AUTUMN 2006 -
part 5: The Bow Street Tornado
BACK TO WEATHER-BLOG MENU
Fine Art Prints & digital images for sale-
Welsh Weather & Dyfi Valley landscapes Slide-Library - Click HERE
- THERE'S NO SUCH THING AS A "MINI-TORNADO"!
up on the morning of Tuesday 28th November 2006 I found there were two
messages on the answerphone. Funny I thought, seeing as it's before
9am. One was from Tony Gilbert, Site Investigations Coordinator for
TORRO (the Tornado & Storm Research Organisation) and the other was
from Carl Yapp from BBC Wales. Both indicated the same thing - the
sooner I could get over to Bow Street the better. There had been a
tornado in the night and a site investigation was needed.
I should at this point explain why TORRO undertakes site
investigations. When a severe and damaging wind, of whatever type,
strikes an area then it will create damage patterns specific to the
type of wind and its intensity. In order to figure out our changing
climate, the more we can look into each extreme weather event and
understand it, the better. A TORRO site investigator, then, is a
meteorological forensic detective. He or she will map out and record
the damage, take photographs and interview witnesses and the data
collected will then enable conclusions to be drawn regarding the event
type and its power.
Each event can then be entered onto TORRO's ongoing database and
statistical examination of patterns of occurence may, in years to come,
be able to tell us whether any trends are emerging.
The investigation quickly confirmed that a tornado had indeed tracked
through Bow Street, with ferocious and damaging winds of over 130mph. A
perfectly natural, if unusual event, and so you will not see either
"freak" or "mini-tornado" used anywhere in the account below. The
latter term, coined by the media, is grossly misleading: it implies a
harmless whirl, like a summer dust-devil. The Bow Street event was
nothing of the kind: it was a violent and potentially lethal vortex
over 50m in width.
WEATHER AT THE TIME
Let's have a look at the weather on the night of the 27th-28th. Here is
a sketch of the synoptic situation at midnight. A deep Atlantic low
sits out to the NW of the UK, with south-westerly winds (red arrows)
over Wales. Just to the west there lies a cold front (pale blue), with
a trough (blue line) following on behind.
Earlier in the day there had been heavy showers and I witnessed a
thunderstorm not far from Tywyn, up the coast from Aberdyfi. Further
activity was forecast overnight.
let's look at the rainfall radar (from Netweather, an excellent subscription service),
for 0100 on Tuesday 28th. The cold front is clearly marked out by a
line of rain stretching from Cornwall across the Bristol Channel and up
across western Wales.
Within the general wet weather (blues and greens) there can be seen
local bright areas (yellows and reds). These indicate torrential
downpours (anything over 20mm/hr is "stair-rods"!), which in turn tell
us that within the cold front there are embedded convective cells -
heavy showers and thunderstorms.
Wherever convection occurs, there exist vigorous upward air-currents -
the updraughts that lift warm moist air high into the convective
cumulonimbus cloud. And if, for a variety of possible reasons, the air
rising into the cloud starts to rotate, or corkscrew upwards, then
there exists the risk of a tornado forming. Sudden updraught
intensification (i.e. a massive jump in the speed of the upward
air-currents within the cloud) will tighten that rotation down into a
narrow but very rapidly spinning column of air, which may extend down
from the cloudbase all the way to the ground. We now have a tornado.
Let us now examine the path of the Bow Street Tornado, and see what it
can tell us about itself.
One part of a TORRO site-investigation is to try to determine the
length of the tornado track on the ground. Here is the first evidence,
on the A487 south of Bow Street. Speed-limit signs have been blown
over; the nearest one has been completely uprooted despite being
anchored in a large mass of concrete.
In the opposite direction, towards Aberystwyth, is the railway with
trees on its embankment. Scanning these with binoculars revealed no
damage: therefore, the first touchdown must have been between the
railway and these road-signs.
The tornado continued along the main road, destroying this road-sign in
the process and flinging part of it towards the railway...
