part 2 - Noctilucent clouds and late June Train-echo....
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half of June 2009 featured the first warm spell of 2009. Temperatures
crept up into the upper 20s widely and conditions became increasingly
humid with showers and thunderstorms featuring by the final week and
some notable storms on June 30th and July 1st, before fresher
conditions came in from the west.
storms gave me some mixed fortunes but with lots of lightning and loud
thunder booming around the hills and some satisfying photos. They also
brought brief flash-flooding to Machynlleth, squally damaging winds
and, for a friend in Artist's Valley, a very close encounter with
to that, though, we had the incredible noctilucent cloud display after
sunset on June 17th. I wrote about these beautiful ghostly phenomena
last year, and I'll briefly recap:
clouds (sometimes referred to by the acronym - NLC) are very high cloud
formations consisting of small ice particles. They
occur in the mesosphere - at altitudes of around 75 to 85 kilometers
(47 to 53 miles) and are thus quite set apart from all other types of
clouds (e.g. a mighty thunderhead is still only 10-20km high). The key time to
observe them is during the mid-May to mid-August period. At this time
of year, it never gets truly dark in the northern sky but instead there
is a deep twilight between sunset and sunrise. At this time, these
normally invisible clouds become illuminated by the sun which remains
over the horizon. Thus, the northern sky is the place to look.
News started circulating on the internet at about 11pm on the 17th that
a good display was becoming visible, so I grabbed the camera and headed
out of the door. This was the view across the road towards St Peters
Incredible! This was the
brightest display I have yet seen, with that classic "nocti-blue" as
it's called by enthusiasts....
...finding somewhere with a
better view was not easy though - in the beer-garden of the White Lion,
a stiff breeze affected the trees and made for a lot of motion-blur
amongst their tops. I think the shot with the church worked better...
...but this one shows how
bright they were! The camera has exposed the scene correctly for the
well-lit street and clocktower, yet towards the horizon that vivid
There has been nothing like that since, although some likely displays
(i.e. they were visible elsewhere in NW Europe) have been clouded-out
here. But there are a few more weeks in which we might see something,
so I'm not saying that's it for 2009. If it's a clear sky around
sunset, keep checking the northern sky for the next hour or two. If
they appear, you will have a job to mistake them for anything else!
Now onto the thunder!
Activity commenced in earnest on June 29th, when an active trough moved
north over Wales. This essentially took the form of an east-west narrow
rainband from which sporadic thunder was ongoing. Its western end
passed Machynlleth by just a few miles. The above shot shows a
torrential raincore moving away northwards, from Dyfi Bridge.
Instability and high temperatures looked to conspire to possibly
produce some afternoon storms too, and I went out in the afternoon,
following the most likely-looking towers and these took me northwards
along the A470, past Dinas Mawddwy towards Dolgellau....
...ending up at a
vantage-point just west of the Bwlch, beloved spot for aviation
....and indeed aviation
was more in evidence than thunderstorms as the above image shows! F-15
heading over Cadair Idris.
Finally the convection started to shoot up
over the Arans....
boiling-up clouds indicative of vigorous activity. These towering
cumulus eventually produced brief heavy showers, but it was clear that
the environment was not favourable for intense thunderstorm formation.
The next day - June 30th - looked to offer a better possibility of this:
On the late afternoon of
the 30th, radar indicated a N-S line of heavy showers building from
near Swansea up to Dolgellau. Atmospheric profiles were indicative of
very high moisture levels: this might lead to hazy conditions but also
would result in the risk of torrential downpours, which developed
quickly. The reason for the storms forming here was almost certainly
due to convergence - coming together - of the synoptic airflow (S to
SSE) and sea-breezes coming in from the coast - being cooler, a
sea-breeze pushes under the warm air inland, lifting it to a height
where it becomes very buoyant, and this allows rapid convection to
initiate. The view above shows two cores of torrential rain approaching
Machynlleth from the south.... whilst below is a sketch-diagram showing
a typical convergence zone:
...meanwhile, this Typhoon jet exited the
"Mach Loop" low-flying circuit and climbed quickly, heading east away
from the storms into clearer skies....
