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 Status: Finalised Material from: Linus, Fernando, Tim


1. Impact

Between 13 and 14 October the tropical cyclone Leslie make landfall in Portugal. The passage of Leslie over Portugal left 27 injured, widespread damage on buildings, thousands of falling trees and an entire  borough (Soure) without power (so serious that the power supply company is considering to ask international help). The IPMA (Portuguese weather service) claimed that the all time record of wind gust was observed in Figueira da Foz  weather station. Accordingly with the IPMA web page a gust of 176 km/h (49 m/s) was recorded at 21:40Z,  exceeding the previous maximum of 169 km/h occurred on 17 of October 2015. The same station reported gusts above 100 km/h in several occasions during 40 mins. Other places reported gusts above 100km/h, in particular, Aveiro (north of Figueira) with 120km/h and Coimbra (to east) with 122 km/h between 22:30 and 23:30 local time.

2. Description of the event

The 15-minute interval sequence of air mass of RGB composite product shows the last hours of the track of Leslie towards Portugal. A comma cloud head is visible from the pictures together with the advection of cold air mass behind (left) as the transition takes place..

Animation showing the airmass product. Valid between 13@18 to 14@00.

The exceptional winds mentioned earlier seem to be related with  the development of a sting jet at the tail of the cloud hook. This is what the IPMA colleagues are claiming thought more diagnostics is needed. Looking at the sequence of images from airmass RGB there is a hint of warm cloud top stripe along the tip of the cloud hook.

Left: Airmass RGB composite at 19:15 UTC. Right: Same as before but valid one hour later. Inset: zoom-in of the previous image showing the sting(?) jet (green) and Figueira da Foz (red arrow)

The two panels below display the airmass RGB composite and the radar maximum reflectivity (dBZ) at 21:50Z. The relative minimum of maximum reflectivity (bended red arrow) in the region of the hook suggests the present of a sting jet.

Left: Airmass RGB composite valid at 21:45 (red arrow show Figueira da Foz). Right: Maximum reflectivity radar (dBZ) valid five min later; arrows represent the sting jet (red) and Figueira da Foz (black)

The maximum gust at Coimbra (=66kts) between 2030 and 2130Z seems to tally with the cause being a sting jet. It coincides on IR imagery with the dark/dry zone that developed hours before (or possibly a mini gust front cloud band that appears within it, similar to the ones Identified by Keith Browning in the October 1987 UK storm). Meanwhile conceptual model considerations, plus radar and satellite imagery above, would ascribe the maximum gust at Figuera da Foz to the boundary region between a sting jet and a cold jet. At inland locations (eg Coimbra) SJ tends to be much stronger than CJ, whilst by coasts (eg Figuera da Foz) they can have similar strengths, due to a different destabilisation mechanism (as described in Hewson and Neu, Tellus, 2015).

3. Predictability


3.1 Data assimilation

 Until the last moment the handling of the storm by the forecast system was a challenge. The HRES model initiated on 13@18 shows initial position errors larger than 100 km when compared with the estimated position by the National Hurricane Centre (the average position error is slightly below 50 km). During the next 3 hours the forecast track deviates from the observed one by 150 km (see slide below). A quick look at the ascat winds available around this time shows there was no direct hit on the storm in the Atlantic which could benefit the re-positioning of the storm in the analysis.

MSLP forecast initiated on 13@18 valid at t+0h (left) and t+03h (right). Blue Dots indicate Leslie position at 13@18 and 13@21 estimated by NHC

3.2 HRES

The plot below show observed 24-hour wind gusts (first plot, not sure all obs are in here!) valid for 13 October 12UTC to 14 October 12UTC and HRES forecasts for 24-hour maximum wind gusts (shade) and MSLP valid 14 October 00UTC (contour). Only the HRES from 12 October 12UTC and later predicted well the landfall on Portugal.

3.3 ENS

The plots below shows EFI and SOT for 24-hour maximum wind gusts valid  on 13 October.

The plots below show the tracks (ensemble -grey, best track - black), position and intensity on 13 October 18UTC (ensemble - squares, best track - hourglass) in forecasts from 13 October 00UTC (first plot) to 4 October 00UTC (last plot).

3.4 Monthly forecasts

3.5 Comparison with other centres

4. Experience from general performance/other cases

5. Good and bad aspects of the forecasts for the event

6. Additional material

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