Give way? Indeed it has done so. This is the A487/A4159 junction. Note
the "Bow Street" sign is undamaged - this area must have been close to
the edge of the vortex...
Beyond the junction lies a builders yard with some trees to its S. The
tornado affected both, indications being that it had veered slightly L
Eyewitnesses reported bright flashes without thunder: these were
power-flashes as utility poles were toppled sending live cables snaking
and sparking about on the ground....
This large Scots Pine stood just N of the utility pole (above) and was
The tornado at this point was wide enough to uproot trees alongside the
main road and also to damage this building at the builders yard,
removing parts of its roof and its shutter-doors...
Beyond the builders yard are houses and the pub - the Welsh Black.
Here, there were eyewitnesses, awoken by the noise (variously described
as a train crash or a plane crash or simply a teriffic roaring) and the
bangs and crashes of their properties being mauled. All reported
torrential rain after the
Pub manager Darren Davies, 24, said:
“I thought a lorry had gone through a house. It was deafening. I looked
out the window and it was raining bricks. It was like an action movie.”
He said a 4x4 was lifted 20 yards across the pub car park. (source: Sun
The arrows in this photo, taken several days after the event (when roof
tarpaulins better mark badly damaged properties) indicates the tornado
width here. It had also clipped a few properties along the main road
opposite the Welsh Black. The caravan in the image was rolled on its
side. The worst damage was to the house with the tarpaulin (L) where
that portion of the roof was removed in its entirety. Numerous
chimney-stacks were felled. And, in a feature typical of many
tornadoes, other properties escaped relatively unscathed. Tornado
damage can be surprisingly fickle in its nature...
Here is a typical example, where the main track is L of the railway.
There is no damage apparent to the trees R of the railway, yet where
the red arrow is a powerful blast of wind has scattered items about and
felled a tree. This may be evidence for a multi-vortex event i.e. more
than one twister within a larger, parent circulation....
The tornado was at its full fury as it crossed the railway bridge on
the B-road to Clarach. Heavy capping-stones were dislodged from the
bridge: it is not known whether this was achieved by wind alone or by
heavy airborne missiles. On the other side is a car dealership. The
roof was torn off the building R and a large Portakabin that would
normally have been in the foreground here was levitated in its
entirety. Beyond there is severe tree damage alongside the railway...
Another shot looking N up the railway a few days later, with the
clear-up well underway. The path of the tornado across the railway and
into the field beyond is clearly evident. Its RHS is demarked by the
intact trees beginning again; within the path the majority of trees
have been destroyed...
Looking over the railway into the fields beyond on the morning itself,
residents survey the damage. By now, people were searching for car-keys
that had been stored in the Portakabin. Many have since been found, all
credit to the villagers...
Down by the railway, this tree is decorated with shredded paper....
Over the bridge and along the road a little, a lamp-post lies bent
over, the fence is flattened and builders are checking for and
repairing roof damage to the houses beyond. To their R lies a small
The playground was surrounded by wooden and mesh fences, both of which
were destroyed. But the swings in the distance have been affected
This is part of the Portakabin from the garage. Mangled almost beyond
recognition, it has flown over the railway, been folded over the swings
and is jammed there quite solidly. This is serious damage....
Another shot with Carl Yapp of BBC Wales. Note how the mesh fence has
somehow become wrapped around the swings (RHS of image)...
From the swings looking along the tornado track to the north. General
tree destruction in the distance and this and the next oak in the field
are badly debranched. Here is a vital clue.
The windfield before and after the tornado would have meant that the
wind would have blown from SW-NE - bottom L to top R in this image. The
oak in the foreground has had its broken branches thrown to the left -
or westwards, against the prevailing wind.
Most tornadoes spin cyclonically, or anticlockwise. When this one was
passing through, therefore, on its leading edge the winds would have
been blowing from R to L. As it passed over, there would have been a
brief calm, before furious winds from L to R set in, marking its rear
flank. That oak has clearly been struck by the leading edge of the
tornado and its branches thrown accordingly: clear evidence that
tornadic, and not other high winds, had occurred.
