Introduction
On 4th October 2007 at 1530 UTC, a large MCS (Mesoscale Convective System), passed over the Balearic Island of Mallorca. Associated to the MCS an F2 tornado was reported. The MCS was accompanied by severe wind gusts that spread the damage in numerous areas of the island.
Figure 1.1: Huge damage in Mallorca streets and highways as a result of the F2 Tornado and severe winds gusts from the MCS itself
The longevity of the MCS was quite remarkable (approx. 12 hours!) , and in some stages of its life cycle, it showed a fast movement, taking a very short time, less than 1 hour, to cross Mallorca island. To show this please have a look at the below illustration of the path of the MCS. Between 07:45 and 10:45 the MCS stay almost stationary before it takes a rapid NE course leaving the Forecasters with almost no time to warn.
Figure 1.2: MCS approximate track and relevant times (UTC). Verify the irregularity of the velocity of the MCS. For example, in the first phase (probably steered by a supercell embedded in the southern flank) the MCS takes more than three times the time employed to cross Mallorca Island, for travelling a similar distance. Also, in this first stage, the direction of movement is not following the steering winds.
Surface wind gusts in excess of 60 knots where registered in a couple of Mallorca weather stations, and also precipitation rates higher than 20 millimetres in 20 minutes in Palma de Mallorca and other spots of the Island.
The convective system initiated at 0700 UTC, in front of Palos Cape, as a Supercell storm, in south-eastern Spain (Murcia region), producing heavy rain along most of its way. It maintained its supercell character until 11:20 - 11:40 UTC approximately; then progressed to NE, crossed Mallorca between 15:30 and 16:15 and reached its dissipating stage by 19:00 UTC, off the northern Catalonian coast.
The social and economic impact of the event was enormous, and can be assessed from many sources, including the internet, newspapers, etc., but this is not the purpose of this case study. This work tries to analyse the meteorological situation from observations and NWP products, and to emphasize the key factors that, according to the authors, could have played a major role when trying to understand the severity of the event. Another aim of the study is to make available to European forecasters as much material as possible, facilitating further research.
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Surface Weather Observations
In this section we will try to summarize the impact of MCS arrival to Mallorca Island in terms of available surface weather reports, including synops, metars, and automatic weather stations observations. Finally, we include a subjective analysis of MCS gust front isochrones while passing over the island.
Synop observations at 15 UTC
Surface Observations over the Balearic Islands show a clearly disturbed mesoscale wind field, with 20 kt westerly winds over Mallorca, in place where easterly winds were forecasted for this time. Only the observation over Menorca (the most eastern Island) is consistent with the 12 UTC 3 hours forecasts. By this time, a tornadic bow echo (see radar chapter) still over the sea was about reaching Palma de Mallorca city, (it did by 15:30 UTC). This lets us think that from the location of the two wind plots over the island, to the MCS gust front, there should be a marked wind speed gradient, not resolved by observation network, but confirmed by the more frequent metar and speci reports at Palma de Mallorca airport, and also by authomatic weather stations. Check that the rest of the environment in the image is not windy at all, stressing the fact that the commented wind field is purely a mesoscale feature promoted by MCS activity, and not forecasted by operational available models.
Metar and Speci reports at Palma de Mallorca airport issued from 15 to 16 UTC
Going to a deeper detail, by zooming into Palma de Mallorca airport, we can easily follow surface weather impact of MCS gust front while passing over it. Notice the sudden increase in wind speed and associated gustiness at 15:30 , and also the marked wind direction shift, from easterly to westerly. The opposite change take place at 16:00 as a result of mesoscale feature leaving the airport. Notice also the sharpness of gust front main core of values above 50 kt, that takes around 10 minutes, from 15:30 to 15:39, to cross the airport.
Subjective isochrones meso-analysis
This analysis is provided by Agustí Jansà (AEMET) who is delegate at the Balearic Islands and gives a quick review of the impact of MCS passage over Mallorca (green and red spots represent damaged electric power towers). Despite we have only observations over land, is not hard to imagine that MCS activity in terms of wind, should have been similar during its whole path, from the beginning off Murcia coast to the end of its life cycle.