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The 4th October Squall line-tornadic or not?
An interesting feature as noted here of a system that developed over the UK on the 4th October was the impressive line convection which developed on it during
the early morning hours. The line convection was associated with a marked wind veer (SSW to W) and some very strong gusts.
Wainfleet (Lincs) reported a 68mph wind gust, Mumbles (near Swansea, S Wales) and Wittering, Lincs (well inland) both
reported a 65mph wind gust. On the surface this looks like a classic case of a rather severe gust front on a
cold front. However various reports of more substantial damage that came in after the event led some to suggest that
in fact tornadic activity may have been involved as well. There was a TORRO tornado watch issued in relation to the squall
line in fact.
Dotted throughout England there were reports of rather localised
damage. In fact the media reported a 'tornado' that occurred at Cawood in North Yorks (south of York),
and Colin Grice living in the Esk Valley, about 18 miles West of Whitby also reported something similar locally (see
box below) with further reports in evidence from nearby Glaisdale, about 11 miles west of Whitby. Reports of similar events
came in from Kingston upon Hull (Lincs) where at about 8.20am many large branches and some trees
fell, closing roads; also at Bilton nearby an Asda store had part of it's roof taken off.
In the south of the UK too, quite substantial damage
was reported just north of Midsomer Norton (Somerset) at Paulton (see text box below).In the north west at Huxley, a few miles SE of Chester, substantial tree damage was reported with
suggestions of a tornado having occurred being reported in the press here too.
An eyewitness report
In the Esk Valley (N Yorks) just
before 8am, the most violent rainstorm passed through here. For about 5 minutes it was almost as if someone had a fire hose
on us. What is more the wind, already strong was whipping the deluge about in all directions. Spray was bouncing off land
and buildings, the overall effect was almost like being at sea!
Going round livestock a couple of hours later revealed
a lot of branches torn from trees, but only in quite small specific areas. Unusually, it seemed as though a lot of the branches
had broken some way from the main trunk, rather than the trees losing a big limb as you might expect. In some cases these
quite large boughs had been carried or lifted maybe 200 metres from the tree.
It looks as though the narrow band ahead
of the larger area of rain was the source of this, and it seems to be a feature of maybe 150 miles length in corresponding
radar images
Colin Grice
Opposite: Pictures from the Esk Valley after the 4th October
event
Credit-Colin Grice(C) 2004 and (below) enlarged radar still from UKww
site at 0730Z on October 4th showing the two squall lines crossing the UK. Note the 'white' pixels near the North Yorks area
indicating extremely heavy precipitation.
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Pictures from the Esk Valley after the 4th October event
Credit-Colin Grice(C) 2004
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One of the jobs of those investigating such events is
to assess whether such events have been caused by straight line winds or tornadic vortices.
Sam Jowett writes: In a situation like a squall line in the UK,
there can often be the threat of tornadic activity embedded within the front, due to the sudden change in wind direction and
strength, which can initiate rotation. This may have been evident on the 4th October 2004 when several reports of "whirlwinds"
etc were recorded. The difficulty is that to verify such reports, organisations
like TORRO (website at http://www.torro.org.uk ) often have only the evidence of damage to determine the existence of
possible tornadic activity.
A fast moving event like a squall line may not only produce winds in a straight
line capable of causing damage, but if there is also damage caused by tornadoes it can be difficult to identify as the
forward speed of the system tends to mean all debris is thrown in the direction of motion. The signs to look for that may
indicate a tornadic event though are that they will always produce a tight swathe of damage, often only a few tens of metres
wide. Damaged trees will likely appear 'mangled' and twisted, with snapped trees often broken above ground level rather than
uprooted.
The debris that is scattered is important too, often being thrown away at a tangent
from the vortex, though the forward speed of the squall front may mask this. Tornadic winds also often cause tiles to be removed
from the lee side of a roof, rather than the windward side, such debris often being catapulted as projectiles and embedded
in lawns etc. The best evidence is obviously that of the eye witness and what they have seen, though.
TORRO manager Mark Humpage adds 'there may well be non-visible
ground touchdown but with ground damage. I have seen myself a tornado with only a visible funnel well off the
ground, but on closer inspection damage is occurring on the ground. Something to be aware of, should anyone
witness such an event, is not just to look at the funnel (which is common practice) but also check out
the ground for debris'.
If unsure whether or not there has been a tornado at your location, contact either
Torro or UK Weatherworld for further assistance.
The Paulton event, near Midsomer Norton, East Somerset. ( Location map here)
Andy Davis, a manager of the UK weathersite 'UKWeatherworld' heard
from his son soon after the squall had gone through. He reported that 'at around 7:15am (4th October 2004)
an incredibly loud noise had woken (him) up, like if a gale was blowing but ten times louder. A seven metre branch
hit the house with great force.The incredible noise lasted for about twenty seconds.During that time many trees in a line
of about 150 metres were ripped about, or snapped clean in half. A twenty metre oak was snapped in half. However, other
trees either side of this line of damage were unscathed.
Andy decided to drive down to conduct an initial site investigation of the event.
He found the swathe of damage was
about 7 metres wide and 50 metres long and that the trees
affected were bigger than first described - well over 20 metres tall in fact. The trees had not been uprooted but
five of them were left with major limbs missing, (see pictures opposite), their main trunks gone along
with the foliage. The few branches that remained were just side limbs from near the bottom of the trunks. The trunks
were ripped off between 2m to 7m from the ground. The limbs show consistant signs of being twisted off,
along with the trunks. The time scale from witnesses ranges from 10 secs to 20 secs; the total time that the trees took
to be damaged.
Andy's son told him more about the debris he saw flying about in the air, into the
house, into the roof and over the roof. Some limbs were carried over the houses and beyond to an extent of about 100 metres
with some debris up to 200 metres away. He had not seen damage to such big trees where the limbs
had been ripped off in such a fashion; seemingly slightly twisted and disorganised. No other trees/property seemed to
have been damaged even nearby.
He also interviewed an eye witness who says she saw
'a tornado out the back'. The witness described how a 'tornado' cloud moved quite clearly moving across her garden;
she said the cloud was not in contact with the ground but started about 20-30' above it and removed her garden
table from the garden clean away. Her house is about 25 metres beyond the White 'VW Caddy' van in the top picture shown
opposite and the tornado, as she described it, was coming towards the area where the man and boy are.
There were other witnesses he didn't speak too, but it seems a few people saw the tornado itself.
Terence Meaden of 'TORRO' is currently investigating the incident further.
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Tree related damage from the 4th October event at Paulton |
With thanks to Andy Davis, Mark Humpage, Colin Grice and
Sam Jowett for contributions to this article.
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