This photo was taken straight into the sun so it's not too good. It
shows the second oak in the previous image and on the ground nearby
lies the heavy sheet steel-clad Portakabin door, having flown through
the air for over 400 metres....
Back down and along the brook a little way, here is another piece of
Further along again, the mangled trees and items of large debris track
the tornado's course...
At this point, it is clear that the tornado veered a little to the NE,
crossing the brook. It then took a course through several housing
estates, causing locally heavy damage to rooves, garage doors, sheds
and greenhouses: mercifully, however, by now it was diminishing in its
Between Bow Street and Rhydypennau two large fields span the distance
between the A487 and the brook, and at their northern end I found this
rolled-up mass of sheet-metal (beyond my jeep, that is!). This came
from the garage with the Portakabin and seems to represent the last
piece of large debris from there - thanks to Eric Ellis-Jones for
emailing me that info! It has therefore travelled, presumably entirely
in the air, for just over a kilometre!
This street, Maes Ceiro, saw the northernmost serious structural
damage. I am indebted to Elwyn Owen for the following notes:
"Our house has a flat roof and at 1.18am on THE night it (ie the whole
roof not just the felt) was almost blown off. In fact half the roof is
off and, according to a builder , the other half would also have been
off were it not for the chimney stack wedging it down. A similar damage
occurred next door and another house will have to be gutted because the
felt roof came off. The roof was "made safe" temporarily by bracketing
it down and the 60-70 mph wind on the following days hardly moved it
which just goes to show the strength of the wind on THE night. The
"bang" as it hit the house was unbelievable."
- THE AMBULANCE DRIVER'S EXPERIENCE
The last bit of structural damage I could identify was to a house in
the estate opposite Rhydypennau School. However the tornado was still
on the ground by the Rhydypennau Inn. This we know from an eyewitness
account from an Machynlleth ambulanceman I interviewed.
He was taking a patient into the hospital at Aberystwyth on that night.
Approaching Bow Street from Talybont, he noted flashes in the distance
which he, quite reasonably, took to be lightning. However, on
approaching the Rhydypennau Inn, the vehicle met the tornado head-on.
"I can only describe it as driving into a black tidal-wave", he told
me. "You literally could not see a thing ahead and I had to stop". A
dustbin flew by from L to R and lots of small debris struck the
vehicle, which was rocking from side to side. All the time this was
going on there was blinding horizontal rain. Then, after what had
seemed an eternity, but was probably only about 15 seconds, it passed.
The ambulance then continued towards Aberystwyth, having to carefully
negotiate bricks, tiles and other debris all over the road in the
southern part of the village. The crew put out an immediate call to
Control, warning them to expect casualties. They arrived in Aberystwyth
shortly before 1.30am.
Earlier in their journey it had been necessary to stop and administer
assistance to the patient, resulting in a delay of a few minutes to
their journey. Had that not occurred, they could have met the tornado
at the other end of Bow Street, where it was at its most savage. Fate
had spared them that ordeal.
The tornado must have dissolved away shortly after the ambulance
incident, but lighter debris, lofted up into the storm-cloud, remained,
kept in the air by the storm's updraughts. Bit by bit it fell to
ground: I tracked paper and fragments of corrugated plastic as far as
Talybont, and more recently some paperwork from the garage has been
found in Corris, near Machynlleth and almost 20 miles to the NE!
LONG AND HOW STRONG? - ASSESSING THE DATA
Here is a 1:50,000 map showing the tornado track and its width, so far
as I could ascertain from my day on-site. It was on the ground for a
little over two kilometres.
Image produced from the Ordnance Survey Get-a-map service. Image reproduced with kind permission
of Ordnance Survey and Ordnance Survey of Northern Ireland.
So, the investigation determined that a significant tornado had
occurred in Bow Street in the early hours of November 28th. It was on
the ground for over 2km and was getting on for 100m wide in places.
