Katia has been called upon much earlier than expected, with the average season tracking 12 tropical storms in total. Currently a category 2 hurricane on the Saffir-Simpson scale, and with wind strengths of 96 to 110mph, she is expected to swing toward northern Britain by Monday 12th September as an extra-tropical storm. The deep and compact low pressure system, with a central pressure of around 960mb, is likely to give rise to severe gale to storm force 10 winds as it charges rapidly westward. According to the NOAA National Hurricane Center in the USA there is currently a 30 to 40% risk of storm force winds wreaking damage in the Western Isles and the North of Scotland.
Weather Logistics UK anticipates that the remnants of Katia will reach the UK from Monday morning onward. It will be combined with wave heights of up to 12 metres and storm surges on the north and west coasts, particularly on the south-east quadrant of the system. Throughout Scotland and Northern Ireland severe gales may also cause widespread structural damage to buildings, with estimated risks of 20 to 30% that sustained winds will reach storm-force and with gale force winds or stronger very likely.
An intense tropical storm in the Atlantic basin is called a hurricane, often possessing an “eye” of clear-sky that can be viewed from space-borne instruments. The tight isobars that pack around these storms are characterised by sustained wind speeds above 76 mph. Hurricanes are a prominent feature of the north-west Atlantic basin and Gulf of Mexico, where tropical disturbances gather latent heat energy from clusters of thunderstorms that infrequently make land-fall in the USA during the early summer to late autumn season. Initially forming as localised depressions, these cloud masses move northward into the sub-tropics where they can spin-up into deep low pressure systems. They are often steered in a north-westerly direction by the sub-tropical jet stream where they are fed by moisture over warm sea surface temperatures, developing in their viciousness and in their girth, with compact radial pressure gradients. In addition, spiralled bands of thundery rain develop around these tropical storm disturbances, twirling outward from the central low pressure. The resulting weather conditions consist of tornadoes and convective instability, causing gusty winds combined with thunderstorms and heavy blustery showers.
Our final seasonal weather prediction for autumn 2011 is currently on Amazon Kindle and by subscription (using the links on the right of this page). Although we expect stormy and wet weather for the British Isles during September, we anticipate that the general weather pattern will settle down dramatically during the latter part of the season with high pressure dominating from late September onward. Early signs from the global forecasting system (GFS) indicate that sunny but cool conditions will begin to set in by mid-September, with the jet stream slipping southward blocking the passage of Atlantic storm systems. From the mid-autumn period onward Weather Logistics UK predicts that hurricanes, and the remnants they leave as extra-tropical storms, will dissipate over the USA as they begin to track further southward.
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