Archive for November, 2012

An extremely rare event for Slovenia – long-lived supercell storm in November!!

Yes, like as said in the title, an intense long-lived supercell travelled across central Slovenia today, bringing intense rain and large hail at some locations. Its November 2nd, yes, early November! One would be surprised to hear thunder so late in the fall season, not being shocked like many of us after seeing radar scans this afternoon. But conditions were there, supportive of rotating storms. I bet it doesn’t need to describe this image… its just perfect! Did I mention November is almost winter month here? 🙂

A large and deep trough is located over NW Europe with very strong mid-level jet rounding it on the southern flank. Over the Alps, in addition, a short-wave trough crossed the region in the mid-day today, providing some forcing needed for lifting. Surface charts showed a weak frontal boundary crossing the Alps as well, which seemed to be another focus in today’s trigger for storms.

During the morning hours, mostly showers and some weakly electrified convection was occuring over the Dynaric mountain range in Slovenia, but at around 12 UTC, a stronger cell formed near Škofja Loka, NW of Ljubljana in central Slovenia. In that area, topography slowly diminishing towards east. Looking over the SFC data from automatic stations from national meteo agency (ARSO), conditions in front of cell were showing light SE-E-NE-erly winds… (time | T | Td | avg wind | wind direction | wind gusts):

Ljubljana – airport Jože Pučnik

13:30 UTC | 11.9°C | 10.0°C | 1.4 m/s | SE | 3.0 m/s
14:00 UTC | 11.2°C | 9.6°C | 0.9 m/s | N | 3.6 m/s
14:30 UTC | 11.3°C | 10.2°C | 0.1 m/s | WNW | 0.9 m/s

City of Ljubljana – Bežigrad

13:30 UTC | 13.6°C | 9.7°C | 0.9 m/s | ENE | 2.5 m/s
14:00 UTC | 14.0°C | 10.0°C | 0.9 m/s | NNE | 4.7 m/s
14:30 UTC | 13.7°C | 9.4°C | 2.4 m/s | NNW | 5.4 m/s

The cell then quite rapidly gain organization and move straight east into the Ljubljana basin. More or less an easterly LL winds provided some inflow towards the higher terrain (a sort of upslope flow from the Ljubljana basin towards the slopes of eastern Dynaric mountains). The cell intensified and radar quickly showed an intese core.

Looking over the closest skew-t diagram, Udine 12z, there was more than 40kt deep layer shear and a few hundreds J/kg of CAPE (Udine skew-t was modified with SFC data from Ljubljana at 13 UTC). Notice also the SREH3, clearly being supportive for rotating updrafts.

It has to be noted that Udine lies around 80km west of Ljubljana basin where supercell occured, not too far. Its on the western side of Dynaric mountain range. But as seen on morning model run, airmass was expected to be more favorable for storms east of the mountain range, MLCAPE was simulated to be between 400 and 700 J/kg. Although CAPE was rather poor and limiting factor according to the sounding, I believe the airmass was indeed different in central Slovenia and CAPE was higher. This is simulated MLCAPE map for 12z from WRF model, showing around 3-400 J/kg in central Slovenia:

Looks like conditions were just appropriate to develop a low topped supercell which then surprisedly remained well organized for almost 2 hours, travelling from its start NW of Ljubljana via Litija towards border with Croatia near Krško. From the animations below it can be nicely seen how the storm was an obvious right moving supercell. Pretty impressive first to be so organized and especially in this time of the year with so limited instability, although within good wind shear profiles.

This is radar animation from OSMER/Fossalon radar from NE Italy…

Here is a 3hrs accumulated rainfalls calculated from OSMER radar, nicely seen quite fast moving cell as scans are made every 10min…

I was unable to chase today, but thankfully there are several webcams in Ljubljana and I was able to spot the supercell on them. Storm was moving just a few km N of Ljubljana, so it was a good view from the city. This is the webcam from Ljubljana pointing towards NW (shot at 1340 UTC); nicely visible tilted mesocyclone with small inflow tail beneath it from the right…

This is the same camera but pointing towards N from Ljubljana (shot at 1350 UTC); impressive structure of a supercell coming into the view from the left, notice a mesocyclone with striations and large low-hanging wall cloud beneath it. Looks like it was only 2-300m AGL!

This is view towards NW from Ljubljana (shot at 1400 UTC); notice the still nice shape of the mesocyclone and RFD cutting into the wall cloud from the left!

A view from the same spot towards E, still visible lowering (wall cloud) while departuring Ljubljana basin into the hilly terrain. But this didn’t kill the storm, it managed to even intensify more as it was moving near Litija and further ESE! Refer to radar below…

This is overlay radar scans animation from ARSO-LISCA location where google maps is placed as a layer…

Some locations reported very intense rain with the storm as well as some marginally lrge hail 1-2cm in diameter, which is quite surprising for this time of the year, though for sure not surprising at all it we look on the radar scans. Here is a radar grab when the storm crossed main road Litija – Zagorje and hail falling was reported:

Additional view from Croatian radar BILOGORA, animation of storm travelling across central Slovenia…

A view from the VIS satellite at 1415 UTC

Some interesting scans… LISCA and OSMER radar and lightning activity along the moving storm. Impressive to see such a high reflectivity at this time of the year! On the bottom left scan you can actually see sort of a hook echo shape!

Additionally, attaching again Udine 12z sounding as well as Zagreb 12z, both locations are in radius 70-80km from the storm’s location. Impressive wind profiles and shear!

My fellow chaser Jure Atanackov was thankfully in Ljubljana, so he managed to jump on a short chase around the city and brough some photos of the structure, here are few of them.

This shot showing a nicely striated mesocyclone with large wall cloud beneath it…

On this photo which was shot on the road from Ljubljana towards Trzin, one can clearly notice the rotation and textbook RFD cut from the left. How impressive is that for November! Thanks to Jure to be there in perfect time!

Further east clear view over the structure, RFD cutting into the wall cloud, Jure described it was quite rapidly rotating and might be even a brief funnel cloud in there; we marked it with an arrow. Unfortunatelly there is no animation of the movement, so we could see it better, but anyway impressive structure for such a time of the year at least to say. Actually, wouldn’t shame such a structured supercell even during summer!

So at the end we could easily say season 2012 doesn’t want to end yet! We’re already 2 months far from the summer end and nature still brings surprises! This event was not extreme by its severity, but for sure was an extremely rare event for this time of the year here, it has to be repeated once again!

Stay tuned for future blog updates.