This is the view looking
from near the top of the pass over to Llanidloes, looking straight
towards Machynlleth. Obviously, it's bucketing down! Thunder was now
audible every minute or two.
Coming up from the south was the next storm on the line, here
...and back on the
Machynlleth side of the pass, looking back homewards again. Lightning
was visible every minute or so now.
The visibility was not great and was
deteriorating. This was the final shot, looking north towards the
Arans, over which some stacked lenticular clouds had developed in the
deck of stratocumulus pushing out east from the storms.
A similar, but drastically narrower and
more electrified convergence-line developed on the late afternoon of
This began life around 1530BST when the growing cloud-tops became clear
on visible satellite imagery. The profiles were still very moist and
flash-flooding had looked likely from the morning onwards - not "if"
but "where". The problem would be in pinpointing the "where" bit.
Sea-breeze convergence would be important and perhaps a bit of
orographic lift from the mountains too. When the narrow, N-S line
became visible I headed out to investigate and storms developed
explosively - this one is running up the line, just west of Plynlimon
The reason for flash-flooding in such situations is because storms
typically form and move along convergence-lines, like trucks going
along a railway. Therefore, if the convergence-line remains in the same
place, areas beneath it will receive storm after storm so long as it
endures. This is a process known by meteorologists as "Train-echoing" -
the "echo" bit referring to the radar returns.
Here's a composite showing half-hourly (BST) radar returns for the
Machynlleth area from 1630 to 2030:
You can see how the storms explode into life by 1700, are intense for
the following two hours as each one runs northwards, then begin to
decay, with the final cut-off at source by 2030 BST. A note about the
rainfall rates: on a normally "wet day", the radar might read up into
the mid-greens (5-14 mm/hour) at the most. Here, at the most intense
phase of development (shown further down the page) the rainfall
intensity was up to ten times that!
Let's have another look at what was happening out there:
Lowerings like this along rain-free cloudbases are indicative of
powerful convective updraughts. They are where to look out for
funnel-clouds and tornadoes - today not to be, unfortunately!
Another storm runs up past Plynlimon. Each precipitating cell spat out
numerous cloud-to-ground lightning bolts.....
In this wideangle shot
looking over Llyn Clywedog, core after core can be seen. Strikes were
occurring right along the line now.
Telephoto showing the edge of a cloudburst!
As activity eased up, I
headed back homewards. Torrential rain visitations were evident in many
places and muddy floodwater was encountered often, as were rocks and
soil washed into roadways....
I liked this strong double-rainbow!
On returning to Machynlleth, I started getting reports of a violent
wind event associated with the heaviest storm of the evening just after
1800 BST, with stuff blown all over the
place, branches & even trees down in places over quite a wide area.
This sounds very much like a strong downburst - when cold air from way
up in the stormcloud is dragged downwards by very intense rain or
hail-fall, and due to its greater density, it forms a gravity-current
that accelerates earthwards and fans out when it hits the ground,
throwing debris in front of it . Here's the radar for 1810:
That is some rainfall! More than enough to cause a downburst. A sudden
flash-flood also resulted, with some property damage and the Fire
Brigade attending to matters. By the time I arrived back, it had
drained away and the sun was out.
It is worth noting at this point that the August 2004 great flood at
Boscastle in North Cornwall happened precisely because a train-echo had
developed over the hills above the town, although in that case it
tipped it down from much earlier in the day - initiation occurred
before lunchtime. But you can see how train-echoes can be potentially
disastrous weather events: with Boscastle, the topography makes it far
more prone to really big flash-floods because of the narrow, steep
sided valleys that take the run-off and because settlements are
situated within them. Machynlleth on the other hand is situated on
slightly elevated fairly flat-lying ground just to one side of the
broad flood-plain of the Dyfi. But had that intensity of rainfall gone
on for several hours here on July 1st, there would doubtless have been
much more extensive damage from flooding.
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