Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
335 PM CST Thursday Jan 29 2015
Short term...(this evening through friday)
issued at 335 PM CST Thursday Jan 29 2015
At 3 PM...a 1004 mb surface low was located over Georgian Bay.
This low brought the drizzle...freezing drizzle...and snow grains
last night and this morning. In addition to the precipitation
winds gusts up to 39 miles per hour at Rochester international Airport and
40 miles per hour at Decorah Municipal Airport late this morning.
The main forecast issue for tonight into Friday morning will be
the clouds. The latest infrared satellite images shows some
partial clearing north of Saint Cloud. A few breaks in the
overcast were also noted in southeast Minnesota...northeast
Iowa...and western Wisconsin. One of these breaks moved through
the La Crosse area between 1 PM and 2 PM. Soundings are providing
mixed messages on the amount of moisture trapped below 900 mb.
The NAM keeps quite a bit of moisture in this layer through
30.12z. Meanwhile the GFS scatters out this moisture early this
evening. With cyclonic flow aloft this evening...am concerned
that whatever residual low level moisture which exists below the
subsidence inversion could expand and fill in The Holes that
currently exist...thus trended more toward the NAM this evening
and then went more toward the GFS overnight. However will have to
watch how this develops overnight. These clouds could have an
impact on the low temperatures on Friday morning.
Long term...(friday night through thursday)
issued at 335 PM CST Thursday Jan 29 2015
On Friday night...the 29.12z models are in good agreement that a
short wave trough will move southeast across northern Minnesota
and northern Wisconsin. Both the Gem and NAM have their moisture a
bit further south than either the GFS or European model (ecmwf). As a result...they
have some light snow as far south as Interstate 94. Meanwhile the
GFS and European model (ecmwf) continue to show that the 925 mb moisture transport
and synoptic lift will remain well to the north of the area. Since
the GFS and European model (ecmwf) have been the most consistent...trended the
forecast toward them.
On Saturday night and Sunday...the models show that Arctic cold
front will move southeast across the area. With the much of
forcing in the southern stream and dry air being advected into
the area from the north...the snow swath continues to move further
and further south with each deterministic model run. A quick look
at the 29.12z GFS ensemble members shows 11 out of 12 members have
the accumulating snow south of Interstate 94. There was even at
least 1 run of the 50 European model (ecmwf) that had its snow north of Interstate
90. With the consensus being further south...lowered or took out
the precipitation chances north Interstate 90.
From Monday night into Tuesday night...the deterministic GFS/Gem/
European model (ecmwf) have 2 quick moving low pressure systems moving through the
region. While there is good agreement amongst these models...the
12 29.12z GFS ensemble family have a wide variety of solutions.
Much of this has to deal with how far north the Arctic cold front
is able to advance northward on the backside of the high. As a
result...still do not have a lot of high confidence on the
location of these snow bands.
From Monday through Thursday...the both the cfs version 2 and
ecwmf show quite a bit of spread in their temperature forecast.
For Monday and Tuesday...the operational European model (ecmwf) is on the warm side
of the ensemble mean. Meanwhile for Wednesday and Thursday... the
operational European model (ecmwf) is on the colder side ensemble mean. The
Thursday European model (ecmwf) low temperature is amongst the lowest of its 50
member ensemble family. With such uncertainty...just stayed with
the model consensus.
Aviation...(for the 18z tafs through 18z Friday afternoon)
issued at 1132 am CST Thursday Jan 29 2015
Main aviation concerns through taf period are MVFR conditions and
strong northwest winds this afternoon into tonight. Latest surface
analysis shows tight pressure gradient across the upper Midwest with
latest metars indicating sustained wind speeds around 20 knots and
gusts up to 30 knots. The pressure gradient remains tight across
much of the region through this evening...as a weak cold front
pushes into southern Iowa this afternoon. This will allow winds to
remain at 15 to 20 knots sustained and gust up to around 25 knots
through 05-07z Friday. High pressure builds into western Wisconsin
and eastern Iowa after 06z Friday and diminishes the winds to 5 to
10 knots by 12z Friday at both rst and lse taf sites. Latest metars
across the area are reporting MVFR conditions. MVFR conditions will
continue at both taf sites through tonight. Then conditions will
improve to VFR Friday morning with high pressure building in and
stratus deck becomes scattered at taf sites.