March weather a tale of two halves
By Dave Wheeler
As with last year, the first half of March was rather unsettled and changeable, with low pressure systems, separated by transient ridges, passing close by, or over, Shetland in quick succession.
The period between the 16th and 21st – again very similar to last year – was more settled, as an anticyclone meandered around the North Sea, Scotland and southern Scandinavia. Similarly, the last 10 days reverted to an unsettled and changeable pattern, as high pressure returned to a position south of the UK and depressions again passed close by Shetland.
Overall Shetland, like the rest of the country, experienced a milder and wetter March, but also a sunnier month than normal. On the 1st, cloud and rain were followed by brighter, showery conditions and a south-westerly breeze as fronts cleared Shetland. A cool dry evening with a patchy frost followed, as a weak ridge declined east. Freshening south-westerly winds brought rain overnight, which persisted for most of the 2nd as a sequence of fronts, driven off the Atlantic by a depression deepening to the south of Iceland, crossed the Northern Isles.
The 3rd saw gusty south-westerly winds and cooler conditions with heavy wintry showers of rain, hail and sleet, giving places a slight covering of wet snow and ice by morning. The day became mostly dry as winds backed south-easterly, later increasing strong as the deep low swung down from Iceland. Heavy frontal rain moved east across Shetland overnight and winds increased to gale-force, with gusts to 60mph. As the low became a filling feature off north-west Scotland on the 4th, the strong to gale-force winds quickly eased. Later in the day, showers merged to give a longer period of rain.
By the 5th a large, slack area of low pressure lay between Scotland and Iceland. With mainly south-westerly winds, Shetland – benefiting from the shelter of the Scottish mainland – was mainly dry and bright. Though clear spells led to local early frost, a weak warm front brought cloud and patchy rain later in the night. This cleared and the 6th was again quite sunny and generally dry with moderate south-westerly winds. Clear spells gave another slight frost overnight.
After a bright start on the 7th, a deep Atlantic low brought periods of occasionally heavy rain or drizzle later in the day, as fronts pushed east over Shetland. As the low tracked slowly north, southerly winds veered south-westerly and strengthened. Rain cleared to blustery, heavy, wintry showers over-night, leaving a patchy covering of wet snow and ice by the morning of the 8th. Gale to storm-force south-west to westerly winds, gusting to 81mph at Lerwick and 89mph in Foula, slowly eased and veered west to north-westerly. Wintry showers continued through the 9th, as the depression moved away over the Norwegian Sea. The 10th was a bright, cold day, with early showers dying out as north-west to westerly winds continued to ease. A cold, clear and frosty night followed as a ridge swung east across Scotland and winds fell light.
After a bright start, rain spread from the west on the 11th as south to south-easterly winds increased strong, ahead of fronts driven east by a deep Atlantic depression. The rain cleared overnight, with sunny spells and showers – some of hail – accompanied by strong south-west to westerly winds following for the 12th, as the low moved away northwards.
A weak ridge crossed Shetland, bringing a mainly dry day on the 13th. Light winds backed south-easterly later as yet another low, tracking east towards the Hebrides, pushed further frontal systems towards the Northern Isles. These brought a little rain or drizzle through the 14th, together with coastal and hill fog patches, as they moved slowly through the area.
High pressure – building across southern Britain – extended northwards on the 15th. Moderate south-westerly winds brought generally cloudy and mild conditions with patchy drizzle to Shetland during the 16th. In marked contrast, as an anticyclone drifted north-west over Scotland, the 17th was a pleasantly mild day with almost unbroken sunshine. Though the next few days remained mainly dry and settled as the high drifted a little further east, cloud amounts tended to increase and sunshine amounts were not so high. By the 20th, while the temperature at Altnaharra (Highland) reached 18.5°C, freshening south-easterly winds were pushing fog banks off the cold North Sea on to southern and eastern coasts of Shetland, and here it was much cooler, with temperatures struggling to reach 8°C.
By the 21st, with the centre of high pressure transferring to a position south-west of the British Isles, freshening westerly winds, though clearing the fog, brought rather cloudy conditions with patchy drizzle. The 22nd was a brighter day with scattered showers, occasionally wintry with hail.
As low pressure moved into Scandinavia, the 23rd was a cooler day, with fresh north-westerly winds bringing sunny or clear spells and wintry showers of hail and sleet. These died out as a ridge declined early on the 24th, with rain and strong south-easterly winds following from the west later. Winds became cyclonic variable early on the 25th, as low pressure moved east over the Northern Isles. Though a ridge made a very brief appearance, this quickly declined, with another Atlantic depression approaching north-west Scotland, resulting in a strengthening south-easterly wind with further rain followed by showers for the 26th.
Frequent showers — some with hail – continued through the 27th, and the strong south-easterly wind backed north-easterly as the depression moved north-east over northern Scotland. Temperatures fell overnight, and by the 28th showers were of snow. Despite some sunny spells, strong north-west to northerly winds resulted in daytime temperatures failing to reach 2°C in Lerwick. As another ridge crossed the area, winds fell light and skies cleared during the evening. A cold night followed, with the air temperature falling to -3.0°C and down to -3.7°C on the grass.
After a bright, cold start, milder south-westerly winds and patchy rain spread from the west later on the 29th, as fronts approached Shetland. These cleared by early on the 30th, leaving bright spells and occasionally wintry showers, accompanied by strong south-westerly winds, as low pressure over Iceland moved northwards.
With high pressure persisting to the south and low pressure to the north, the month ended with a mild south-westerly airflow over northern Scotland. While this brought damp, drizzly and misty conditions with extensive low cloud to Shetland, elsewhere in Scotland it was dry and warm with some sunshine. The temperature at Dyce reached 17.7°C, the warmest March day there for 10 years. In Shetland, temperatures at best just reached 8° C.
MARCH STATISTICS RECORDED AT LERWICK OBSERVATORY
March 2009 : Averages 1971-2000
Mean maximum temperature: 7.2°C : 6.3°C
Mean minimum temperature: 3.4°C : 1.9°C
Daily mean temperature: 5.2°C : 4.1°C
Mean sea-level pressure: 1003.3hPa (mb) : 1008.3 hPa (mb)
Total rainfall: 131.4mm : 122.3mm
Wet days => 1 mm: 23 days : 19.0 days
Sunshine (electronic sensor): 89.6 hours : 85.6 hours
Air frosts: 2 days : 6.5 days
Ground frosts: 8 days : 14 days
Snow/sleet: 9 days : 12.9 days
Days with gale: 3 days : 5.6 days
Maximum daily mean temp: 7.8°C on 15th
Minimum daily mean temp: 1.0°C on 28th
Highest maximum 9.5Â°C on 17th
Lowest day maximum: 1.8°C on 28th
Highest night minimum: 6.4°C on 15th
Lowest minimum: -3.0°C on 29th
Lowest grass minimum: -4.4°C on 10th
Wettest day: 26.7mm on 8th
Sunniest day: 9.6 hours on 17th
Highest mean hourly wind: 48 knots on 8th
Highest gust: 69 knots on 8th