The wet and windy
start to 2005 continued as an active Atlantic depression ploughed across the North of the UK.
The storm was centred far to the West of Ireland on the 11th and began to deepen rapidly as it approached the Northwest of
Scotland during the day bringing a spell of damaging winds, these following on from those that hit the North on the 8th.
Mean wind speeds approached sustained hurricane force and gusted to record breaking speeds in the far north and west for a
time during the late evening and early morning period of the 11th/12th.
At midnight the
low was centred close to North Rona (a remote island station located in the far west of the Western Isles) predicted to be
close to 944mbars as seen on the 00Z synoptic chart: http://www.wetterzentrale.de/archive/2005/brack/bracka20050112.gif). It was during this hour that the station recorded a mean
low level wind speed of 94kts (108 mph) and a phenomenal gust of 116kts (134 mph) – which would be the 2nd highest
low level gust ever recorded. However there is concern about the station exposure and discussion continues as to the
veracity of the report. Although the station is very isolated and exposed, it was not alone in recording a gusts in excess
of 100 mph during the night as is shown by Table 1 below. The strongest lowland gust aside from at N Rona, of 116mph, was recorded
by a trusty Davis Vantage Pro 1 at Carloway on the Isle of Lewis. The wind rig had been put into service only the day before
the storm and the owner wasn't sure it would survive it's first testing; however it clearly did!
Table 1: Selected wind gusts recorded during the 11th and 12th
|
Station |
Wind Gust
(mph) |
|
Aanoch
Mor (3000ft)* |
142 |
|
Cairngorm
(4000ft)* |
139 |
| North
Rona, W Isles* |
134 |
|
Carloway,
Isle of Lewis
Sule
Skerry |
116
115 |
|
Barra |
106 |
|
Benbecula |
103 |
|
Stornoway |
101 |
|
Loch Glascarnoch |
96 |
|
Fair Isle |
92 |
|
Lerwick |
91 |
* Not accepted as lowland readings
Hurricane force
winds which swept the Western Isles during the evening as the centre of the low (still falling at the time) passed just to
the North - caused a considerable amount of structural damage and flooding in the area as the tide surged inland. Tragically
a family of five lost their lives in the storm as they attempted to escape from rising floodwaters at their home on
Benbecula - the car in which they were travelling was washed into the sea by the tide and severe winds.
Much of Scotland was affected by the storm – roads were blocked,
main bridges closed, ferry services and rail operations abandoned. Over 60,000 were left without power as trees and power
lines lay strewn across the countryside in the storms wake. During the afternoon a driver was killed after a lorry blown over
the A1 near Burnmouth crushed their car, many were evacuated from their homes as 4ft flood waters engulfed parts of Oban,
a Spanish fishing boat which went missing off the Hebrides was rescued by the RAF – all 19 passengers safe. Further
flooding also occurred in the Western and Northern Isles – particularly badly affected was Ronaldsay in Orkney where it
was estimated to be the worst flooding for 20 years.
A discussion of models forecast synoptic development and the actual
synoptic set up
WV imagery at midday on the 10th January already showed that a low approaching
mid Atlantic was fairly well developed. Upper level forcing had already engaged the low level surface centre and generated
some deepening and the forcing appeared to be overrunning the low as it is moved NE towards the British Isles. All
the available models agreed on another bout of deepening before it reached the UK and it was this second phase of engagement
which produced the likelihood of explosive deepening. Synoptic charts for midday on the 10th showing SLP, 500mb , 700m
and 850mb levels are available here- http://www.wetterzentrale.de/archive//ra2/2005/Rrea2005011012.gif
As a broad and sharpening upper trough moved east from Newfoundland a
jet streak rounded the base of this trough and engaged the surface low to the SW of Ireland resulting in powerful
cyclogenesis for about 24 hrs. Model guidance all indicated an
exceptionally strong gradient developing to the south of the low centre with core surface winds from peaking at
80knts to the west of the Hebrides and 925mb winds near 100Knts.
By 21Z on the 10th the most intense
upper forcing associated with the sharpening upper trough was about to engage the surface low with continued explosive
cyclogenesis occuring over the extremely tight thermal gradient. The
depression was thus expected to "bomb" over the following 30 hrs - and would be one of the strongest depressions
to affect the UK for some considerable time.
