Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
614 am CDT Monday Sep 22 2014
Short term...(today and tonight)
issued at 339 am CDT Monday Sep 22 2014
At 3 am...a 1026 mb surface high was centered over eastern
Nebraska. While the area is under clear skies...we saw a rapid
increase in our radar returns from near La Crosse to between
22.02z and 22... they have remained over those areas
throughout the night. While some of this was our normal anomalous
propagation of the radar beam that we see as the nocturnal
inversion develops...the 30 to 40 dbz returns are migratory Birds
through the area. We know this because the correlation
coefficients are running between 0.65 and 0.9. With no
precipitation in these areas...this means that these radar targets
are biological in nature. We know it is migratory Birds because of
the time of year and that the VAD wind profile is showing 30 to
45 knot winds between 2 and 15k feet. Typically in these synoptic
situations we do not get this many wind levels aloft without
clouds. In addition these wind speeds are running about 15 to 25
knots higher than what show up in the soundings. This additional
speed is from the migratory Birds. The highest concentration of
Birds are running between 2 and 8k feet.
Besides the Birds tonight...been watching the slow development of
fog. Dense fog is already reported at Boscobel and Black River
Falls. With light winds /less than 10 knots/ up to 2500 feet and
our web cam already showing a layer of stratus in the valleys...we
expect to see a continual development of areas to widespread fog.
Followed the 930 to 10 am climatology with the dissipation of
Tonight...looks to be even more favorable for the development of
fog in the river valleys and central Wisconsin with light winds
up to 25k feet and clear skies. As a result...went with widespread
dense fog from 2-3 am through 930-10 am.
Long term...(tuesday through sunday)
issued at 339 am CDT Monday Sep 22 2014
Tuesday night...the 22.00z models are in good agreement that weak
850 mb moisture transport will develop across southeast Minnesota
and northeast Iowa ahead of a trough moving out of the plains.
BUFKIT soundings between 06z and 12z are saturated between 5 and
20k feet and there is Omega between 5 and 10k feet. This results
in the models generating some light rain across this area. Since
all of deterministic models were in agreement...added a 15 to
24 percent chance of rain to the forecast.
From Wednesday into Wednesday night...show that the 500 mb short
wave trough shears apart across the area. One part moves north and
is absorbed by the polar jet stream. Meanwhile the southern piece
moves southwest and forms a closed 500 mb low. While there is
agreement that this will occur. There are differences in
precipitation. Both the European model (ecmwf) and Canadian produce precipitation
across the forecast area. Meanwhile the GFS and NAM keep this
precipitation across south central Minnesota and western Iowa.
Since European model (ecmwf) soundings are not showing much lift Colorado-located with
the moisture...kept the forecast area.
For the remainder of the forecast period /Thursday through
Sunday/...it looks dry as high pressure remains located across
the area. 850 mb temperatures are running 10 to 14c which is about
2 Standard deviations above normal. This result in temperatures
about 10 degrees warmer than normal.
Aviation...(for the 12z tafs through 12z Tuesday morning)
issued at 614 am CDT Monday Sep 22 2014
A surface ridge of high pressure over the taf sites is promoting
favorable conditions for fog formation...especially in river
valleys and low lying locations. Lse for the most part has been
impacted with LIFR stratus...with visibilities varying between IFR
and VFR. Rst has avoided the vlifr fog impacting Dodge Center and
Austin. Both sites should stay status quo for the next few hours...
perhaps lse dropping a little more in visibility given the long
night right now. By mid-morning...9-10 am...daytime heating will
allow all fog and stratus to mix out and result in VFR conditions
for the rest of the day. The ridge of high pressure remains in
place tonight...and all signs point to a repeat of what\s going on
now. In fact...the wind profile over lse is very light all the way
up to 15000 feet...greatly favoring valley fog/stratus.