Weather extremes of 2005: cold spell of mid-late November/early snowfall of the 25th-26th November

November was a month of two halves; the first half continuing on from the near record-breaking autumn warmth of September and October, was warm, but also very stormy (see previous reports), itself being the warmest such period since 1994. This was followed by a dramatic change to a much more quiet and colder anticyclonic period, bringing the coldest second half of November since 1993 with temperatures up to 4C below average in some districts. This more than compensated for the early warmth to bring the first below average month in the CET series since July 2004 (ending a record-breaking run of 15 months of above average temperatures). The cold spell also brought about some disruptive falls of snow to the southwest in late November. Incredibly (and mostly due to the sunny, anticyclonic mid-month period) November 2005 was the sunniest on record for England and Wales, a record that stretches back to 1881.

The upper air situation across northwestern Europe on the 13th, following on from the stormy period of the week previous, was that of a deep long wave trough ‘digging in’ south to the east of the UK, with a cut-off upper vortex forming over Spain. This allowed cooler air to flow south across the UK in a N’ly flow behind a depression and for conditions to turn cool, frosty and bright. Although the upper level flow flattened out on the 14th allowing milder maritime air back in, the trough soon re-established itself over the coming days to the east of the UK across central Europe. An omega block formed (an omega block is an upper level high that has been displaced north of the jet, preventing the progression of Atlantic weather systems across northwest Europe). This forced depressions to the north and the south of the UK, allowing an anticyclone to dominate the weather for an extended period of time.

Synoptic set up by the 18th November

http://www.wetterzentraleforum.de/archive/2005/brack/bracka20051118.gif

Following a northerly plunge, from the 18th, high pressure established itself over the UK extending north towards Iceland and due to the air mass being relatively dry throughout the lower troposphere, this brought about unusually clear skies to most, if not all places. It wasn’t particularly cold by day with temperatures typically close to, or a little below average, however, overnight temperatures dropped quickly with the lack of cloud cover leading to frequent widespread moderate or even severe frosts from the 18th to the 23rd. The lowest temperatures recorded during the cold spell are displayed below in Table 1. This air mass stagnated over the UK, diurnally cooled and moistened in the lowest layers.

Table 1: Summary of lowest temperatures recorded on 18thand 19th November 2005

Station	Date Recorded	Min Temperature (°C)
Tulloch Bridge, Lochaber	18.11.05	-9.2
Braemar, Aberdeenshire	19.11.05	-9.2
Kinbrace, Sutherland	19.11.05	-9.0
Aviemore, Highland	18.11.05	-8.8
Glenlivet, Moray	19.11.05	-8.6
Redesdale Camp, Northumberland	19.11.05	-8.1
Redhill, Surrey	18.11.05	-8.0
Altnaharra, Sutherland	19.11.05	-8.0
Strathallan Airfield, Perthshire	19.11.05	-7.8
Shap, Cumbria	18.11.05	-7.7
Benson, Oxfordshire	18.11.05	-7.7

Under clear skies it was sunny by day, however some large variations in maximum temperatures occurred with persistent freezing fog in some valleys of the southwest Midlands across Gloucestershire, Oxfordshire, Warwickshire and Worcestershire leading to subzero maxima. (see the high resolution satellite image showing the persistent freezing fog on 19th November 2005). Church Lawford, Warwickshire managed just –0.5C on the 20th; the same day Pershore, Worcestershire reported a maximum of –0.6C.

High pressure remained over the UK until the 23rd, with the pressure reaching a high of 1041.7mbar at Lake Vyrnwy late on the 22nd. The high then retreated northwestward quickly on the 23rd/24th, replaced by a depression driving south through the North Sea, causing some very large 48 hour pressure falls in excess of 60mb in the east of the UK. The depression brought a brief and very cold arctic blast from the north which went on to cause some disruptive snowfalls across the southwest of England, west of Wales and much of northern Ireland during the latter part of the 24th and early part of the 25th.

Synoptic set up on the 25th November http://www.wetterzentraleforum.de/archive/2005/brack/bracka20051125.gif

An Arctic front preceded by a low level flow directly from the Arctic moved quickly south on the 24th, and was very extremely active, bringing a short period of very gusty winds, hail squalls and ice pellets, followed by a large fall in temperature of up to 5C in just a matter of minutes. Temperatures didn’t fall much below freezing behind the front because of a strong northerly wind helping to mix the air, however this made it feel bitter and helped to drive heavy snow showers inland across northern Scotland and northern Ireland throughout the night where falls of up to 5cm were reported across the higher parts of Northern Ireland, 15cm at Aviemore and 20cm at Glenlivet.

A trough developed in the Irish Sea, with convergence enhancing pre-existing instability from the cold Arctic outbreak aloft overrunning relatively high sea temperatures. This developed a near continuous band of heavy snow showers through the west of Wales and down into southwest England.

The effect has become known as the ' Pembrokeshire Dangler' and general only occurs during late autumn and winter, since the environmental factors required for its formation (warm enough sea temperatures aligned with cold arctic air aloft) are usually only met at this time of year. As a northerly flow is forced between the sea and the Antrim Plateau it meets a land breeze as the winds blowing fromjust east of north off of England and Wales and from west of north off of Ireland converge right down the length of the Irish Sea. The convergence line spawns deep convective cells as they flow over progressively warmer waters creating further instability and prime conditions for prolonged convection across Pembrokeshire, Cornwall and West Devon.

Devon and Cornwall were particularly badly hit during the day with as much as 8 inches (20cm) of snow falling in the higher parts of Cornwall and Devon on Bodmin Moor, paralysing the A30 between Kennards House and Bodmin after a incident, requiring army helicopters to come out and rescue motorists who were stranded in their vehicles.

Further disruptive snow fell on the 28th. A relatively narrow band of snow showers developed during the morning on an occlusion across NE England, through to Lancashire, Cheshire and across the Manchester area. This moved southwest throughout the afternoon and intensified briefly to bring a covering of snow the Midlands and Gloucestershire where up to 8cm of snow fell. Over 300 cars were abandoned after heavy snow made the some of the rural and even major routes impassable, worst affected was the A417 between Gloucester and Cirencester.

(C) David Jameson Jan 2006 (with additional info from Nigel Bolton on Wikipedia).