Winter
2004-05 PART 2:
The Atlantic awakes - and brings tornadoes and
landslides!
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On the evening of December 27th an
active cold front passed over England and Wales, with
embedded thunderstorms. Such features are known as
squall-lines and they can produce extremely violent
weather, although they tend to be narrow and pass through
quickly. Here are the notes I hastily wrote that evening:
"At about 9.55pm this evening I
was enjoying a glass of wine and watching a DVD when
suddenly the power went off, then came on again within a
few seconds. Shortly after there was a rattle of hail at
the window and then all hell broke loose. I rushed to the
front door and watched as sheets of torrential rain came
swirling horizontally up the street (i.e. in a N to S
direction), playing on the walls of the buildings
opposite as if being fired from a hosepipe. At times the
rain was actually moving diagonally upwards. Above the
howling of the wind and rain was a second distinct sound
- a deep roaring that passed in the darkness, seemingly
up the valley below me (I live on a slight hill just
above the flood-plain of the E-W trending Dyfi Valley).
Within only a couple of minutes an inch of water was
running down the street. I then noticed several
neighbours on their doorsteps looking out into the night
and asking what on earth had passed through and made that
roaring. The rain continued, but less heavily, for a few
more minutes while winds remained squally but not with
the violence of that first assault. Thereafter things
quietened down to how they had been prior to the power
cut."
The following morning I set off across the fields towards
the Dyfi looking for one thing - tornado damage. That
deep roaring is a sound often reported with tornadoes, so
I was somewhat disappointed to find no trace of damage at
all! However, elsewhere in Wales this was not the case
and a day or two later I investigated a tornado damage
track near Trefenter, south of Aberystwyth in the rolling
hills of mid-Ceredigion.......
The Trefenter event took place close to 10pm on
the night of the 27th and was accompanied with
that same terrifying deep roar, "like
Concorde taking off". There the similarity
ended. This was a tornado that caused damage. The
above image is looking back along the damage
track from the small farm it hit. The arrow
points to where it carved its narrow (10m wide)
path of destruction through the small wooded
valley .....
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....and
this is within its path. Multiple tree-damage
with branches and crowns snapped off. Tornadoes
often cause a lot of snap-off damage. Most of the
branches were thrown NE from the parent trees:
given that the prevailing wind that night was
WNW-ESE and most tornadoes have anticlockwise
circulation this is consistent with the strongest
tornadic winds, on its SE flank, where they blow
SW-NE.
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At the top of the field the tornado struck a
windbreak of sturdy Leylandii conifers and
snapped them off close to ground-level, again
throwing them slightly NE. The largest affected
tree had a trunk 25cm in thickness. Leylandii are
notoriously strong - these were growing squarely
in the path of the prevailing wind - and the
force to do this takes some imagining! Also note
in this image the polytunnel and greenhouse in
the background - utterly unscathed. They were not
in the tornado's path but some distance to its
north - fortunately.
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The tornado then removed the ridge-tiles and the
steel chimney-flue from the barn in the
background (arrowed). This image is taken in the
area where the couple living here picked up most
of the tiles, mainly broken! Very fortunately the
tornado missed the farmhouse itself.
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This is a close-up of the objects lying in the
foreground of the above image! Prior to the
tornado's arrival, this 1 x 1.5m piece of 10mm
ply was the other side of the barn. It had
travelled up over the barn and cast down, snapped
clean in two, on this side! I tried to break the
larger piece and could only bend it slightly!
Beyond that there was no more damage, indicating
that the tornado had lifted after hitting the top
of the barn, but bits of roof insulation and
Leylandii foliage were lying about on the ground
for a little further.
Rating-wise this was in the upper end of T2 on
the TORRO scale:
T2 - Moderate Tornado: wind 33 - 41 metres/second
or 73 - 92 mph. Heavy mobile 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.
This was one of several tornadoes from that cold
front. Others were confirmed after TORRO site
investigations in South Wales, Somerset and
Gloucestershire, making this a significant
tornado outbreak by UK standards. And only days
later a second active cold front brought with it
another outbreak, on January 1st 2005: a grim
start indeed to the new year for some. Here,
meanwhile, the next lot of weather-related
trouble came on the 7th.....
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A rapidly deepening Atlantic depression tacked
across the UK bringing a swathe of storm-force
winds and prolonged heavy rain on the 7th and
overnight into the 8th.
By the following morning things had calmed down
and I went out for a look. Firstly the Dyfi - as
it tends to in such conditions - had burst its
banks and the main road north out of Machynlleth
was closed. As often happens several times each
winter, some drivers had attempted to drive
through the deep floodwater and had run into
trouble. The Dyfi Bridge section of the A487 is a
graveyard for diesel engines wrecked when water
gets sucked up into the air intake - beware!
What was of interest was the landslide that
affected the A470 just below the top of the pass
between Dinas Mawddwy and Dolgellau. This was a
classic bog-burst type, when an intense rainwater
buildup occurs under a peaty bed of reeds,
sphagnum moss etc, sat upon an impermeable
glacial drift subsoil. This pushes the peat and
vegetation up until the consequent
"dome" ruptures. It shot ca. 80m down
the steep hillside, with a track 15m wide at the
top, widening to 25m at the road and fanning over
a broad area in the valley below. 80m of steel
crash-barrier was torn from its mountings, though
still in one length, and as seen here it lay in a
big arc across the debris well below the
road.....
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....here looking up
in the opposite direction....
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.....and here looking straight up at the slide
along its path....
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....and here looking down from its point of
initiation. The road only suffered minor
damage as most of the debris was peat and
vegetation, but boulders weighing upwards of 75kg
were also transported. Clearly a forceful
debris-surge that would have had very serious
consequences if any traffic had been passing as
the A470 can be a very busy trunk-road.
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The
storm also had its consequences at sea. Here,
days later, a vast raft of torn-up seaweed lies
near the low water mark at Borth beach. In
amongst it were bivalves torn from the seabed and
quite a few dead fish - mainly whiting, gobies,
flatfish and wrasse, plus a solitary (and very
dead) small octopus. Big storm-seas are lethal
for anything unwise enough not to get out of the
shallows....
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The same seas had also scoured much sand from the
beach, revealing Borth's famous "fossil
forest". These are the remains of pines and
birches, in a bed of peat lying on grey clay. The
"forest" is believed to have flourished
about 6000 years ago, when sea levels were lower
- in fact, since the ending of the last ice-age,
11500 years ago, it is estimated that sea level
off the Welsh coast have risen by as much as 90m!
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Sobering stuff, given the way that the arctic
seems once again to be in a melting-dominated
phase of its climate, but that aside the
"forest" is a peaceful and photogenic
spot on a calm midwinters' afternoon. The
Atlantic had gone back to sleep....
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...and so it dozed through January, with cool
rather gloomy anticyclonic weather dominating.
The middle of February finally brought a change
in the shape of a Northerly blast and convective
wintry showers, and the first storm-chase of the
year! As I write we're a day or two away from the
coldest period of winter weather for a long time.
Enjoy!
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