Windstorm Friedhelm

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Posting Date: December 9, 2011, 1:15:00 PM

Current Situation Windstorm Friedhelm has passed into Scandinavia today after wreaking havoc across Ireland and the UK yesterday with hurricane-force gusts and heavy precipitation. The UK Met Office issued a Red Alert for the region, its strongest warning, characterizing the storm as the worst to hit the northern UK in ten years. The storm’s impact was felt most heavily in parts of northern England and throughout Scotland, where the strongest winds were measured—gusts as high as 165 miles per hour. Rail companies currently are operating with reduced services and imposing 50 mph speed restrictions following yesterday’s many disruptions. Fallen trees and debris have caused widespread road blockages, but at this time warnings to motorists to avoid travel are being lifted. High-sided and heavy-goods vehicles have been of particular concern, as fierce crosswinds yesterday toppled several vehicles, also adding to road closures. Many schools remained closed today because of the extreme weather, while hundreds were closed yesterday. Localized flooding remains a danger, and the Environment Agency has issued flood warnings for 12 separate lakes and rivers in Cumbria. Storm Development Windstorm Friedhelm began as a depression over the northern Atlantic, but as it approached the British Isles, it experienced a rapid drop of pressure that caused its wind speeds to soar, reaching in places the maximum of “Force 12” on the Beaufort scale. This kind of rapid deepening of a low pressure system—when pressure falls by as much as 24 millibars within 24 hours—is also known as a “weather bomb.” In Friedhelm’s case the pressure dropped by 44 mb as it approached land, eventually hitting a low of just 944 mb on Thursday at midday. Satellite imagery indicates the occurrence of a sting jet, a localized strong downdraft that can develop on the southern flank of an extratropical cyclone and bring cold, dry air from the mid-troposphere down to the surface. Sting jets generally cause high, localized wind speeds, and this development may be responsible for the very high gust speeds (of 165 mph) recorded at Cairngorm in Scotland. These speeds are just shy of the highest wind speeds ever recorded in the UK, which was 173 mph on March 20th 1986, 25 years ago. Maximum wind speeds in more heavily populated areas typically reached gust speeds as high as 80 to 90 mph. Building Stock: Residential building stock in Scotland is predominantly of masonry construction. For commercial exposures, however, the construction type is approximately 50% masonry with the remaining construction split between steel frame and reinforced concrete. Little structural damage to these construction types is expected for windspeeds of the order widely experienced from Friedhelm, although damage to cladding, signage, and some isolated roof covering damage could occur. Reported Impacts Reports of structural damage are few at present and in any case the level of damage is not expected to be severe. While structural damage may be low, reports of minor superficial damage to cladding, store display windows, and signs have been common. Power outages, however, were extensive, with an estimated 70,000 homes losing power during the storm—roughly two thirds of which (50,000 homes) are still without electricity. Also, a 300-foot wind turbine, located at Ardrossan, North Ayrshire, was badly damaged by the storm when its locks failed; left to the mercy of the strong winds, it exploded. Turbines typically are locked during high-wind events, both to avoid damage to the turbines and to avoid causing uncontrollable power surges through the national grid. Forecast At present Winterstorm Friedhelm is moving across the North Sea, bringing high sea levels and the threat of coastal flooding to parts of Scandinavia before it eventually dissipates. Friedhelm comes on the trail of windstorms Xaver (also called Berit) and Yoda, which impacted the region in late November. This clustering effect of windstorms in Europe—storms following closely on one another—is a well-studied phenomenon and is explicitly accounted for in the AIR Extratropical Cyclone Model for Europe.

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Posting Date: December 9, 2011, 1:15:00 PM