Severe Atlantic storm batters Ireland and UK

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A very deep area of low pressure resulted in a North Atlantic cyclone (named Alexandra) which caused massive ocean surges and brought strong winds to parts of Ireland and the UK.

Severe Atlantic storm batters Ireland and UK
Date & Time
9 December 2014 16:00 UTC–10 December 06:00 UTC
Satellites
Meteosat-10, Metop-B
Instruments
SEVIRI, ASCAT
Channels/Products
Airmass RGB, ASCAT winds

Gale force gusts of more than 129 km/h (80 mph) were recorded at Tiree in Scotland and ocean waves over 15 m (50 ft) high were recorded by the K5 buoy off the north west coast of Scotland (59.10N, 11.40W) .

The strong winds caused transport chaos and power outtages across the Northern parts of the UK.

The gales and ocean surges were caused by rapid or explosive cyclogenesis — an intense low pressure system with a central pressure that falls 24 hPa in a 24-hour period, referred to colloquially as a 'weather bomb'.

In his weather blog, ITV weather forecaster Liam Dutton explained how the jet stream caused the cyclogenesis.

Download full resolution RGB image, Meteosat-10 Airmass RGB, 9 December 16:00 UTC
Download animation, Meteosat-10 Airmass RGB, 8 Dec 10:00 UTC – 11 Dec 10:00 UTC (MP4, 17 MB)

The Airmass RGB image from 9 December 12:00 UTC (left side) shows the system had a central pressure of less than 950 hPa. The ASCAT instrument on Metop measured 60 knots (111 km/h), but the real winds were probably higher as ASCAT winds saturate at around 60 knots (higher winds than 60 knots do not produce a higher ASCAT signal).

Also striking in this image are the strong winds over the Western Mediterranean caused by Mistral winds (two storms caught in one image).

The Airmass RGB image from 10 December 06:00 UTC (right side) shows the very large fetch zone of the storm, which is ideal (conditions) for forming high (monster) waves and very large ocean surge.

Met-10/Metop-B, 09 December 2014, 12:00 UTC
Airmass RGB, with surface pressure and ASCAT winds (source: EUMeTrain)
Full Resolution
Met-10, 10 December 2014, 06:00 UTC
Airmass RGB with 10 m ECMWF model winds (source: EUMeTrain)
Full Resolution
 

Wave heights graphic

 
Plot of significant wave heights over the Atlantic

The plot (right) shows the significant wave height in the Atlantic recorded by SARAL/Altika and Jason-2 satellite altimeter passes from 15:00 UTC to 21:00 UTC on 9 December.

The colour scale ranges from 0 to 20 m, with the areas coloured in orange, brown and red showing significant wave heights of more than 10 m. Significant wave height is defined as the mean height of the biggest third of all the waves.

 
 

'Weather Bomb' - Explosive Cyclogenesis in the Atlantic, video of EUMETSAT infrared imagery (YouTube)
A closer look at ‘weather bombs’ (Met Office)
'Weather bomb' hits power and travel in northern UK (BBC News)
Huge waves crash on Orkney Islands (BBC News)

Previous cyclogenesis case studies

 
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