Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
520 am CST Friday Dec 13 2013
Short term...(today through Saturday night)
issued at 240 am CST Friday Dec 13 2013
An upper level shortwave will track out of the Desert Southwest
today...sliding across the Ohio River valley by Sat.
Meanwhile...another...stronger shortwave will slide across southern
Canada...dipping across the northern Great Lakes Sat night/Sunday.
Looking at the southern system...good amount of low level isentropic
upglide out ahead of it...the bulk of which holds across IA/ill.
Some qg with it...strongest in the 500-300 mb layer...and like the
thermodynamics...well to the south. A 40+ knots low level jet will push
moisture out ahead of the shortwave...some of which could get
wrapped north and west of the eastward moving low...into colder air.
Current track of the system keeps the bulk of its accumulating precipitation
to the south...but some light snow farther north seems pretty
likely...albeit limited in amounts.
With the northern system...models paint a stronger/deeper qg
response...but keeps it across northern parts of Minnesota/WI and to the
north. Some isentropic upglide ahead of this system...but
again...its north of the forecast area. No moisture tap...but enough
of its own saturation that areas of light snow are likely.
So...as it sits...there will be two snow producers moving across or
near the region later tonight/Sat...but one generally stays south
with the other north. That said...enough lift and saturation that
there should be a periods of light snow from tonight into Sat night.
Amounts will range from a dusting to around 1/2 inch.
One potential caveat to the precipitation chances is that NAM BUFKIT
soundings suggest that there could be a problem with ice in the
cloud initially...which would bring freezing drizzle into the
picture. Its a tenuous suggestion at the moment...and going to keep
all snow for now. Certainly something that bears watching though.
Long term...(saturday through thursday)
issued at 240 am CST Friday Dec 13 2013
A few more pieces of upper level will move along the busy northwest
flow...moving across northern parts of the region Sun night/Monday.
Decent amount of 850 mb temperature advection moves across the upper
Mississippi River valley Sunday night...ahead of the shortwave...and
would be a good focus for some light snow production. Looking like
another light snow/low amount kind of event.
Significant differences in how the European model (ecmwf) and GFS sculpt the forecast
toward the end of upcoming work week. The GFS spits a piece of upper
level energy out of the Desert Southwest Thursday...while a broader
trough slides across the northern plains. These features eventually
merge as the move across the upper Mississippi River valley...with a
surface low developing across the plains and tracking over the eastern
Great Lakes Friday night. The storm evolution would favor a broad area
of snow for much of the forecast area from Thursday night-Friday night. Some
suggest that there could be a wintry mix across the south at onset.
The European model (ecmwf) is a different story. It has a similar scenario...but is
stronger and quicker with its features in its 12.12z and 13.00z
solution. This scenario favors a northern track to the low...more
warm air across the region...and more mixed precipitation with the main snow
band well north. Quite a flip from some of its previous runs...but
might be latching onto this trend. Not confident in either solution
so will hold course with the consensus solution for now.
Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 520 am CST Friday Dec 13 2013
Generally good VFR expected this morning. Northwest to northeast
flow in the surface-850mb layer will spread lower level moisture/clouds
southward through the day. These broken-overcast MVFR clouds already in the
kmsp-Keau-kmfi area...expected to reach the taf sites early this
afternoon. Moisture/clouds will continue to increase/thicken through
tonight as moisture from Lake Michigan is pulled westward and low
pressure moving into the middle MS valley sends moisture northward. May
be some flurries in the area through the late afternoon/evening
hours...however did not add these to tafs at this time.
Lift increases by late evening and for the overnight hours...along
with the deeper moisture. -Sn expected to develop across the area
around midnight...then persist well into the morning hours of Sat.
Included 3sm -sn at both taf sites after 05-06z tonight...with ceilings
lowering into IFR/lower MVFR ranges. Around a half inch of snow
possible by daybreak Sat. May see some 1-2sm visibilities in the -sn late
tonight...but left this out for now as plenty of time to add it with
future updates as the -sn details become clearer.