CLIMATE-UK.COM'S REVIEW OF THE BRITISH METEOROLOGICAL SCENE

MONTHLY SUMMARY

No. 621

For SEPTEMBER 2007

DRY AND QUIET FIRST HALF, RATHER CHANGEABLE LATER. NOTABLY COLD FINISH

Text Box: High pressure was located close to southwestern Britain throughout the first half of the month with frequent northwesterly winds over UK. The second half was more changeable with southwesterlies and northwesterlies alternating, but winds veered northeasterly during the last few days. Sea-level pressure was 3 to 8 mbar above the long-term mean over the British Isles, and the anomalous flow was north to northwesterly; this was the seventh most 'northerly' September in 135 years of records.

An anticyclone centred southwest of Cornwall on the 1st drifted slowly northeast to lie close to Ireland from the 4th-12th, before declining. A new high transferred quickly eastwards across southern Britain on the 14th/15th, followed by a well-marked cold front. Winds blew from between west and north throughout this period.

The weather remained quiet and generally dry throughout the first half of the month, and although there was occasional rain in western and northern Scotland, even here amounts were trivial until the 12th. In many parts of the UK sunshine was variable rather than plentiful, but the coasts of southeast Scotland, northeast England, and southern England were well favoured, and Portland (Dorset) recorded 129h of sunshine between the 1st and 15th. The highest daily durations were 12.6h at Camborne (Cornwall) on the 4th and 12.5h at Leuchars (Fife) on the 7th. Cool and warm spells alternated: highest temperatures occurred on the 5-7th and 12th-13th, and Howden (East Yorks) registered the month’s highest reading of 25.8°C on the 7th. Coldest nights were the 3rd/4th, 10th/11th, and 14th/15th, with -2.2°C at Dalwhinnie (Inverness-shire) early on the 15th. Northern Scotland was windy on the 14th with a peak gust of 56kn at Foula (Shetland). The depression which tracked between Scotland and Iceland on the 15th contained the remnants of Tropical Storm ‘Gabrielle’, and it brought prolonged heavy rain to the highlands and islands of Scotland with 72mm at Broadford (Skye) on that day.

A brief northerly outbreak on the 17th quickly gave way to a rather changeable west to southwesterly regime from the 18th-23rd, but another very vigorous cold front swept eastwards across the UK on the 24th. A northerly flow developed in its wake, and this flow veered northeasterly as an anticyclone moved from the Atlantic across northern Scotland to reach Scandinavia on the 27th.

Text Box: The 17th was cold, and Braemar (Aberdeenshire) reported a maximum of just 7.8°C, while hail and sleet showers fell in northern Scotland, but temperatures returned to normal by the 19th, and the next few days were mostly cloudy with occasional rain, chiefly in northern and western regions where some heavy falls were logged over the hills; Capel Curig (Snowdonia) recorded 210mm in the ten days starting on the 16th. Amounts in the east and south were mostly small, and the rain that fell at Filton (nr Bristol) early on the 19th was the first measurable rain there for 31 days. It became rather warm in central and eastern districts on the 22nd and 23rd, but an active cold front brought 10-20mm of rain to much of the UK on the 24th, and the frontal passage was associated with widespread wind damage from severe squalls and a number of minor tornadoes. A gust of 58kn was recorded at Cranwell (Lincs). From the 25th-28th a cold north wind brought showers to eastern districts which gave way to longer periods of rain on the 27th-28th in East Anglia and the Southeast; meanwhile western and northern districts enjoyed long sunny spells. The 26th was particularly cold - probably the coldest September day since 1993 - with a high of 7.1°C at Braemar, 7.7°C at Carter Bar (Roxburghshire), and 10-13°C even in southern England, and the following night the mercury dropped to -3.6°C at Saughall (Ayrshire) and to -3.3C at Tyndrum (Perthshire). The last two days of the month were mostly dry and slightly warmer.

Both mean maximum and mean minimum temperature for the month were within 0.5degC of the long-term average in Scotland and Northern Ireland, and 0.5-1.0degC above in England and Wales; nevertheless, it was the coolest September for six years. Rainfall was above average in parts of northern and western Scotland also locally in northwest England and north Wales, but it was a dry September everywhere else; a swath from London to Wiltshire and Oxfordshire had less than 20mm. Sunshine totals were 20-40 per cent below normal in northern Scotland, but 10-20 per cent above in most other regions.

TEMPERATURE

SUNSHINE

RAINFALL

Highest and lowest totals

Highest and lowest totals

on record for month

on record for month