Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
846 PM CST Friday Mar 6 2015
rather quiet out there this evening. Temperatures are
falling...but much slower than the previous evenings. Warm air
advection and some high clouds moving through are keeping
temperatures mostly in the 20s so far. We/ll still see some teens
out there overnight...then rise into the 30s for Saturday. No
updates needed for the next 24 hours.
a low pressure system will track across Lake Superior Saturday
morning...dragging a cold front across the area during that time.
In general...cigs/vsbys will remain VFR through the
period...however...there could be a narrow band of MVFR clouds
that accompany the passage of the front after it moves through.
These clouds would likely only last a few hours...if they form at
all. Look for winds to shift from southwest to northwest in the
wake of the front.
Previous discussion... /issued 314 PM CST Friday Mar 6 2015/
Tonight and Saturday...forecast confidence high.
As one shortwave exits this afternoon...another one will approach
from the northwest...arriving tomorrow morning. Warm advection aloft
ahead of the approaching wave will result in middle and high level
clouds tonight. The clouds and warm advection will also keep temperatures
from falling very far...with lows expected to be within a couple
degrees of seasonable normals.
Some lower clouds may move in tomorrow behind the shortwave as
colder air aloft filters in. Models differ a bit with the extent of
the cold air though...as the European model (ecmwf) keeps 925 mb temperatures fairly steady
through the day...while GFS and NAM cold advect. GFS/NAM soundings
not very deep with the low level saturated layer...so kept sky cover
at least partly sunny. With some sunshine possible...trended temperatures
toward milder European model (ecmwf) and Canadian solutions...as GFS/NAM undercutting
temperatures today by several degrees. The colder NAM/GFS solution is
likely due to the model snow pack having too much of a cooling
effect on surface temperatures.
Saturday night through Monday...forecast confidence medium.
The cold advection Sat evening will weaken toward sun am with a ridge
of high pressure extending from Iowa toward the southeast USA. A weak
clipper system is then expected to track from the northern Great
Plains to either northern or central Illinois by 00z Monday. The European model (ecmwf) and
Canadian keep the frontogenesis axis and snow band mainly from northern
Iowa into northern Illinois...while the other models track it slightly farther
south. The trend is shifting south as well so maintained lower
chance probability of precipitation in the far southern County Warning Area with a dry forecast over the remainder
of the area. Given the system is very weak and probably going
south...chances for more sunshine are possible...and along with
weak warm advection...increased high temperatures to the upper 30s to
A weak surface ridge will follow for sun nt but will shift to the east
and south for Monday. Southwesterly winds...warm advection...and full sunshine
are expected. High temperatures should reach lower 40s.
Long term...Tuesday through Friday...forecast confidence medium.
A split jet stream will prevail with the northern branch mainly in southern
Canada and the southern branch in the southern USA. This will bring mild
temperatures to the area with the 1000-500 mb thicknesses ranging from
546-552 dm. The details of the surface temperatures will depend on a trailing
Canadian cold front that will stall or move through the region
but then lift back north as a warm front. The best estimate is for
the front to gradually move south through southern WI on Wednesday or Wednesday nt
and lift back north Thursday nt and Friday. Highs in the 50s are expected
for much of this period over at least south central WI with the
potential to move farther east for some of the days.
VFR conditions are expected through at least tonight. Could see some
lower clouds move in by Saturday afternoon as colder air aloft
arrives behind a departing shortwave. Saturated layer is not very
impressive on models soundings...so will stick to mainly scattered lower
clouds for now.
Low level wind shear could get to around 30 knots from 0-2kft by
daybreak tomorrow...particularly across the western forecast area.
Surface winds will be coming up by then though...so will hold off
mention in the tafs for now. Something to keep an eye on though.
New forecast yields wind gusts right near Small Craft Advisory
criteria into the evening...so 03z expiration time for the advisory
still looks good.
West to northwest winds will approach Small Craft Advisory criteria
Saturday...but overall looks too borderline to go with an advisory
at this time.
Still a good amount of broken ice in the nearshore waters. Milder
temperatures and persistent offshore winds the next couple days will likely
get rid of a lot of this ice though.
Lm...Small Craft Advisory until 9 PM CST this evening for lmz643>646.
tonight/Saturday and aviation/marine...ddv
Saturday night through Friday...gehring