Updated 22nd April
28th April - 20th May
Temperatures will come out within 0.5C of the 1981-2010 average in most parts of the country, though temperatures are likely to be slightly below average overall in eastern parts of England, and slightly above in Northern Ireland and western parts of Scotland.
Rainfall totals will be above average in most parts of England, Wales and south-eastern Scotland, with excesses generally between 20 and 50%, but some parts of the Midlands and southern England may have larger excesses, of 50-100%. Most of Scotland and Northern Ireland will be drier than average, with deficits of up to 50% in western and northern Scotland.
Sunshine totals are likely to be up to 20% above average in western Scotland, but up to 30% below in central, southern and eastern parts of England and a slight shortage is also expected in eastern Scotland. Intervening areas of the country will have close to average sunshine.
Monday 28th April - Sunday 4th May
A complex synoptic weather pattern will arise early in this week, with high pressure over Greenland and low pressure developing over Scandinavia, bringing cold Arctic air masses southwards to the north of the British Isles, but in the meantime, low pressure systems close to the south-west of the British Isles will bring relatively warm air masses up from the south-east.
Between the 28th and 30th, it will be unsettled over most of the British Isles, with showers and longer spells of rain. Persistent rain belts are most likely to become slow-moving in the Midlands, northern England and southern Scotland, and also central parts of Ireland, where temperatures will be rather below average by day, but above average by night. Southern parts of England and Wales will have more in the way of sunshine, mixed with slow-moving downpours, some of which will be heavy and thundery, and here temperatures will generally be slightly above average. The driest weather will be reserved for northern Scotland, and there will be plenty of sunshine for north-western Scotland, but north-eastern Scotland is likely to be relatively cloudy due to winds off the North Sea. Temperatures in northern Scotland will be slightly below average.
Between the 1st and 4th, the low over Scandinavia and high over Greenland will increasingly send colder air masses down from the north. It will turn less wet in the southern two-thirds of the country as a result, but it will still remain generally unsettled, particularly in eastern parts of the country, with low pressure often close by in the North Sea. Eastern parts of Scotland and England will continue to be affected by rain belts at times, with some brighter, showery interludes in between, but there will be a fair amount of sunshine at times in western parts of the country, accompanied by fewer showers. The cold Arctic air is unlikely to take hold strongly enough to bring lowland snowfalls although there will be snow showers in the Scottish Highlands.
It will be wetter and cloudier than average during this period in most parts of the country, especially so in eastern and southern parts of England, but western Scotland and Northern Ireland are likely to be both drier and sunnier than average, particularly in north-western Scotland. Temperatures will generally be slightly below average.
Monday 5th May - Sunday 11th May
This period will start with low pressure over Scandinavia and the North Sea and a weak belt of high pressure in the eastern Atlantic, which will maintain cool and showery conditions in the east of Britain and some dry sunny weather in the west, but I expect pressure to fall to the west of the British Isles, which will allow low pressure systems to head eastwards from the 6th/7th onwards, and maintain the generally unsettled theme. The lows will track a bit further north than in the previous period and this means that all parts of the country will have some periods of wet weather associated with frontal systems, but it will not be as wet in the southern half of England. There will be some brighter showery interludes in between the rain belts, and some of the showers may be heavy and thundery.
Temperatures are likely to recover slightly above the long-term average during this period, but it will remain cloudier and wetter than average in most parts of the country.
Monday 12th May - Sunday 18th May
The weather is likely to turn quieter due to high pressure ridging north-eastwards from the Azores region towards the middle of May, although there is considerable uncertainty over where the high will end up and how strong it will be. I expect that highest pressure will be located mainly to the west of the British Isles, with low pressure over central Europe and Scandinavia, which will result in relatively cool conditions spreading down from the north, and the driest and sunniest weather will consequently be reserved for western parts of the British Isles. Southern, eastern and central parts of England will be most prone to cloudy and damp conditions associated with the low pressure to the south and southeast of the country.
As a result I am expecting this period to be drier and sunnier than average in most western parts of the country, and it will be warmer than average in most parts of Scotland, Wales and Northern Ireland, but most central and eastern parts of England will have close to or slightly below average temperatures, and while it will most likely be drier than average, sunshine amounts will be near or slightly below average.