In early June 2019 Storm Miguel brought high winds and heavy rain to a number of countries in Western Europe.
By Ivan Smiljanic (SCISYS) and Sancha Lancaster (Pactum)
On 5 June, AEMET, the Spanish Meteorological Agency, named a rapidly developing low pressure system as Storm Miguel, it was the 23rd named storm of the 2018/19 European windstorm season. Part of north west Spain and Portugal were battered by heavy rain, high winds and large waves. Winds of up to 147km/h hit northern Spain, then the storm swirled around the Bay of Biscay, before moving on to France.
On 7 June Storm Miguel produced gusts up to 80 mph (129 km/h) along the Atlantic coast of France. At Île d'Yeu, France, one of these gusts set a record for the month of June, last set in 1981.
By 8 June the tail end of the storm had hit parts of the UK (with gusts up to 65 km/h and heavy rain), the Netherlands and even as far north as Finland.
This extratropical cyclone developed in a rapid cyclogenesis process, where the initial disturbance on the polar front produces very quick formation of a deep low-pressure centre.
Figure 1: Meteosat-10 Airmass RGB with MSLP overlaid, 6 June 00:00–21:00 UTC
One of the criteria defining rapid cyclogenesis is that the pressure drop is more than 1 hPa an hour. The MSLP (Mean Sea Level Pressure) values in Figure 1, show pressure drops of more than 16 hPa in 12 hours, during the transition from the development to the advanced stage of this cyclone.
Rapid cyclogenesis normally happens at the left exit region of the upper-level jet stream (Figure 2) and it goes through three major stages:
- Development of a ‘cloud head’, sitting on the cold side of the main cloud band — the result of a cold conveyor belt split, creating typical convex-shaped cloud edge.
- Strong dry intrusion in the region between the cloud head and cloud band — the result of subsidence of stratospheric air, leading to more positive vorticity and deepening of the cyclone.
- Deepening of the low pressure centre and wrapping of the cloud bands around it (advanced stage).
These stages are well seen through animated Airmass RGB imagery (Figure 3), since this RGB product nicely shows warm and moist air masses in green shades, cold ones in blue shades, and stratospheric intrusion (e.g. tropopause folding) in red stripes.
Figure 3: Meteosat-11 Airmass RGB, 5 June 12:00 UTC–8 June 12:00 UTC
In the advanced, mature stage, when the cloud spiral is well developed and circling the cyclone's centre, parallel cloud lines can appear close to the innermost part of the cloud spiral. These cloud bands are parallel rain bands.
Although rain bands are, by their nature, more visible on radar than in satellite images, five minute rapid scanning imagery, capable of showing some cloud properties connected to rainfall rates, serve as a valid proxy.
Rapid scanning does not provide a lot of additional value for analysis of systems of that size, however, more images in a certain time period can aid in the nowcasting of individual convective cells embedded in associated cloud bands.
A good proxy for rain rates is normally the IR10.8 SEVIRI channel, which informs on the height of the thick cloud, hence its ability to produce more, or less, rain (see animation in Figure 4).
Figure 4: Meteosat-11 IR10.8 RSS, 6 June 12:00 UTC–7 June 12:00 UTC
Cyclones such as Miguel are normally associated with strong rain and wind episodes, so this cyclone was not exceptional, nor was it classified as major. However, it was unusual because it developed during a warm season, such cyclones usually form in the winter months. Also, this storm is a noteworthy example of the rapid cyclogenesis process.
- Three dead after a rescue lifeboat overturned off the French coast.
- French fishing boat missing, with crew assumed dead.
- 28,000 homes in the west of France left without electricity.
- In Jersey several trees were blown down.
- An Irish yacht crew, sailing off the Spanish coast, had to be rescued by helicopter.
Wind field at 850 hPa pressure level, overlaid over HRVIS image, reveals the intense wind episodes on the west coast of France (Figure 5).