# Severe Convection with Tornado over Mallorca - 4 October 2007

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## Chapter V: Conclusions

### Table of Contents

- Chapter V: Conclusions
- Conclusions

### Conclusions

Main conclusions, key ingredients and relevant considerations related to this severe weather episode are presented below

#### 1 - Causes of main MCS initiation and first stages peak intensity

- The setting of a low level mesoscale cyclonic circulation, off Murcia coast (see mesoscale overlays), before MCS initial stage, which was quite well forecasted by ECMWF model in time and place.
- The presence of an upper level vorticity maxima collocated nearly in place, just a bit to the west, above the before mentioned low level circulation, close to the left exit region of an upper level southerly jet (see chapter 2.1 - WV6.2 animation).
- High values of low and deep layer shear together with high values of storm relative helicity in the area of initiation, that helped the system to develop rotation.
- CAPE values above 800 J/kg for a quite deep layer in the easterly low level stream entering the cyclonic circulation, together with non buoyancy driven vertical accelerations, helped the convection to be very intense and particularly deep (see chapter 2.1 - Severe Storms RGB) in these first stages.

#### 2 - The onset of supercell phase: impact on model performance

- It is believed by the authors, that first stages of the MCS (from 08 to 11 UTC) were in supercell form, more specifically, a supercell with a quite slow right mover character, that produced very large amounts of accumulated precipitation during its 3 hours of life (see exercise 4).
- Although no doppler information was available, other radar and satellite signatures, together with background experience, support further this hypothesis.
- It is also believed that because the anomalous propagation to the right, operational numerical models lost track of the system from this initial stage onwards, therefore reproducing most of its impact (including precipitation) well to the west from where it really happened, leaving the forecasters with no reliable model guidance, not in terms of precipitation, neither in terms of surface wind.

#### 3 - MCS unexpected track and reasons for longevity

- After the MCS left its supercell stage, it got catched and steered by the mid level flow and ended up in Mallorca island, by 15:30 UTC, having followed a path clearly shifted to the east with respect to operational forecasts, mainly because the right moving character of initial phase.
- Also, a not enough accurate short range forecast of upper level jet position (see chapter 2.1 - WV6.2 animation) could have played an important role in reducing the accuracy of the forecasted MCS track.
- Reasons for MCS longevity can be derived from the presence of a well defined low level thermal boundary (see mesoscale overlays) attached and to the east of MCS track. This thermal boundary, very importantly, was nearly parallel to the mid level flow, allowing the system no to get cut off from the low level ageostrophic moist and warm easterly advection during its whole track. Also, the boundary, acted as a "slider" preventing the MCS from entering the large CIN area to the eastern side of it (see comments below fig. 2.1 in Chapter 2).

#### 4 - Tornadic nature of MCS at its arrival in Mallorca

- Although we are somehow blind to see the insides of the system when arriving in Mallorca, because of the lack of nearby radar information, the radar structure of the cell by this time, as shown by the distant Barcelona radar, is very similar to a bow echo (meteograms at Santa Ponça station helped us to support this idea; see exercise 1)
- The tornado itself, could have been produced or favoured by interaction of the leading edge of MCS gust front, with a deep, sharply defined low level moisture ridge (see exercise 7) that provided the large buoyancy / low NCA environment usually needed for tornadogenesis.
- Speculations on prior rotation provided by book-end vortices in the bow echo structure or baroclinically generated rotation derived from the sharp thermal and moisture gradients, can be made, but are difficult to demonstrate, because of the lack of nearby radar wind derived information.

#### 5 - Utility of proximity forecast soundings and hodographs information

- The analysis of vertical profiles shown in section 4.3, and the evolution of them over Palma de Mallorca, from 12 to 15 UTC, has proved to be quite valuable, although not conclusive, for the assessment of tornado potential, showing clearly favourable tendencies of the main convective parameters commonly used for this type of assessment, although, we again want to remark, this can never be taken as conclusive.
- Also the 3 hours evolution of the hodographs over Palma seems to be quite of help in the assessment of kinematic parameters tendencies, specially in the task of the assessment of source levels with large rotation potential (large streamwise vorticity values), which in this case were coincident with levels with very large potential buoyancies.