The
technical background to the storm of 27th October
The storm of the 27th / 28th October was a fine
example of a classically developing storm in the Atlantic, from a humble shallow low pressure to a possible record breaker
in just over 24 hrs. By mid morning on the 27th the low centred to the west of Brittany was
estimated to have a central pressure around 949-950mbs (one of the deepest in recent years below 50N in October)
To enable us to fully understand the rapid deepening
of the low we need to look back at the synoptic set up over the North Atlantic during the preceeding 36 hrs and examine
some of the features present in the upper atmosphere that led up to this storm. Looking at the 300mb flow over
the North Atlantic at 00Z on the 26th October (image top right, click for a larger image, opening in a new window)
(300mb is an excellent level for synoptic meteorology, and is frequently used to determine the height of the polar front
jet stream), we can see that a large upper trough was sinking south over the mid Atlantic, an already deep trough was present
over Biscay and Western Europe and a highly mobile but rather warm sub tropical low latitude flow was moving over the Azores.
The relatively low latitude of the jet stream
(40-45N at 25-50W) led to warm sub tropical air moving north associated with a complex but relatively shallow low which had
formed north of Bermuda over the previous 30 hours. During the 26th a breakaway low developed and moved east towards the Azores
whilst at the same time a deep layer cold upper air trough was moving SE from SE Greenland and the Davis Straight. A sharp
temperature contrast was therefore establishing itself across the Central Atlantic. Massive
amounts of upper air cold advection were pouring into the rear of the upper trough whilst warm ridging ahead of the low level
thermal zone threw a breif ridge across Ireland and the UK. The net effect of all this was to sharpen the upper
trough by generating large amounts of vorticity advection on the ascending side of it.
All the features outlined above can be seen in
the WV image (second image from top on right; courtesy of Dundee University) which has been annotated to show the
major players. Over the next 12 hours increasing interaction took place between the low level thermal zone and the trough
sinking south. As the low crossed from the warm side of the jet (little development) to the cold polar side (more development)
it coincided with the trough swinging in from the west and the PVA began to play its part. A
100 kt jet was present on the forward side of the upper trough. During the morning the low pressure area came under the influence
of the diffluent left exit of the jet, PVA was locked in place by the developing low and rapid deepening began.
The WV image for 00Z on the 27th (third image
from top, right) shows the low clearly undergoing explosive deepening, the upper trough rapidly moving in and PVA being
locked in place (the dry wedge (darker area) cutting under the cloud head, usually a sure sign that rapid cyclogenesis
is about to occur. The cold air cloud leaf is separating the cold polar air to the NW over the Atlantic. At 12Z
on the 26th the shallow low was analysed at around 992mbs over or just of the NE of the Azores; over the next 12 hours it
deepened to around 953mbs (a fall of 39mbs in 12 hrs -easily classified as "explosive deepening"). Some further slight
deepening is likely to have taken place during the morning with maximum depth probably acheived around 06-09Z with the central
pressure probably of around 949/950mbs. A visible satellite image for 12Z on the 27th is shown (fourth image down on
right) the classic swirl storm with convection running into the low with a wrap around coming around the south side of
the low).
The upper trough and jet overran the low later
in the day and the big upper level high over NE Europe led to a gradual slowing of the surface centre. The low quickly became
vertically stacked and with the jet moving on ahead the low stalled to the South of Ireland and began to slowly fill. A plethora
of ships were available to aid the analysis process. Ship 'DEDI', the Heidelberg Express, (image, right) out from
its home port of Hamburg, provided some of the most useful.
Huge pressure falls (>15mbs/3 hrs) were occuring
at the ship overnight (26th/27th) and SE'ly winds increased to a mean of 56knts during the middle of the night. A slackening of the winds occured following the passage of the front. The ship came very
close to the centre during the early morning (954.5mbs), then directly under the backwash around the southern flank of
the low. Winds quickly increased Storm Force 10, then Violent Storm 11, then between 10 & 11 am winds increased to Hurricane
Force 12 at the ship. Sustained winds were recorded at 66 Knts or 78 mph. (it should be noted the ship is in the list
of UK Met Office 'reliable' ships). The Sea state in these kinds of winds would be very violent indeed. Hurricane Force 12 Warnings were hastily issued for sea area 'Fitzroy' by the Met Office as it became apparent
just how strong this storm was on its southern flank.
This level of winds was not surprising given the
level of deepening in the previous 12 hrs; the windfield typically will take a few hours to catch up with the pressure falls.
During the afternoon however with the windfield expanding these kinds of winds did not rotate around the low to
affect the extreme SW of the UK as had been suggested might have been possible.
Huge seas were generated by this low and this
combined with large waves and tides created some serious coastal flooding along the south coast from Dorset to Cornwall. Highest
gusts appear to have been near 70 mph at Llanbedr and Brixham's Berry Head coastguard station.
NW France saw gusts to near 80mph (see table above), whilst elsewhere across the UK, it was just another wet and
windy day. However, if the set up had been just 5 degrees further east, then the story would have been very different
indeed.
This low was a classic text book example of explosive
cyclogenesis occurring when all the upper air parameters come together. Although this does occur rather frequently
in the North Atlantic- and in that respect this low was no different- to get such explosive development south of
50N in October is something a little rarer.
Paul Blight (C) 2004.
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