at this point for any
concern about precipitation with the warm air advection.
Wednesday night and Thursday...forecast confidence is medium.
Upper level northwest flow continues and we/ll see a complex
merger of short waves from Canada and the central U.S. Occur over
the Great Lakes during this period. An upper level trough gets
carved out over the western lakes...while the associated surface
low tracks across the northern half of Illinois. Quantitative precipitation forecast amounts should
still remain mostly below .10 inch. Temperature profiles support a
wintry mix Wednesday night with a decent elevated warm layer.
Light freezing rain...sleet and snow are possible. Maybe all rain
early on across the southwest...before changing over to a mix.
The precipitation will diminish from west to east on Thursday...with the
afternoon looking drier than the morning.
Friday...forecast confidence is high.
High pressure will bring dry weather...but temperatures will run below
normal for a change.
Saturday and Sunday...forecast confidence is medium.
There is a lot of model uncertainty for the weekend. Both the GFS
and the European model (ecmwf) develop an upper level large amplitude low pressure
trough that gradually drifts east from the western High
Plains...through the middle of the U.S. The GFS has the
associated surface low pressure system moving across northwest
Wisconsin Saturday night while the European model (ecmwf) has the low well to our
south. Precipitation type shouldn/T be a question with either solution as
temperatures will be cold enough to keep it all snow.
Monday...forecast confidence is medium.
High pressure should follow with dry but very cold weather for the
early part of next week.
Precipitation will slowly push east across southern Wisconsin this
evening. Expect light freezing drizzle to continue in south-central
Wisconsin through the early evening. Light snow is also possible at
times with little if any accumulation. There is uncertainty on how
late into the evening precipitation will persist. Expect light snow
to continue in south-east Wisconsin into the evening with up to a
couple tenths of an inch of additional accumulation after 0z.
Flurries may linger in far southeast Wisconsin through the night.
IFR ceilings expected overnight in south-central WI. IFR or MVFR
ceilings expected in the southeast. Think visibilities will be IFR
or MVFR. There is some uncertainty with timing on when conditions
will improve Tuesday morning.
tonight/Tuesday and aviation/marine...marquardt
Tuesday night through Monday...Davis