|Autumn 2010 part 5 -
Record cold spell:
Mid-Wales 27C colder than Greenland!
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ended the last post with the comment: "
Here, the charts are hinting at colder conditions as we head towards
the end of November..." - little expecting the spectacularly cold
plunge that occurred!
By November 23rd, a strong blocking high pressure system had
established itself over the North Atlantic and southern Greenland,
bringing in a northerly airflow all the way from the eastern Arctic. As
the high started to extend a little eastwards, the flow modified itself
to a NE then by the 29th an Easterly, but by then very cold air had
been carried in over much of the UK.
Cold air over warm seas is a classic late Autumn-early Winter pattern,
and the colder the air the more intense the showers that are generated
as it moves over the sea. Unlike summer thunderstorms, which often
depend on warm surfaces heated by sunshine to initiate convection, in
winter it is the warm body of seawater around our coasts that acts as
the generator. Thus it is the windward coasts and the districts just
inland from them that see the most shower activity and therefore snow.
Showers of snow affected Machynlleth on the morning of November 25th,
although in the image below, the main activity is out west, over
Cardigan Bay, where thundery snow showers were pretty much ongoing
somewhere throughout the period....
In such airflows, small depressions can develop and bring with them
more extensive, prolonged snowfall. This happened on the morning of
November 26th, and by lunchtime the Machynlleth area had variably 4-8
centimetres of snow covering all surfaces: with no wind it stuck pretty
It was strange to see snow on only partially-turned Autumn leaves!
I went for a
good walk - over Y Wylfa to Glaspwll then out along the Llyfnant Valley
to the main road at Glandyfi, where I thought I might hitch a lift
home. Along these narrow back-lanes, it was a classic "winter
I got a bit carried away with photographing the intricate and unique
patterns in the snow-encrusted vegetation:
Nearing Glaspwll, I recalled the pleasing Autumnal shot I had composed
at this vantage-point on November 16th, just ten days ago:
And just ten days later!
Passing through Glaspwll, I set off down the Llyfnant Valley. Here is a
The Llyfnant Valley runs straight as a die for several kilometres and
is a very steep-sided feature in the landscape. Normally there is the
usual geological explanation for such things and this is no exception:
a major east-west fault cuts the strata here. In the vicinity of such
faults the rock tends to be shattered and broken-up, so that it may be
more easily removed by the agents of erosion - hence the
orientation of the fault has controlled that of the valley.
The road out to Glandyfi runs right alongside the stream at first and
is not a place for a skid! People were today sensibly avoiding it
completely. Beneath the snow there were patches of sheet-ice which
caught me once or twice...
In many places, the side of the road has been hewn out of the steep
Continuing westwards and the road starts climbing away from the stream,
opening views of the opposite side of the valley....
...and so it was back to finding patterns in the snow-covered
Tree-ferns thriving along a bow of sessile oak:
Very seasonal - had it been a month later!
plantation-trees look interesting in snow!
Reaching the main road I had to walk a mile or two, before finally
someone picked me up and took me to Machynlleth, with news of
horrendous conditions around Aberystwyth, cars on their roofs and so
on. I was back early afternoon to get a pan of my special stew on the
stove - it is the traditional Welsh Cawl, but with added red wine,
three cloves of garlic and a couple of hot red chillies. That makes it
The 300hPa chart for midday on the 27th neatly shows what was going on.
The jetstream (bright orange colours) exits Newfoundland (L), but then
splits. One half moves south-east towards North Africa but the other
moves northwards over Greenland before plunging south to the west of
Ireland and then flattening out across Europe. It has the shape of a
Greek Omega symbol, and this is indeed a good example of an
Omega-Block. Instead of letting our weather come from the south-west,
as occurs in non-blocked situations, it is bringing everything down
from the very far North - hence the cold!
By the evening of November 27th, some extremely cold air had spread in
over the UK and, over the snowfields of Wales, that night the
temperatures plummeted to phenomenally cold levels. According to the
Last night saw November
minimum temperature records fall across the country. Most notably
both Wales and Northern Ireland recorded the coldest November night
since records began. In Wales, temperatures fell to -18.0 °C at
Llysdinam, near Llandrindod Wells, Powys. Northern Ireland recorded
-9.5 °C at Loch Fea. Scotland recorded minimum temperature of -15.3
°C at Loch Glascarnoch, whilst England recorded -13.5 °C at
Topcliffe in North Yorkshire. The UK’s lowest ever recorded temperature
in November was -23.3 °C recorded in Braemar, in the Scottish
Highlands, on 14 November 1919.
Here's an example, from Llangorse in Powys for the morning of the 28th:
the source is the Weather Underground Weather Stations site at the
The same high-pressure block that was pumping frigid air southwards was
bringing mild air northwards on its western flank: these figures (same
link above) were for Narsarsuaq, Greenland on the morning of the 28th!
With minus 15 recorded at a number of sites across
Powys and 12C at the same time in Narsarsuaq, the latter was, at the time, 27C warmer
than Mid-Wales!!! Having mused on the Saturday morning that Trawscoed
(SE of Aberystwyth) was the same temperature as Cairngorm Summit, I
found the Mid-Wales-Greenland contrast mind-blowing!
I can tell, in my non centrally-heated house, that it is seriously cold
i.e. below -10 outside. In scenes reminiscent of my childhood, at such
levels of cold the insides of my upstairs windows typically ice-up:
These were taken on the morning of the 29th but both
mornings were very similar: pile out of bed, put on layers, get fire
lit, then do all the usual stuff. The place heats up quickly enough
with a fire merrily crackling away, but the downside was that I had
made a major incursion into the woodpile over the past few days. With
no wood-collecting arrangements yet in place, I had to plump for a few
hours' driftwood foraging down the Dyfi Estuary....
Setting out at lunchtime on the 28th, I noticed some
distant cumulonimbus clouds so I pushed on to Tywyn to see if they were
photogenic. They were:
Quite spectacular, with the new Tywyn Breakwater making for a bit of
I stayed a while as the scene constantly evolved....
Eventually the driving convection seemed to weaken and shortly after
this last image was taken the whole lot fell apart. It was time to get
that driftwood before nightfall!
I stopped briefly at Aberdyfi for a look across the Estuary to the
The hills above Tre'r-ddol: Foel-Goch, Moel y Llyn and Moel y Garn:
Later, I located a good wash-up of driftwood, enough to require two
trips to collect it all. On the 29th I returned to do just that, but
again noticed cumulonimbus clouds out to sea so it was back to Tywyn.
These were snow-showers that were brewing-up over St George's Channel,
between Anglesey and Dublin, and later that afternoon they became
Further up the coast this was the view from
....where the old chapel looks out over the sea:
A final zoomed-in shot as the sun started to get lower and the need to
get that wood beckoned:
This morning (November 30th) I woke to the
pleasing sight of ice-free windows. An Easterly wind had got up
overnight bringing cloud and snow flurries. Looking out of my front
door in the half light, it felt positively balmy - and a quick look
online revealed a welcome warm-up: Trawscoed had gone from -6.6C to
+0.1C between 0500 and 0600. Compared to the sub -10 shock of Sunday
morning, no wonder it felt mild - it's all relative after all!
Looking ahead, it does not seem especially mild (in the normal sense)
in the weeks to come, and more snow is possible at times, but it looks
- at the moment - as though we are unlikely to see temperatures plunge
to the unseasonal - and downright uncivilised - depths that the past
weekend inflicted on us! But at the same time, Winter officially starts
tomorrow. It's bound to have something interesting to throw at us at
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