Fallen, broken tree in a park. Credit: RenegadeStudio

Strong low level winds in Portugal

22 December 2009 22:00 UTC–23 December 15:00 UTC

Fallen, broken tree in a park. Credit: RenegadeStudio
Fallen, broken tree in a park. Credit: RenegadeStudio

A rapid cyclogenesis came from the Atlantic and entered Portugal during the night 22-23 December (with deepening stronger than 20hPa/24 hr).

Last Updated

31 May 2022

Published on

22 December 2009

By Javier García-Pereda (AEMET)

The storm caused important damages (several people wounded, damages in houses and infrastructures, big trees fallen) along a narrow coastal area between the Tagus Mouth and Cabo Carvoeiro, moving north-east into the inner country. The damaged areas were all located south of the centre of the low, with a minimum observed surface pressure of 969.4 hPa at Cabo Carvoeiro at 04:20 UTC, 23 December.

Strong Low Level Winds in Portugal
Figure 1: Meteosat-9 IR, 23 December 00:00-23:00 UTC

For the above described case, the Nowcasting SAF (Version 2010) software has been run for the days 22–23 December 2009 to show its possibilities for the watch and warning of dangerous strong wind situations.

A larger difference than the default one has been permitted between the HRW wind components and the ECMWF wind guess used for their calculation, up to 144 km/h through configuration parameters LLAG_HRV = ELAG_HRV = 72; LLAG_IR = ELAG_IR = 24).

Filtering out the HRW output, and considering only the strongest winds in the very low layer (850-1000hPa), a narrow area of hurricane force winds (ie 15-minute mean winds with speeds between 125 and 150km/h) is detected coming from the Atlantic Ocean into Portugal, related to the area where the main damages in Portugal occurred.

Hurricane force winds at this layer (850–1000hPa) caused by the pass of a prior low during the afternoon of 22 December, can also be seen from the Strait of Gibraltar eastwards throughout all the Spanish Mediterranean, including the southern part of the sea area of Cabrera where Category II hurricane force winds (15 minute mean winds stronger than 154km/h) were measured by HRW product.

Only two surface wind observations were available for the study region in Portugal, with wind gusts around 140km/h: Cabo Carvoeiro and Dois Portos.

A later analysis of surface winds , considering the extrapolation of Doppler Radar winds by the Portuguese Instituto de Meteorologia, shows small areas within the study region with 10m maximum winds stronger than 150km/h and 200km/h around 04:30 UTC, 23 December.

Strong Low Level Winds in Portugal
Figure 2: Map of affected area
Strong Low Level Winds in Portugal
Figure 3: Meteosat-9 IR image with winds overlaid
Strong Low Level Winds in Portugal
Figure 4: zoomed-in image shows maximum winds stronger than 150km/h and 200km/h

The very low layer hurricane force wind areas in Portugal and the Mediterranean, shown by NWCSAF/HRW product, were not identified by the ECMWF model wind guess used by the algorithm. Throughout both days, the wind fields shown by the model guess were clearly weaker than the winds shown by HRW product.

ECMWF model only shows 925hPa hurricane force winds in a small area in the southern flank of the low at 23 December 00:00 UTC, with a reduction in the wind force and a more southerly location of the wind maximums at 06:00 UTC (with maximum winds located over the Gulf of Cádiz instead of the described area in Portugal).

Because of this, while no hint can be extracted from the model about the areas suffering the strongest winds in Portugal, they are clearly detected with HRW output.

Strong Low Level Winds in Portugal
Figure 5: Model HRW winds

Considering the wind fields in the Mediterranean the afternoon and evening of 22 December, the hurricane force winds at the layer 850–1000hPa shown by HRW product throughout all the sea areas between the Strait of Gibraltar and the Balearic Islands are much stronger than the corresponding ECMWF 925hPa wind fields, which only show maximum wind speeds around 110km/h at 18:00 UTC on 22 December, and smaller to 100km/h at all other times.

Because of this, HRW product can be very helpful in the nowcasting of dangerous wind situations, beyond the possibilities of the NWP models used for HRW calculation.