With winds up to 100 miles an hour in places, Xynthia, the weekend storm that drowned scores of western Europeans in their houses, must have felt like a hurricane on the ground. In the air aloft, what satellite imagery shows is an extremely powerful mid-latitude cyclone.
What gave the storm such destructive power and such prodigious rains was the confluence of two strikingly different air masses -- a wave of cold, relatively dry air that is typical of the North Atlantic and a massive, concentrated flow of unusually warm moisture from the Tropics.
Piled up against the African coast in the eastern Atlantic is a wedge of sea surface temperatures that are roughly 1.5 degrees Fahrenheit above average that is spread all the way to South America. This North Atlantic storm formed far enough south to entrain this plume of extra warm moisture. The southwest-to-northeast trajectory of this plume suggests what researchers have been calling an "atmospheric river," a tropical firehose that is capable of causing flooding just about anywhere it hits the ground.
Tropical meteorologist Jeff Masters, in his Weather Underground blog on the subject, notes that "total precipitable water" in this plume was up to 300 percent above average. Writes Masters:
As this extra moisture flowed into the storm, the moisture condensed into rain, releasing the "latent heat" stored up in the water vapor (the extra energy that was originally used to evaporate the water into water vapor). This latent heat further intensified Xynthia. The storm's central pressure fell to 966 mb at the storm's peak intensity, reached at 18 GMT Saturday after it passed over Spain's northwest corner.
Come spring or no spring, as long as this lethal combination of air masses are in motion in the Atlantic, the potential for more weather extremes remains high. Masters makes another observation: "If this pool of very warm water is still around in July, it could lead to an earlier than average start to the Atlantic hurricane season."
IMAGE: Meteo-France
Tags: Floods, Meteorology, Natural Disasters, Winter
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