Warm, changeable in the NW, fine spells in the E
May 2013

High pressure will be persistent over central Europe during the first half of May, but it will often be far enough to the south to allow weak weather fronts to head into Britain from the North Atlantic Ocean. The 3rd will have an active rain belt over Scotland and Ireland, which will slowly edge southwards into Wales and northern England late in the day. The 4th and 5th will be mostly cloudy with a little drizzle for western areas, thanks to slow-moving fronts, although some sunny spells will break through in the northern half of England late on the 4th. However, the 6th, the Bank Holiday Monday, will be dry and sunny and warm over most parts of the country thanks to southerly winds, and temperatures will rise to 20-22C in some central and southern parts of England.

Between the 7th and 9th, a new low pressure system will move in off the North Atlantic and will bring a belt of rain eastwards on the 7th, followed by cooler brighter weather on the 8th and 9th with some scattered showers, these mostly in Scotland and northern England on the 8th, and in eastern areas on the 9th. A fine spell of weather is expected in most places between the 10th and 14th as high pressure is expected to build over eastern England and into the North Sea, with dry sunny weather and temperatures mostly between 16 and 20C, though the northern half of Scotland will remain prone to frontal systems moving in off the Atlantic and bringing cloudy skies and outbreaks of mostly light rain.

The third week of May is likely to be cool and showery with low pressure set to drift south-eastwards from Iceland towards northern Scotland. Fronts are likely to bring a breakdown around the 15th-17th, with some thunderstorms possible towards the south-east. The weather from the 18th-21st will most likely end up cool, bright and showery.

Into the last third of May, low pressure over Iceland will probably be prominent, and belts of rain will periodically fire in off the North Atlantic Ocean, particularly affecting Scotland and Northern Ireland. Rainfall amounts will often be small in eastern and southern parts of England, and temperatures are likely to be near or a little above normal. A cooler bright showery episode is likely at the end of the month as high pressure builds from the south-west.

Overview
May 2013 is likely to be a fairly warm month, with a Central England Temperature of 12.7C. Temperatures will be between 0.5 and 1.5C above the 1981-2010 average across the British Isles, with East Anglia likely to see the greatest positive temperature anomalies.

Rainfall totals will generally be above normal over western, central and northern Scotland and also over Cumbria and north Lancashire, with excesses of 50-80% in south-west Scotland. In contrast much of eastern Scotland and eastern and central England will have a dry month with rainfall 20-50% below normal.

Sunshine anomalies will generally mirror the rainfall anomalies. Much of eastern and central England will have a slight excess, of around 10-20%, but there will be a general shortage of 10-20% over the western third of Britain, and 20-30% over western parts of Scotland.

Forecast issued by Ian Simpson (aka TWS) on the 3rd May 2013.