Archive for the ‘Europe case studies’ Category

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.

Reanalysis of European 2011/12 winter period

Winter is officially over, so it is time to have a look at the European 2011/12 winter patterns.

 

The most monitored area is the ENSO region. After the King La Nina (I refer to it as the “King La Nina”, because it was really something else than a normal La Nina) in 10/11 winter, there was a short period of neutral values in the ENSO 3.4 region.

This chart shows the actual sea surface temperature. I have marked the ENSO areas (regions):
Blue – ENSO 3 (Recently ENSO3 warmed up quite a bit in the east part)
Violet – ENSO 4
Red – ENSO 3.4 (The main ENSO region, where the cycles are determined)

In this graph (ENSO 3.4 SST Anomaly) we can see the 07/08 La Nina, the cold ENSO Neutral in 09, the El Nino in 09/10, King La Nina 10/11 and a weaker La Nina in the 11/12 winter. This years La Nina was a bit different than last year. It was weaker, more short lived and it had a slightly different effect, as I will show later on.

Important to point out, is also the ENSO3 region, with interesting dynamics in the last month or so.

The 30-day Southern Oscillation index. There is a well visible difference between 10/11 La Nina and 11/12 La Nina, with much less positive values with the recent La Nina. Values above 8 are in favor of a La Nina, while values below -8 are in favor of an El Nino.

The SOI positive values from Jul 2010 to Apr 2011 were quite outstanding. If we look at the surface pressure anomalies, we can see why. MSLP difference between Darwin and Tahiti was quite big, resulting in a prolonged strongly positive SOI anomaly.

 

If I focus my attention on the patterns.
As we can see this March, a persistent “death ridge” is present over much of Continental Europe. This composite shows the 500hpa height anomaly from 1st to 16th March:

Interesting to point out here, is the that this pattern seems to be more common in El Nino years. This is a composite of March pattern in some El Nino years:

Its hard to say, but maybe there could be a connection between the dynamic ENSO3 region in the last 2 months, going up into positive values on the limit of an El Nino. The heavy rainfall in Texas and Oklahoma in USA, kinda corresponds with situations during a warm ENSO3 region.

 

Now that the March reanalysis is out of the way, I am going look back at the start of the pattern, in October-November. La Nina came in effect around August/September, so considering the 2-3 month Lag for Europe, It probably started to reflect in the before mentioned time span of Oct-Nov.

First lets have a look at October. This is a composite, featuring years with similar La Nina/ENSO development.

And October 2011:

I think you see the connection here.

Next up is November. In November we had a persistent blocking over much of Europe, with the jet stream diverted up to north, around the strong ridge.

I had to look into older La Nina years, to find a similar blocking pattern in November (although less intense).

December had a big turn, with the ridge finally weakening and the zonal flow taking over.

Finding a pattern like this, was a bit easier. I have found it in La Nina years of course, this time in the years, featuring a bit stronger La Nina events.

January to February period was also not that easy to find, but I have found it in range of fading/weak La Nina, or strong neutral negative ENSO.

Now, we all remember the cold shot in February, which was mostly a result of an SSW event and Polar blocking, reversing the AO into negative, and featuring a zonal reversal over the continent.
I would like to point out, that in jan/feb 1986 and 2006 there was a fading La Nina + SSW combination, like this year.

The cold period in February really looks brutal. And actually, it really was brutal.

2006 and 1986 were similar years with the same setup: Weaker La Nina and an SSW event. The outcome was quite similar too. A cold shot over continental Europe.

To wrap up the winter reanalysis, here are some temperature composites. The official winter period for reanalysis is DJF – December, January and February.

January was quite average over Europe. It stared off with above average anomaly, but the temperatures started falling later in the month. The last week of January was the start of the cold period, and overall resulting in normalization of the monthly temperature anomaly.

February started with the very cold period. Towards the end of the month, temperature anomaly was slightly above average. But monthly anomaly is still below normal because of the strong cold shot in the first half of the month.

Temperature wise, the final outcome was an average winter, a bit cooler in North Africa and SE Europe.

This winter reanalysis is quite superficial, and with all this composites, I have to point out a few things. Basically, as we can see, this year there was a combination of many patterns, of which all happened in the past before, with the same thing in common: A weak/moderate La Nina. And also worth mentioning is the obvious inter-event variability of La Nina. Basically all the composites that I made as a comparison to this year, featured different variations of possible La Nina patterns. But we have to consider that ENSO is not the only thing that effects the patterns. We have the PDO, AMO, MJO, IOD, QBO, WPO, EPO, PNA, ITCZ, etc… And other oscillations and factors. But keep in mind this: PDO, ENSO, MJO, IOD,… are in a way dominant to AO, AAO and NAO. Considering the AO and NAO as “tools” of the main oscillations and other big factors.

Stay tuned for further updates on global weather! More details on the latest pattern will be published soon.