While mean temperatures for the month were close to the 1971-2000 normal across Scotland, it was slightly colder than average in Shetland.
In common with the rest of Scotland, Shetland had a drier than average May. However, while sunshine totals were generally close to average, here it was markedly sunnier than normal.
For much of the period from the 1st to 11th, with an anticyclone to the north-west and pressure relatively low over Scandinavia, a cool north to north-easterly airflow was maintained over Scotland, bringing wintry spells across Shetland. During the 5th and 6th the high slipped a little further south, allowing a short spell with a more westerly airflow.
On the 12th the centre of high pressure returned to mid-Atlantic and temperatures recovered, as a more mobile south-west to westerly airflow became established across northern Scotland. Though high pressure developed across the UK and North Sea shortly after mid-month, the Northern Isles – on its north-western flank – remained in a moist and very mild south-westerly airflow until the 23rd.
At this point the high moved away into Continental Europe and, with the East-Greenland high ridging over Iceland, a cool northerly airflow returned to the Northern Isles. Apart from some minor breaks, as shallow disturbances moved south from the Norwegian Sea, this situation remained unchanged until the end of the month.
A cold front moved south into Scotland on the 1st, with moderate or fresh north to north-westerly winds bringing cool, cloudy conditions and mainly light, wintry showers of rain, hail, sleet and snow across the Northern Isles.
With an anticyclone persisting over the Atlantic and pressure low over Scandinavia, these conditions continued through the 2nd-3rd. Overnight, as the high slipped further south, a front approached from the Atlantic, bringing rain and drizzle accompanied by misty conditions. It became drier and clearer later on the 4th as the front cleared east.
On the 5th a shallow low drifted southeast from Iceland, bringing outbreaks of rain, misty conditions and hill fog. Light winds then freshened northerly and clearer, drier conditions arrived later in the day, as the low slipped south between Shetland and Norway.
With pressure high across Iceland and low over the southern Baltic, cool north to north-easterly winds persisted during the next few days. The 6th was cloudy with a few scattered showers and a strong north-easterly wind gusting to 40mph. The 7th was mainly dry with plenty of sunshine and lighter winds. Fair Isle was the sunniest place in the UK with 14.7 hours.
Northerly winds strengthened on the 9th as another low moved from the northern Norwegian Sea into southern Sweden. It was a cold, cloudy day with wintry showers of sleet, hail and snow.
Winds backed fresh north-westerly on the 10th as the high over Iceland transferred back into the Atlantic and another low moved south down the Norwegian coast. It was a very cold day with wintry showers, the temperature reaching only 6.0° Celsius in Lerwick and falling to below freezing during the following night.
The 11th, almost as cold, felt less so with lighter winds, but it was cloudy with further – occasionally heavy – wintry showers. The 12th was much more pleasant, being mainly dry with sunny spells, and milder, as moderate north-westerly winds backed southerly.
Cloud thickened and south-east to easterly winds freshened on the 13th, with rain spreading east later, as the high to the west moved closer to the Azores and a depression near Iceland pushed fronts towards Shetland.
With this low tracking south-east to become slow-moving off north-west Scotland, the Northern Isles fared reasonably well as, with mainly south to south-westerly winds, we benefited from the shelter of the Scottish mainland. Apart from the odd shower, it was mainly dry.
The 14th was rather cloudy, but there were lengthy sunny periods on the 15th and 16th. With 15.4 hours sunshine, Fair Isle was again the sunniest location in the UK.
A ridge crossed the area during the next two days, as the Azores high migrated north-east towards the UK, bringing further dry and sunny days with light winds.
On the 18th Lerwick was the sunniest place in the UK with 15.9 hours of sunshine. As the high moved east along the English Channel, a south to south-easterly breeze developed on the 19th and 20th, bringing an end to the sunny spell, as low cloud and sea-fog patches rolled off the sea on to coasts and headlands.
As the high drifted north-east into the North Sea a light southerly airflow carried warm air northwards. A temperature of 20.1° Celsius was recorded at Kirkwall Airport on the 21st, the warmest May day there for 28 years. A mostly cloudy Lerwick managed 15.4° Celsius. The 22nd was a cooler day with light and variable winds. Mist and fog continued to affect some coasts and hills.
Though the high over the North Sea declined south on the 23rd high pressure, ridging south over Iceland, took its place. This, together with falling pressure over Scandinavia, resulted in freshening northerly winds and lower temperatures, as showery troughs moved south from the Norwegian Sea across Shetland.
The next few days were bright with scattered showers, these a touch wintry at times with hail, as a depression – encapsulating quite chilly air – moved south over the Northern Isles.
Between the 27th and 28th pressure remained low and complex over the Northern Isles and northern North Sea, bringing a mixture of sunny or clear spells, scattered showers and mainly light and variable winds. It remained cool, with a grass frost early on the 27th.
As pressure began to build over the Northern Isles, the 29th was mainly dry and bright with an early grass frost. However, a depression tracking east over southern Britain pushed a front northwards, bringing some overnight rain, as freshening south-east to easterly winds backed north-easterly.
As the low moved away into Denmark and a small anticyclone – developing near Faroe – moved east close to Shetland, the 30th and 31st were dry and mainly sunny, with fresh north-easterly winds veering to a lighter south-easterly direction.