Updated 3rd May
Covering the period 9th May - 31st May
Mostly warm, sunny and dry, but cooler changeable last week of May
Mean temperatures, relative to the 1981-2010 reference period, will be about 1C above average in most parts of the country, with a cool last week of May partially offsetting generally warm weather earlier in the forecast period. Western Scotland may come out nearer 2C above, with south-east England closer to normal.
Rainfall totals will generally be below normal, particularly in northern and western Scotland, although some locations in Wales and western England may end up with near or above-average rainfall, where totals are swollen by thundery downpours early in the forecast period.
Sunshine totals will be above normal, particularly in western and northern Scotland, but only slightly above normal in eastern England.
Monday 9th May - Sunday 15th May
Easterly winds will be dominant across the UK during this week, with lowest pressure gradually transferring eastwards across France and into central Europe. Thus, winds will blow from a warm south-easterly direction early in the period, but gradually changing to a less warm north-easterly. The winds are also expected to become lighter later in the week.
Cloudy skies and some light rain will affect western Scotland and Northern Ireland early on the 9th, but this will slowly clear westwards. Otherwise it will be a warm and sunny day for most, with some scattered thundery showers developing across Wales and the western side of England. It will be cooler near North Sea coasts, but mainly sunny, with low cloud confined to coastal fringes. The 10th will be warm and sunny in western Scotland, but otherwise it will be a similar day to the 9th. Some thundery showers are likely to affect south-east England and inland parts of Northern Ireland, as well as Wales and western England.
The 11th is expected to be cloudier, with some outbreaks of rain and local thunder spreading from the south to affect most parts of the country, although eastern coastal areas and the north of Scotland will miss the rain. Temperatures will remain above average, but less so than on the previous two days. Between the 12th and 15th, a north-easterly breeze will bring some low cloud and drizzle in from the North Sea, this peaking on the 13th/14th, but this will be mainly confined to eastern Britain, with central and western areas remaining sunny and quite warm for the time of year. Further thundery downpours are likely to recur in Wales, western England and Northern Ireland but with the chance of thunderstorms generally declining as the week progresses.
The mean temperature during this period will be 2 to 3C above the 1981-2010 average in most parts of the UK, with western Scotland seeing unusually high maxima, but it will be just 1C above in south-east England and near to North Sea coasts.
Rainfall totals will be well below normal in Scotland and down the eastern side of England. Elsewhere, they will mostly be below normal but with considerable localised variations due to the thundery nature of the rainfall that will affect some western areas. Wales and south-west England will tend to have close to average rainfall, with locally high totals where thunderstorms hit.
Sunshine totals will be close to or a little below normal in eastern and southern England, but it will be sunnier than average elsewhere, particularly in northern Scotland.
Monday 16th May - Sunday 22nd May
This week will begin with a settled spell and a weak ridge of high pressure covering the British Isles, but the general trend will be for pressure to fall to the north as the week progresses, bringing increasingly unsettled conditions, particularly to northern parts of the UK. Temperatures will be rather above the seasonal norm early in the week, but falling close to average as cooler and less settled conditions spread from the north-west.
Despite the change to cooler and more changeable conditions, temperatures will be 1 to 2C above the long-term average in most parts of the country, with eastern coastal areas becoming warmer relative to normal as the winds will blow more frequently from a westerly or south-westerly direction, rather than an easterly. Rainfall totals will mostly be below normal, with the relatively unsettled second half of the week not producing particularly high totals. Sunshine totals will be above normal in central, southern and eastern England and in eastern Scotland, but mostly close to average elsewhere.
Rest of the month
Monday 23rd May - Tuesday 31st May
I expect this period to be quite cool and changeable with low pressure over Scandinavia bringing mainly north-westerly winds to the British Isles. Rainfall totals will tend to be above normal but not exceptionally so, with frequent bright showery weather in between the rain belts coming in off the Atlantic, and some western parts of the UK may see slightly below-average rainfall. There is, however, a fair amount of uncertainty over rainfall amounts at this stage. Temperatures will tend to be a degree or two down on the 1981-2010 average, but less than 1C below in some southern and western parts of Britain. Sunshine totals will be close to normal in most regions, but probably above normal in western Scotland and in south-western Britain.