So, how strong was it?
The International Tornado Scale ranks tornadoes from 0 to 10 in terms
of their windspeed. Because of their violence, it is not possible to
measure tornado winds conventionally, but instead we may estimate their
strength from the level of damage found. Here is the scale:
litter raised from ground-level in spirals. Tents, marquees seriously
disturbed; most exposed tiles, slates on roofs dislodged. Twigs
snapped; trail visible through crops.
small plants, heavy litter becomes airborne; minor damage to sheds.
More serious dislodging of tiles, slates, chimney pots. Wooden fences
flattened. Slight damage to hedges and trees.
homes displaced, light caravans blown over, garden sheds destroyed,
garage roofs torn away, much damage to tiled roofs and chimney stacks.
General damage to trees, some big branches twisted or snapped off,
small trees uprooted.
overturned / badly damaged; light caravans destroyed; garages and weak
outbuildings destroyed; house roof timbers considerably exposed. Some
of the bigger trees snapped or uprooted.
levitated. Mobile homes airborne / destroyed; sheds airborne for
considerable distances; entire roofs removed from some houses; roof
timbers of stronger brick or stone houses completely exposed; gable
ends torn away. Numerous trees uprooted or snapped.
vehicles levitated; more serious building damage that for T4, yet house
walls usually remaining; the oldest, weakest buildings may collapse
houses lose entire roofs and perhaps also a wall; more of the
less-strong buildings collapse.
houses wholly demolished; some walls of stone or brick houses beaten
down or collapse; steel-framed warehouse-type constructions may buckle
slightly. Locomotives thrown over. Noticeable de-barking of trees by
hurled great distances. Wooden-framed houses and their contents
dispersed over long distances; stone or brick houses irreparably
damaged; steel-framed buildings buckled.
steel-framed buildings badly damaged; locomotives or trains hurled some
distances. Complete debarking of any standing tree-trunks
houses and similar buildings lifted bodily from foundations and carried
some distances. Steel-reinforced concrete buildings may be severely
The most intense
damage was from the road junction to the Welsh Black Inn, on to the
garage where it was particularly wild, and into the playground and
fields beyond. This I would rate as T4. Beyond that as it veered into
the housing estates E of the brook, it dropped quickly, doing T3, T2
and T1 damage until it dissipated somewhere N of the Rhydypennau Inn.
It was probably at low T1 intensity where the ambulance met it.
People familiar with documentaries on Tornado Alley in the USA will
likely be aware of the American F- (Fujita) scale and here it is for
to chimneys; breaks branches off trees; pushes over shallow-rooted
trees; damages sign boards.
limit is the beginning of hurricane wind speed; peels surface off
roofs; mobile homes pushed off foundations or overturned; moving autos
pushed off the roads; attached garages may be destroyed.
damage. Roofs torn off frame houses; mobile homes demolished; boxcars
pushed over; large trees snapped or uprooted; light object missiles
||Roof and some
walls torn off well constructed houses; trains overturned; most trees
in forest uprooted
houses leveled; structures with weak foundations blown off some
distance; cars thrown and large missiles generated.
houses lifted off foundations and carried considerable distances to
disintegrate; automobile sized missiles fly through the air in excess
of 100 meters; trees debarked; steel re-inforced concrete structures
On the F-scale I would therefore rate the Bow Street Tornado as an F-2.
Some other questions that I was asked during BBC Wales interviews on
the day are:
"How was it caused?"
Tornadoes can form by several processes, perhaps the best known being
in association with a supercell thunderstorm, in which an organised,
longlived deep rotating updraught, or mesocyclone, is present. However,
such storms are uncommon in the UK and we frequently have to look to
other mechanisms of formation, one important one being ingested
vorticity. Let's have another look at the map because it makes it
easier to explain:
Vorticity is a scientific term pertaining to the amount of circulation
or rotating currents present in a fluid or gas, and it can often be
induced as the fluid or gas passes a fixed object.
