Over 3.5 inches (90mm) of rain fell in Cambridge during Sunday 21st October, causing widespread flooding in the city. The rain was brought about by a small depression which had drifted north from France overnight on the 20th, and had drifted northwards across the eastern parts of England and Scotland during the 21st. This small depression was part of a larger scale feature lying to the southwest of the UK, over the Atlantic Ocean. This low pressure area has been present for a few weeks now, and has been responsible for the mild southerly airflow that has covered the UK and Ireland this month.

One of the downsides to mild southerly winds at this, or any, time of the year is the amount of moisture they contain, as the winds travel over the relatively warm seas. The air has been coming from a long way south at times, and mainly from the ocean rather than the land. This has meant that some places have seen a large amount of rainfall, especially in the west (where it is usually wettest anyway). The totals recorded in Cambridge are nearly twice their October mean monthly rainfall.

Cambridge wasn't the only place to see very heavy rain. The rain, which moved northwestards as a fairly large band about 100km wide, and stretched from north to south right up the eastern half of England and later Scotland, brought a month's worth of rainfall to many other parts of the East Midlands and southeast England, causing widespread flooding and much damage to properties. The M11 motorway was badly affected, causing huge tailbacks as traffic had to pass through flooded roads in single file in places. Many places saw over an inch of rain, but the rain was heaviest in the Cambridge area.

It was only last October that everyone was talking about constant heavy rain and flooding, brought about by a similar setup to the current conditions. Last year it was central southern parts of England that were generally worst hit, but this year the normally dry southeast has borne the brunt of the rain. Until Sunday it hadn't been that wet in the southeast, but the monthly statistics will tell a very different story now.