16th August 2018
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Better end to December after series of lows brought windy conditions to isles

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Though the focus of low pressure was in its normal December position over Iceland, it was considerably deeper than usual. Despite this, mean atmospheric pressure was above average at Lerwick.

As a consequence of the in­creased pressure gradient, for much of the month the Northern Isles experienced rather windy and cyclonic conditions, as a sequence of lows, separated by brief ridges, tracked either north-east or east close to Shetland.

The last week, from Christmas Eve until Hogmanay, was much more settled, as an anticyclone drifted north across the UK, to become slow-moving over Scandinavia and the northern North Sea.

It was provisionally the coldest December since 1999 for Scotland as a whole. However, for Lerwick, December 2003 was marginally colder, with a daily mean of 4.7° Celsius. While cold anticyclonic conditions brought average temperatures below the long-term mean for much of mainland Britain, on the maritime periphery of the high, Shetland was still significantly milder than normal. For England and Wales it was the coldest December since 2001.

Rainfall was below or well below average in most areas, only around 50 per cent of normal over parts of England and Wales, but locally wet with 150 per cent in eastern Scotland. Sunshine was above normal virtually everywhere, approaching 200 per cent in some parts of England.

During the first three days, slack low pressure over the Norwegian Sea and North Sea brought cool northerly winds with wintry showers of sleet and snow and an early gale on the 1st. Winds quickly eased, days were often sunny and nights, with clear skies, rather cold and frosty.

South-east to easterly winds then strengthened, ahead of fronts driven east into Scotland by an Atlantic depression, bringing rain, preceded by sleet, on the 4th. Winds gradually backed north-easterly and rain turned showery as the depression tracked east to the south of Orkney. On the 5th the showers died out, as easing north-easterly winds backed further to become light north-westerly, as the depression moved away over the North Sea and pressure built to the west.

A ridge swung east over Shetland bringing a reasonably fine day on the 6th. However, the next three days were unsettled, as the next depression, tracking east through the Iceland region, drove its fronts across northern Scotland.

Rain on the 7th cleared to showers by the 8th as strong south-westerly winds veered westerly. Showers turned wintry overnight, as the depression moved into the Norwegian Sea and winds veered to a cooler north-westerly direction. These resulted in a thin covering of snow or ice, and the month’s only snow-cover at Lerwick early on the 9th. The 10th was a better day as another ridge, moving east across the area, brought drier conditions with lighter winds.

With an anticyclone over Scandinavia supporting a ridge across Scotland, the 11th started fine, but showers soon spread across the isles as south-easterly winds freshened. As a deep depression near Iceland pushed fronts east towards Scotland, south-easterly winds increased severe gale-force on the 12th.

Slowing down as they came up against high pressure to the east, the fronts lingered near Shetland for the next 24 hours, bringing heavy overnight rain which, accompanied by strong to gale-force south-easterly winds, persisted through much of the 13th.

The rain turned lighter and patchy overnight as fronts eventually cleared eastwards, with the winds easing markedly by the 14th, as the filling depression slipped south-east over northern Scotland. Though mainly dry conditions followed overnight as winds veered south-westerly, a depression near Iceland pushed an occluding front east over the area on the 15th, bringing more rain later in the day.

With a large, complex area of low pressure to the north- west, the next three days saw unsettled conditions with outbreaks of rain or showers. Mild south-westerly winds increased gale- force from the 17th. Showers turned more frequent, heavier and wintry with hail on the 18th, with thunder reported from Fair Isle.

After a bright start on the 19th, a deep low centre moving towards Faroe brought heavy rain, with southerly gales increasing severe gale, gusting to 77mph on Fair Isle and 86mph in Kirkwall. As the low moved east to the north of Shetland on the 20th, rain gave way to wintry showers, as gales – temporarily moderating – veered westerly.

Showers continued for much of the 21st, Lerwick having a thunderstorm and hail, as the main centre moved east from Iceland and winds, veering north-west to northerly, increased gale to severe gale and locally storm-force, with Lerwick reporting a gust of 86mph.

A ridge swung east across Shetland and winds eased early on the 22nd, but more rain followed later in the day, and again on the 23rd as a new depression, moving into the Iceland region, pushed fronts east over Shetland.

As an offshoot of the Azores high moved over southern Britain and extended a ridge north across Scotland, the three days over Christmas were mainly dry, though a mild south-westerly breeze brought rather cloudy conditions.

As the high drifted across the UK into southern Scandinavia weak fronts, associated with low pressure to the north-west, brought some mainly light showers and freshening southerly winds on the 28th and 29th. The 30th was dry though rather cloudy but, as the high drifted closer to Shetland, the 31st was dry and bright with light winds.

Dave Wheeler

DECEMBER AVERAGES 2008  :  1971-2000
Mean maximum temperature 6.2 °C  :  6.2°C
Mean minimum temperature 3.4 °C  :  2.0°C
Daily mean temperature 4.8 °C  :  4.1°C
Mean sea-level pressure 1010.9 hPa (mb)  :  1006.2 hPa (mb)
Total rainfall 133.4 mm  :  150.0 mm
Wet days => 1 mm 24 days  :  22.2 days
Sunshine 17.5 hours  :  14.9 hours
Air frosts 5 days  :  6.8 days
Ground frosts 13 days  :  14.3 days
Snow/sleet 9 days  :  11.3 days
Days with gale 9 days  :  7.4 days
Maximum daily mean temp 7.9°C on 23rd
Minimum daily mean temp 0.9°C on 2nd
Highest maximum 10.3°C on 16th
Lowest day maximum 2.0°C on 1st