The second half of December 2006 was dominated largely
by a vast anticyclone which was anchored close to or over the British Isles for several days. This itself was noteworthy in
that it caused widespread travel problems due to fog at London Airports. Christmas began to see the high presure move slowly
east whilst at the same time the jet Stream over the western Atlantic strengthened and began to drive fronts towards the UK
to erode the block. The last few days of December saw the remnants of the dull grey anticylonic block swept out of the UK
as a major change headed in from the west.
The diffluent omega blocking which had persisted for nearly
two weeks collapsed SE into Europe, as cold advection weakened to its east and a strengthening Atlantic jet and trough
literally 'shoved' it out of the way. Zonality was returning to much of the North Atlantic and N Europe.
Discussion
During the period of the 29th to the 31st December a series
of depressions swept across Britain. The first of these crossed northern parts overnight on the 29th/ 30th.
During the day on the 29th, a major branch of the cross Atlantic
jet rounded the base of the large upper trough located in the central Atlantic and headed north-east towards Ireland. Midday
air reports (aireps) measured the jet speed as being close to 150kts. During the day a developing shallow wave in the
baroclinic zone south-west of Ireland came under the left exit of this jet core, At the same time a kink in the upper flow
at 250mbs developed into a shortwave trough and as such both jet IPV and vorticity advection from the trough begin to act
on the developing shallow surface low. The net result was a quickly deepening depression, which tracked across Northern Ireland
and north-west Scotland overnight. The sequence of events is shown in the water
vapour (WV) imagery shown below.
The depression deepened to around 970mbs. Wind gusts were
not that extreme, but were strong ahead of the cold front as it moved south east across England and Wales, along with some
heavy rain. In the 24 hours to midnight on the 30th, Tulloch Bridge (Highlands) saw 36mm, 35mm was reported at Capel Curig
in North Wales and 34mm as far south as Boscombe Down (Wilts). The highest gusts can be seen below:
Notable UK maximum gusts on 29th December 2006
This depression moved
north east and the cold front cleared the south-east on the morning of Saturday, 30th. This was initially forecast to be a day of sunshine
and showers, however models had started to trend towards developing a secondary depression which was then forecast to
race across the south of England during the day. This originated from another minor shortwave in the mid Atlantic which
raced east and became associated with a PVA Maxima. At the same time another jet core shot east (this one measured at over
160kts) and the minor trough increasingly came under its left exit, and hence deepening began. The problem forecasters
had here was in the timing. The cold air low was developing from a very small
scale disturbance in both the upper and lower atmosphere in a area of the Atlantic, (west of the Azores) that was sparse in
data. In fact the developing area barely covering more than a few gridpoints in the global models hence they initially struggled
to resolve it.
The trend though was to develop a deeper low
and during the day wind warnings of up to storm force ten (circa 50 kts) were issued for
the southern North Sea shipping areas (these primarily not because of the depth of the low, but because of the
tight squeeze between the low and the high pressure block over southern Europe).
By 12Z on Saturday the low was developing to the south-west
of Cornwall. Midday observations from the Seven Stones Light Vessel (west of Lands End) suggested a pressure of 1001.9mbs with a mean 47kt wind and a gust to 62kts (71mph). Therefore it seemed there was a very tight
gradient developing to the south of the depression. During the afternoon the depression tracked across South Wales to Leicestershire
and then into the southern North Sea. The depression produced gusts to over 70mph
in the English Channel and northern France and to over 60mph across southern coastal counties. During
the afternoon some particularly heavy convective cells developed over central southern England and tracked north-east, with
at least one producing a mesocyclone / tornado across Berkshire with other reported tornado's across Suffolk causing damage
to property.
Other notable gusts recorded on the 30th included Dover's
Langdon Bay station with 88mph.
Notable UK maximum gusts reported on the 30th December
