17th August 2018
Established 1872. Online since 1996.

Dry and sunny but rather windy

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September saw a reverse of the pressure field of July and August, with below normal pressure over Greenland, the Norwegian Sea and northern Scandinavia, and above normal pressure over the British Isles and much of central Europe. As a result, the general trend was for a south-west to westerly airflow over Shetland.

With pressure low and complex over the north-east Atlantic, cyclonic conditions dominated the weather of the Northern Isles during the first third of the month.

While not producing the fine and settled spell seen further south, the middle third was at least less unsettled as an anticyclone moved over the UK (giving a September record of 1041mb on the 11th).

On the northern edge of this high, Shetland lay in a broad west-south-westerly airflow off the Atlantic.

The last third of the month saw the high pressure move further away from the UK, with a more changeable south-westerly airflow over the Northern Isles.

The month began with a complex area of low pressure over the north-west of the British Isles and fronts slow-moving across north-east Scotland. Early rain and misty conditions cleared and, apart from a few well-scattered showers, the 1st was then mainly dry with some sunny spells and moderate south-westerly winds.

As a secondary low tracked east across mainland UK, Shetland remained bright with a few scattered showers and light west-south-west­erly winds. The low deepened as it moved out over the northern North Sea, and winds backed fresh north-easterly as heavy rain spread north later on the 3rd. Although – as the low tracked away into Scandinavia – rain cleared early, the 4th remained cloudy with showers and a fresh to strong north-westerly wind.

A weak ridge followed from the west, bringing a dry, bright start on the 5th. As this declined, the cool north-westerly winds were replaced by milder, cloudy south-westerlies, bringing patchy rain and drizzle later. Heavy rain followed over­night.

A front remained slow-moving close to the Northern Isles on the 6th, with cloudy conditions, patchy rain or drizzle and easing south-westerly winds. Cloud and outbreaks of rain cleared to occasional showers and strong to gale south-westerly winds on the 7th, as a wave-low on the front ran past north-west Scotland.

On the 8th a deep Atlantic low – running quickly northeast over Faroe – brought strong to locally gale-force southerly winds and heavy rain. Showers followed the rain as the fronts cleared through, and winds veered westerly and increased gale or severe gale, with gusts to 69mph reported at Lerwick and 62mph on Fair Isle. Westerly gales moderated and showers eased away on the 9th, as the low moved away and the Azores high extended into southern Britain.

The 10th to the 12th saw mainly cloudy conditions with moderate or fresh west to north-westerly winds and patchy rain or drizzle, as weak fronts drifted around the northern flank of a small anticyclone moving from the south-west over central areas of the UK. The 13th was also cloudy, but mainly dry with easing north-west winds, as the high meandered away to the west of Scotland.

Between the 14th and the 17th Atlantic high pressure maintained a ridge over Scotland, bringing mod­er­ate north-westerly winds and mainly dry – though often cloudy – conditions to Shetland.

As the anticyclone began to ease away eastwards, a complex area of low pressure developed south of Iceland, while southerly winds freshened, Atlantic fronts remained out to the west, their progress blocked by high pressure to the east. As a result, the 18th to the 20th were mainly dry days with sunny or clear spells. A front eventually reached Shetland, bringing some light, showery rain on the 21st as south-westerly winds increased strong to gale-force.

From the 21st to the 28th pressure remained high across southern Britain. As a sequence of quite deep Atlantic depressions tracked north-east between Scotland and Iceland, fronts affected the Northern Isles from time to time, bringing rain or showers separated by brighter drier interludes. South-westerly winds were generally strong, reach­ing gale-force on the 21st and 22nd – with gusts to 69mph in Lerwick – and also the 23rd and 25th.

On the 29th pressure built to the south of Iceland, the developing high extending a ridge over Shetland. Showers died out as easing south-westerly winds veered north-west­erly. A large area of high pressure was established over the North Atlantic and North Sea by the 30th, bringing light winds and a cool start to the day with a touch of grass frost locally. However, a slow-moving front did produce a little showery rain at times.