If you go and stand on an arched bridge over a river (i.e. one that has
supporting pillars built in the river-bed), and look down into the
water on the downstream side, you will see that, immediately downstream
of the pillars, the water is not flowing as evenly as it is elsewhere,
but is full of little whirlpools all the time. The interaction of the
water passing the pillars, and curving around them, has caused this.
That water full of little whirlpools has greatly increased vorticity,
and you can see it!
Now let's look at the air blowing along towards Bow Street on the night
in question. It was being forced through the broad topographical
wind-gap that exists in this area, and is partly followed by the Nant
Peithyll. Exiting this gap there would be a tendency for some of the
air to continue turning round to the NNW, which in turn would cause the
low-level air to develop numerous cyclonic (anticlockwise) eddies -
i.e. the air downstream from this gap would have strongly enhanced
cyclonic vorticity, just like that water downstream from the bridge in
our imaginary example above.
A developing convective storm with a strong updraught and low base,
crossing this area, as was the case in the early hours of the 28th,
would have pulled the air below up into its cloudbase, as all
developing storms do. But, if the updraught was particularly vigorous
at the time, and it pulled the high-vorticity air into its base, the
vertical updraught movement combined with the rotational vortex
movement would have both stretched and speeded up the eddying winds to
the point that they became a narrow but furiously-rotating tornado.
Seen a spinning ice-skater bring her arms in and then up above her
head? She is getting narrower and longer, and starts to spin faster and
The tornado would have only lasted a matter of minutes before it
dissipated, probably due to the storm updraught weakening -
individually these were short-lived storm cells. It just happened that
this one crossed an area packed with low-level vorticity at exactly the
right moment in its development, ingested all that vorticity, stretched
and sped it up and this was the result.
My thanks to Nigel Bolton of TORRO for
an illuminating discussion on this mechanism for tornado formation.
"How likely is it that Bow Street will
have another tornado?"
The answer is that it is possible, because tornadoes can touch down
anywhere, but it is statistically unlikely. However, winning the
Lottery is statistically unlikely, but people do win it, so it is
impossible to 100% guarantee it will not happen again.
"We keep hearing that Global Warming
will bring more extreme weather. Will that include more tornadoes?"
This is impossible to answer on a "yes or no" basis. The truth is that
we do not yet know enough about the effects of climate change on any
one part of the globe. It may be the case that more energy in terms of
heat may bring more severe storms our way: then again warming may
change the synoptic pattern in ways that spare us the worst of the
TORRO are not heavily involved in the climate change debate. Instead,
the organisation monitors and records extreme weather events such as
this one in detail that is not possible to achieve elsewhere. The
organisation is unique in that, instead of having all its employees
based at one office and often deskbound, it has a large network of
volunteers throughout the UK. This means that when a severe event like
this strikes, someone is usually available to go and collect that vital
data. On this day, that somebody just happened to be me.
Site investigations can be deeply moving occasions. One is awestruck by
the fury of nature, and there is always the need to be sensitive to
residents who have seen their properties badly damaged or worse. I
found this scene particularly disturbing. I love photographing
thunderclouds and here is one - there were a number around on the day
after - with the trashed swing in the foreground. It sums the day up
for me. Great natural beauty, but also such furious, merciless
destruction, can both feature in the weather.
Shortly before I took this photograph, a BBC reporter asked me why I
love tornadoes. I replied firmly that I do not. They are interesting,
and we need to gather more data and work out more about them, just as
we need to better understand our changing climate and how to cope with
it. It is my belief that the work of TORRO will contribute to that in
the long term.
Finally, I would like to express my deep sympathy with anyone adversely
affected by this event and my heartfelt thanks to everybody who helped
me in this investigation. If anyone has information they think would be
worth adding to this report, please email me HERE.
BACK TO WEATHER-BLOG MENU
Fine Art Prints & digital images for sale-
Welsh Weather & Dyfi Valley landscapes Slide-Library - Click HERE