Status:Ongoing analysis Material Status: Finalised Material from: Linus, Fernando
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, Tim
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Discussed in the following Daily reports: |
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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.
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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
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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.
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4. Experience from general performance/other cases
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5. Good and bad aspects of the forecasts for the event
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