Record-breaking storm lashes United States

Thursday 04th Nov 2010 by theWeather Club

At the end of October, the United States experienced one of the most extraordinary storms ever recorded. Although it was centred on the Upper Midwest, the storm stretched from the Gulf of Mexico to Canada and from the Rocky Mountains to the Atlantic Ocean. The NASA satellite image of the storm is extraordinary – it looks as though somebody has knocked a bottle of Tippex over a map of America.

The storm formed very quickly on 26th October – so quickly that it can be categorised as a 'bomb': the fabulous term given to a storm which drops at least one millibar of pressure per hour for 24 hours. At the storm's peak at 5:13pm on 26th, the weather station in Bigfork, Minnesota recorded barometric pressure of 955.2 millibars – the lowest pressure not associated with a hurricane ever recorded in the US.

Extratropical cyclones tend to form in the US in spring or autumn, caused by the large temperature difference between the north and south of the country. Warm air rushes from a high pressure area in the south towards a cooler, low pressure area in the north. The larger the difference in pressure between the centre and the outer edges of the weather system, the more intense the storm. With such extreme low pressure recorded in the centre of this storm, the intensity of what followed is not surprising - strong winds and heavy rain lashed the country, 61 individual tornadoes were reported over two days, and 33cm of snow fell in Dakota. More than 500 flights were grounded at Chicago O'Hare Airport.