The GFS was consistent in bringing a core of 70kt+ 10M winds just off the
Hebrides between 21Z and 03Z. It is very rare for a model to be consistently forecasting such strong low level 10 metre
winds and along with a core of 80kt 925mb winds (a short way above the surface) meant that mean winds
in sea areas Malin and Hebrides could exceed 85mph and gusts reach 105mph plus.
By 09Z on the 11th explosive deepening was underway west of Ireland. The UKMO 06Z analysis had a sub 960mb centre to the west of Valentia.
Analysis already pointed to a rather deeper centre than expected, models
already having some trouble keeping up with the development. The centre was probably around 8mbs deeper than suggested
by the T+6 Model Suite, the likelihood was of it playing 'catchup' throughout the day.
Of greatest concern was the shape of the low; imagery and the models did not
correlate terribly well. WV imagery in particular showed a more elongated low centre NW/SE than the models would ideally like.
This fact was born out by the observation from SHIP SBFC at 06Z to the west of Ireland. The ship reported a mean wind
of 64knts, somewhat higher than the UKMO guidance from T+6 had intimated (which kept that level of winds further NW, along
with an almost due westerly gradient, whereas the models had initially preferred a WSW'ly). The concern was that this rather elongated centre would propogate NE and thus bring the risk of the very
damaging winds closer to NW Ireland and W Scotland. The troughing
on the SE side of the low remained a large uncertainty though it was recognised that its development would
likely become clearer as more observations become available through the morning.
By midday the low continued
to look very impressive and deepen. (Charts are available here http://www.wetterzentrale.de/archive//ra2/2005/Rrea2005011112.gif)
Satellite representation of the centre continued to appear slightly more elongated
to the NW /SE than the models actual representation, with the strongest winds just to the south of the bank bent
occlusion; satellite 'scat' winds showed a core of 70 kt winds associated with it. There had also been a gust to 82 kts
at Belmullet- worrying, as it was rather higher than model gusts had been expecting at this time. Whilst it appeared to have
been associated with a hail squall, the strongest core of winds was now recognised as being likely to clip
NW Ireland and affect more of N Ireland than had previously been expected.
Pressure rises from Buoy 62108 had also exceeded 16mbs in a 3 hour period
during the morning in the confluence post upper trough, these pressure rises expanding as the low continued to deepen.
However there were few ship observations to back up the buoys, as ships had been advised to avoid the area. The
low was now expected to reach peak intensity some time around midnight to the NW of Scotland with a pressure near or
below 940mbs.
By 21Z the intense
depression to the NW of Scotland was nearing its maximum depth. WV Loops showed a decrease in forcing, as the storm became
vertically stacked. Central pressure was estimated at or slightly below 940mbs. At 21Z the isolated island station of
North Rona was reporting pressure as being 946mbs. The depression had slowed as it extended upwards and now moved
under its own momentum, as opposed to the jet driving it - which was about to move on.
Winds had been extreme across the NW during the evening. North Rona had already
reported mean winds of just short of 100mph with gusts to 115mph. Barra in the Hebrides had reported a 92Knt gust before the
automatic station had stopped reporting at 17Z.. Stornoway had reported a gust over 100mph in the last hour. Gusts of up to
75/80mph were also being reported along Scotland's West Coast, whilst gusts peaked in the Glasgow area just shy
of 70mph.
There was also some intense convection rotating around the southern flank
of the low, some very vigorous indeed with a cluster approaching 15W associated with a PVA Max. Trajectories moved this feature NE
to clip NW Ireland and then affect Western Scotland overnight. Snow levels were generally about 300m but were temporarily
lower in any squalls so blizzards were occurring over the Scottish mountains. Sferics west of Ireland highlighted the instabilty.
Winds rose a little more during the next few hours as noted above.. a peak wind of 134mph being
reported at N Rona though it should be emphasised that this is a very remote and unpopulated island station; however
generally after 00Z they started to abate.
By the morning of the 12th winds were subsiding as pressure rose quickly and the low moved away
to the NE. (Midday charts are available here- http://www.wetterzentrale.de/archive//ra2/2005/Rrea2005011212.gif ). However by this time the damage had been done as reported in the section above. If
there was a consolation it was the track of the low at least meant that the most destructive winds were spared from the
most populated areas of N Ireland and the Scottish Forth/Clyde valley. However it was certainly one of the most intense
depressions to affect the west coast of Scotland for many years.
The information is reproduced with the kind permisiion of Paul Blight and was abridged from
his discussion and reports of the storm on 'UK Weatherworld'.
