Change to normal September fare as high pressure dominates scene
WHILE during most Septembers we normally expect to see Atlantic depressions pass by in quick succession, this year the month was unusually anticyclonic. Though not often situated close to Shetland, the centres of high pressure were generally close enough either to keep fronts away or those that did reach our shores were often weak affairs.
On a broad scale, high pressure – initially centred over Scandinavia – became more or less confined to the south as the month progressed. The result was that wind directions were often south or south-easterly at first, but became south-west or westerly later. Because of our peripheral position on the edge of the anticyclones, cloudy conditions often prevailed. However, Fair Isle, with 108.3 hours, was somewhat sunnier than Lerwick.
Only three depressions of note affected the British Isles during the month. The first tracked east across southern Britain, with little effect in the north. Another, just before mid-month, turned back into the Atlantic after approaching western Scotland. The third tracked east just to the north of Shetland on the last day of the month. However, its full significance only became apparent a few days later, as unseasonably early snow fell across parts of Shetland.
A shallow area of low pressure lay across the north of Scotland during the first four days of the month. Initially there was a lot of moist air bringing heavy rain and murky conditions on the 1st. The 2nd was brighter and clearer with a mix of sunshine and showers, some heavy, prolonged and locally thundery. It then became mostly dry, with warm sunshine by day and cool, clear nights, resulting in the only grass frost of the month on the morning of the 4th.
The low moved away to the east on the 5th and a deep low developed off south-west Britain. While this low tracked slowly east across southern England as a filling feature on the 6th, an anticyclone over the Atlantic ridged north-east across Faroe and the southern Norwegian Sea, resulting in strengthening north-easterly winds which brought a continuation of the mostly dry conditions, with well-broken cloud cover.
This eased during the 7th as the remains of the low moved out over the southern North Sea, backing north-westerly by the 8th as high pressure transferred into Scandinavia. As the high declined on the 9th, a south-easterly airflow began to freshen, as a deepening Atlantic low – running in towards north-west Scotland – pushed fronts north-east to bring heavy overnight rain to Shetland.
Strong south-easterly winds eased and veered southerly for a time early on the 10th, as the filling low moved away and fronts cleared Shetland. However, south to south-easterly winds soon strengthened again as the next depression – rushing in off the Atlantic towards western Scotland – drove its fronts towards the Northern Isles. Just managing to reach Shetland on the 11th, the fronts then became blocked by high pressure to the east. They remained slow-moving – though steadily weakening – close to Shetland for the next few days, bringing outbreaks of generally light rain together with some mist and low cloud. The fronts were eventually pushed clear as the intensifying Scandinavian anticyclone extended a ridge west across the northern North Sea. The ridge persisted for the next two days, with light winds then freshening on the 18th as it declined, and Atlantic fronts began to push in from the west.
However, as the equinox approached – a period that can often see the first significant gale of approaching winter – high pressure continued to dominate our weather. While one anticyclone to the east slipped south over the North Sea, another developed to the south-west of the British Isles. A mild south-westerly airflow covered northern Scotland between the 19th and 22nd, with Atlantic fronts close to Shetland. However, the proximity of high pressure meant that these were fairly weak and – apart from some patchy rain and occasional misty conditions – it remained mostly dry with some brightness. In fact, the second sunniest day of the month occurred on the 22nd, when Lerwick reported 9.1 hours of sunshine: a welcome relief in what was a virtually sunless period extending from the 11th to the 27th.
On the 23rd the anticyclone to the south-west drifted slowly north-east over Scotland, re-invigorating the waning Scandinavian high for a time as the two merged. High pressure to the east – maintaining a ridge south-west across the UK – then drifted south. Between the 24th and 27th the airflow was mainly south-westerly, with Atlantic fronts trailing around the edge of the high pressure again affecting the Northern Isles from time to time. This became quite a strong westerly accompanied by heavy blustery showers on the 29th, as a depression, tracked east from Iceland and fronts crossed Shetland. Fair Isle reported a thunderstorm and hail during the afternoon. The 30th saw the strong westerly winds veer to a cooler north-westerly, as the low moved east into Scandinavia.
Initial Statistical details for Scotland as a whole (1961-1990 average) for September:
Mean Temperature – 11.1°C (0.6°C above average).
Rainfall – 105.8 mm (73% of average).
Sunshine – 81.7 hours (83% of average). Dullest September since 1985, when 80.8 hours were recorded.
SEPTEMBER STATISTICS RECORDED AT LERWICK OBSERVATORY
September 2008 : Averages 1971-2000
Mean maximum temperature 13.8°C : 12.4°C
Mean minimum temperature 10.2°C : 8.2°C
Daily mean temperature 11.9°C : 10.1°C
Mean sea-level pressure 1014.6hPa (mb) : 1009.5 hPa (mb)
Total rainfall 60.0mm : 115.3mm
Wet days =>1 mm 12 days : 17.4 days
Sunshine (electronic sensor) 66.5 hours : 100.8 hours
Air frosts 0 days : 0.0 days
Ground frosts 1 day : 1.7 days
Snow/sleet 0 days : 0.2 days
Days with gale 0 days : 2.1 days
Maximum daily mean temp. 13.8°C on 9th
Minimum daily mean temp. 8.7°C on 30th
Highest maximum 16.8°C on 21st
Lowest day maximum 11.1°C on 30th
Highest night minimum 13.1°C on 1st
Lowest minimum 6.4°C on 30th
Lowest grass minimum -0.1°C on 4th
Wettest day 11.8mm on 9th
Sunniest day 9.5 hours on 3rd