Storm Leslie batters parts of Western Europe

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The extratropical cyclone known as Storm Leslie brought hurricane force winds and severe floods to parts of Portugal, Spain and France in mid-October 2018.

Date & Time
13 Oct 00:00 UTC–15 Oct 12:00 UTC
Airmass RGB, ECMWF Significant Wave Height, ECMWF Total Water Column

By Ivan Smiljanic (SCISYS), Ian Mills (Ian Mills Consulting) and Sancha Lancaster (Pactum)

Leslie was the strongest extratropical cyclone strike the Iberian Peninsula since 1842. Leslie, a large, long-lived, erratic tropical cyclone, was the 12th named storm and sixth hurricane of the 2018 Atlantic hurricane season.

Figure 1
Figure 1: Storm Leslie's journey from 23 Sept 12:00 UTC to 13 Oct 18:00 UTC.

For few weeks in fact this low pressure system (hurricane Leslie) moved around central North Atlantic before it was picked up by a jet stream that guided it towards Europe. Figure 1 shows the position of the system during its life cycle, where the very straight path it took the last few days indicates the influence of the jet stream.

Although, Leslie was no longer a hurricane by the time it had travelled across the Atlantic, it was still a storm-force extratropical cyclone, with winds of 110 km/h (70 mph).

In advance of Leslie, IPMA issued red warnings for high winds or dangerous coastal conditions for 13 out of its 18 districts, including the capital Lisbon. On 13 October at 21:10 UTC, Leslie made landfall in Figueira da Foz, where winds up to 175 km/h (109 mph) were recorded.

The storm brought heavy rains and strong waves across the country, leaving 324,000 homes without power, more than 60 people had to be evacuated and two people were killed.

Looking at the moisture content and wave height estimations, only a few hours before the landfall, it becomes obvious why the heavy rain and winds/waves episodes contributed greatly to the alarming situation (Figures 2 and 3).

Figure 2: ECMWF Total Column Water estimate, 13 Oct 18:00 UTC
Figure 3: ECMWF Significant Wave Height estimate, 13 Oct 18:00 UTC

On 15 October moisture from Leslie's dissipating extratropical remnant fed a quasi-stationary front over southwestern France, generating the conditions favourable for intense rain events, leading to flash flooding in the area. Carcassonne had 160–180 mm of rain in five hours and at least 14 people were reported to have died, mainly in Villegailhenc, Aude.

The last phase of this hurricane, together with landfall and the transition of the disturbance towards France, is covered by the Airmass RGB animated imagery (Figure 4). It is obvious that the system became more disorganised after the landfall — reducing in strength and being advected towards the east as a typical low pressure disturbance.

Figure 4: Meteosat-11 Airmass RGB animation, 13 Oct 00:00 UTC–15 Oct 12:00 UTC

The Airmass RGB animation (Figure 5) from 14 October 18:00 UTC to 15 October 06:00 UTC, shows a bright white cloud band moving northeast from northeast Spain into southwest France. This front brought the heavy rain to southwest France, causing severe flooding.

Figure 5: Meteosat-11 Airmass RGB animation, 14 Oct 18:00 UTC–15 Oct 06:00 UTC

Although there were some signs of large convective cloud embedded in this cloud band contributing to the heavy rain (Figure 6), the CAPE product from ECMWF’s model (Figure 7) showed only modest levels of CAPE over southwest France. There must have been additional factors which contributed to the high rainfall totals.

Figure 6: Met-11 Airmass RGB, 14 Oct, 23:45 UTC
Figure 7: ECMWF CAPE model, 15 Oct, 03:00 UTC

Comparing two different rainfall products from NWCSAF (Figure 8) shows that stratiform rain was the de facto cause for the floods in southwestern France. Convective development was relatively shallow and that is why the convective rainfall product was signalling marginal rainfall accumulations in that area.

Image comparison
NWCSAF Precipitating Clouds overlaid with CCR, background SEVIRI IR10.8 channel NWCSAF Precipitating Clouds, background SEVIRI IR10.8 channel
Figure 8: Comparison of two NWCSAF products - Precipitating Clouds and Convective Rainfall Rate, with the SEVIRI IR10.8 image in the background, 15 Oct, 06:00 UTC

This tropical cyclone was accompanied by a considerable moisture advection from the tropics, mostly in the lower levels around 850 hPa (see Figure 9). This moisture contributed very much to subsequent heavy-rain episodes in France.

Figure 9
Figure 9: ECMWF Total Column Water overlaid with wind data at 850 hPa, 14 Oct 10:14–12:00 UTC

The 2 m Dew Point and wind from the ECMWF model (Figure 10) shows very high dew point air being fed in from the Mediterranean on the southeasterly low level winds. This would mean high levels of moisture moved across southwest France. As long as this flow persisted it would continue to be feed this moisture laden air over the land.

This low level moist air was forced to rise as it met the high ground in south west France — forming low level cloud.

Figure 10: 2 m Dew Point and wind from the ECMWF model
Figure 11: Met-11 infrared, 15 Oct, 00:30 UTC

As the frontal cloud moved over this low level flow of moist air, as seen on the Meteosat-11 infrared image (Figure 11), the precipitation from the medium level cloud would fall through the lower level cloud sweeping up the smaller droplets of rain or drizzle and, hence, increase the droplet size dramatically, leading to heavy rain at the surface. This effect, known as the seeder-feeder mechanism, can dramatically increase rainfall amounts and lead to flash floods.

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