Fair Isle Weather - May 1999

A month of near average temperature (+0.3ｰ Celsius) and sunshine (104.5%) but, thanks to 36 hours of continuous rain, the wettest (243.2%) May on record for Fair Isle.

The month began dry and mainly sunny with high pressure to the west of the UK and a flat ridge extending across Scotland to the northern North Sea.   After a few days the main centre of high pressure transferred northwards, as an anticyclone developed over the Norwegian Sea with a ridge extending south across the Northern Isles.   This was maintained across Scotland during the next few days with a SE値y airstream developing across the Northern Isles as frontal systems moved north into southern areas of the UK from Biscay.

High pressure to the north of Shetland prevented frontal systems, associated with a deepening Atlantic low tracking north to the west of the UK, from moving into Shetland until the 10^th when a weakening front, becoming slow-moving over the Northern Isles, brought outbreaks of rain with fog and strong E値y winds to the area.

The middle of the month saw a return to mainly sunny conditions as high pressure, developing to the west of the UK, extended east across Scotland on the 15^th.   Clear skies resulted in relatively low nighttime temperatures with a ground frost recorded early on the 17^th.   The high then transferred east into southern Scandinavia as low pressure, developing over Biscay, pushed fronts north into southern areas of the UK.   The Scandinavian high maintained a ridge over Scotland for the next few days before collapsing on the 20^th as a depression moved east to the south of Iceland.

This low continued east across Faeroe, driving fronts through the UK and bringing gale to severe gale force SW to W値y winds and blustery showers to the Northern Isles later on the 21^st.

The strong winds backed S to SW値y on the 23^rd as a low deepened between Faeroe and Iceland.   The low, becoming slow-moving to the north of Shetland, brought blustery showers with hail and strong W値y winds before gradually filling on the 25^th as another deepening Atlantic low ran quickly northwest towards the Hebrides.   This low tracked northeast across northern Scotland on the 26^th with a frontal system becoming slow-moving over the Northern Isles, giving 25.4mm of rain between 09h-21h on the 28^th, and making this (with a 24-hour total of 26.6mm) the wettest May day on record.   The 36 hours of rain from 21h on the 27^th to 09h on the 29^th gave 36.8mm (almost equal to the average rainfall for May) and resulted in this being the wettest May on record for Fair Isle.

Another Atlantic low, following a similar track, deepened as it ran northeast across Scotland on the 28^th and introduced strong N値y winds as it moved away towards Norway and high pressure developed to the west of Britain.   A ridge, moving east across northern Scotland, gave a couple of settled days before declining south and frontal systems, associated with low pressure moving east across Iceland, brought rain and strong SW値y winds to the Northern Isles.

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Last edited on 20 September, 1999   Dave Wheeler weatherman@zetnet.co.uk
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