Schweizer Sturmforum

Hello everyone,

Having received many phone calls in our Geneva weather office on the morning
of August 11th concerning a possible tornado in Massonnens the night before,
I decided to drive there myself after my morning forecasting shift. After
inspecting most of the damage I could find and talking to as many
eyewitnesses and villagers as possible, all the findings seem to point to a
tornado (a narrow and relatively long ~2km SW to NE oriented damage path,
snapping/twisting of large diameter trees, insolation found high up in
trees, eyewitnesses describing the event as sounding like a train or
airplane engine, curvature effects in a wheat field). From what I could
gather from the farmer who lost part of the roof on his house, the event
occurred very close to 22h00 local time which would coincide pretty nicely
with the shear zone observed at 21h50 LT on the radial velocity display of
the ETHZ radar described by Willi Schmid. Unfortunately, since the event
occurred at night, none of the villagers I was able to talk to actually saw
the funnel. I asked if hail had fallen before the wind event but the people
I spoke to said no. They did say that very heavy rain had fallen previous to
the wind event.

Based on the damage I was able to survey, a maximum rating of F2 appears to
be the most appropriate. This decision is based on the damage inflicted to a
patch of forest (~100m x 40 m) where 30-40 LARGE diameter trees were
completely snapped in half at their midsections and some others uprooted.
This coincides nicely with F2 type damage
http://www.tornadoproject.com/fscale/fscale.htm. Downstream of the damaged
forest, the tornado's intensity seems to have decreased as the damage to
trees is less widespread but still significant. This region also corresponds
to the area where the roof was partially blown off the barnhouse and where
F1 damage seems to have occurred. Downstream of the house, the tornado seems
to have decreased yet further with only F0 type damage visible such as
chimney and roof tile damage as well as large broken tree branches. I have
constructed an approximate path that the tornado most likely took along with
intensity ratings based on the information I was able to gather while on
site. It can be visualized at
http://www.meteosuisse.ch/fr/Previsions ... lite.shtml
along with a written summary in French. I am also awaiting the return of the
2 rolls of film that I took of the damage with my reflex camera. I will scan
and forward you some of these pictures once I receive them.

Here at MeteoSwiss I was able to visualize the PPI reflectivity images
corresponding to the approximate time of the event and like everyone, I was
not very impressed. Of course, since the convective cell responsible was
embedded in a vast area of stratiform precipitation from the MCS,
attenuation of the radar beams undoubtedly occurred, minimizing the dBz and
R values. Unfortunately, as far as I'm aware, we only have real-time radial
velocity data currently available from our Doppler radars with a Nyquist
velocity of 8.25 m/s. I will try to contact Locarno to see if they can
extract the velocity data from the "La Dôle" radar from this event. In terms
of beam height though, too bad the radar is at 1600m.

In terms of dynamics, with 30-35 kts of deep layer shear, an approaching
upper-level trough, jet streak destabilization and high sfc dewpoints for
low LCLs, conditions were certainly at least partly conducive for
supercell/hybrid type convection. Perhaps the hilly terrain in the region of
Massonnens with its rolling hills and small valley-like depressions
partially aided in converging the air and attributing some low-level spin to
the air underneath the main updraft (maybe similar to the tornado cases
observed in the small tunnel-like Jura valleys in the past) and locally
enhancing the SRH. The damage path certainly seems to be located within a
broad valley-like depression with other small embedded depressions located
along part of its path.

I will certainly let you know more once I receive new relevant info
regarding this case and would greatly appreciate any updates on your side as
well.

Best and friendly regards,

Lionel Peyraud
Forecaster / Research
MétéoSuisse - Genève

I am seriously considering preparing another webpage of the event (which
could be official if ok with you) on the MétéoSuisse webpage so that
continuity can be upheld. This webpage could also be inserted and translated
on the MeteoSchweiz and MeteoSvizzera webpages as well with proper citation
of ETHZ and Tordach/Skywarn banners on the webpage of course. The idea is to
provide as much information as possible especially to the Massonnens
community who instinctively calls our Geneva office or taps our website and
also to all scientists, students, chasers and passionate individuals
interested in the event. I will get back to you asap (probably tomorrow)
regarding the webpage after talking to some of my colleagues (I have the
night shift tonight).

In the medium and long terms, it would be great if we could
all collaborate together and perhaps publish a case study on this event.