|Spring 2011 part 2: from
dessication to deluge!
& Dyfi Valley landscapes Slide-Library - Click HERE
May 10th and all gardeners in the vicinity are feeling a lot more
relaxed about life after three days' worth of thundery downpours. It's
been a phenomenally dry Spring up to this point and although we need
more rain again, the past weekend has really helped with over an inch
falling in the Machynlleth area.
The convective activity provided a few chances to get out with the
camera with some success (images to come below); prior to that my
attention was focussed on keeping the garden going. I've upgraded my
rainwater-collector with a butt I picked up at the local recycling
centre for a quid - the one in the middle - which after the weekend is
a third full already. This is a surprisingly effective method of
harvesting rainwater if you have a sloping area. Part-burying the butts
helps with the gradient but also importantly reduces evaporation in
warm weather. The only other things you need are some bits of old
hosepipe, something to bodge a round hole near the top of each butt and
some plumbers' putty to seal things up. When full, this will hold one
hell of a lot of water!
Although the weather has been dry, the garden has managed surprisingly
well. Given that reeds and sallow naturally grow there, I suspect there
is a reasonable supply of groundwater - these are not things that will
establish themselves on dry banks. As a consequence everything is
coming on well.
It's been a great Spring for butterflies. The resident Commas, Peacocks
and Tortoiseshells are back, as are the Speckled Woods:
This one is mopping up water on a recently-watered patch: although the
stones have dried out, the soil is giving it enough....
Orange Tips have been there every day - the male:
...and the female. Both seem to prefer wild flowers, of which there are
now plenty, with Cranesbill blooming around most beds and Red Campions
now well-established in places....
Green-veined whites also like the Cranesbill
A first since I started the garden - a Small Copper.
All welcome (apart from the Cabbage Whites, but they've not shown just
The vicinity of the compost-heap is home to several slow-worms which
like to bask just under the lid and enjoy the heat given off by
During one of my storm-photography sessions at the coast, I came across
plenty of seaweed, giving the compost-heap a big recharge. Can't beat
seaweed for composting. The slow-worms don't seem to mind particularly
- they come back every year once the nights turn colder....
The dry spell culminated in a week-long blast of warm Easterly winds of
exceptionally low humidity. One afternoon the winds could actually be
seen to be whipping up clouds of dust - winter flood-silt - from the
pastures near Dyfi Bridge. I dared not have any weed-burning sessions
for fear of the bone-dry forestry above the plot.
Finally, the Atlantic started to win the airmass-battle and the winds
veered south-easterly and then southerly. Now, with the source of air
not from the dry Continent but from the SE Atlantic, warmth and
moisture really cranked up. On the evening of Friday 6th May, the first
plume destabilised, bringing spectacular elevated thunderstorms to the
southern UK and bands of thundery rain affected many areas overnight. A
further injection of warmth and moisture arrived on Saturday 7th and
this was forecast to be convectively unstable so that afternoon, I
headed up the Machynlleth-Llanidloes mountain road to see if much was
happening. From the top of the pass, the air was a mass of broken
convective cloud, here viewed over the twin Bronze-age cairns of Carn
Wave upon wave of convective towering cumulus passed through, the
abundant wind-shear evidenced by the R-wards tilt of the towers. There was a very real chance of severe
thunderstorms should any cell actually get it together...
Storms eventually fired late afternoon,
becoming electrified as they moved up into North Wales during the
evening. This massive, cauliflower-shaped cluster was one of them at an
early stage of development, but they were too far away through the haze
for detail to be seen....
That night, an upper vorticity-maximum swung in from the SW, causing
large-scale lift within the plume, with further bands of heavy,
thundery rain resulting (hoorah!) and finally a cold front moved in
from the west, clearing the remains of the plume away eastwards and
introducing a maritime airflow, with much better visibility and still
with plenty of convective instability present as cold upper air pools
On the afternoon of the 8th, with masses of showers and some
thunderstorms plotting out upwind (to my south), I headed to Borth
(with current fuel prices, all my storm-intercepts have to be strictly
local): on arrival there was little going on and what cells were
heading through were in a state of decay. I went and looked for seaweed
Half an hour later the scene had changed dramatically, with a very
heavy cell out to sea:
Here's a simple telephoto: I like the contrast between the clear sunlit
sea and the impenetrable gloom of the storm behind:
Coming along from the south, a clump of towering cumulus clouds had
started to glaciate i.e. their tops are turning into wispy cirrus,
composed of ice-crystals. These were all shortlived storms, with tops
going straight to fluffy anvils, as opposed to the solid-looking
cloud-tops that you get when storms really mean business...
However, the storm still had a lot of torrential rain coming out of it!
Here's a zoom-in to the south of Borth. The curved slabby rock-face L
is known as Harp Rock.
Out due west at the same time, rain-shafts indicated another convective
cell had matured and was raining-out:
As the cell to my south also matured and rained-out, it sent a
gust-front moving out sideways towards me:
Here the gust-front is very close. The car-radio was at this point
making a crazy amount of static-noise - a clicking, popping sound,
indicating the environment to be pretty highly-charged, although there
were no strikes associated with this storm.
Over it came,
followed hard on its heels by lashing heavy rain that cleared the beach
pretty much there and then. Beyond, another gust-front was following,
this time from a completely decayed cell to the SW:
In view of the lack of action now evident
upwind, I followed the outflow-boundary marked by this gust-front
inland, where it collided with a developing updraught.
Outflow-boundaries are like little cold-fronts, so they lift the
environmental air ahead of them, and can cause further storms to fire
up or accelerate the process with developing ones. Outflow meets inflow
- and off it kicks:
I followed this system as it tracked across the hills towards
Machynlleth. By the time I got out to the golf-course on the eastern
side of the town, it was spitting out lightning bolts every so often,
the thunder rolling around in the way it always does in hill-country,
echoing from one side of the valley to another. A good end to a stormy
The unstable conditions continued on Monday 9th, with the ingredients
for thundery weather of a more organised nature in place, as suggested
by Paul Knightley's forecast on UK Weatherworld:
"Large upper low to the
west of the UK is pretty much vertically stacked with surface low.
Numerous vorticity maxima are rotating around the system within the
well-mixed returning polar maritime airmass. Deep layer shear is
generally fairly low, especially across Eire/N Ireland /20-25 knots/.
However, low-level shear is reasonable /15-20 knots/ across much of the
risk area, hence a chance of tornadoes. The best chance of more
organised convection, with a somewhat higher wind/hail/tornado threat
appears to be from SW England through Wales and into parts of the
Midlands/northern Cent S England. Here 0-6km shear of 25-35 knots is
sufficient for organised structures, perhaps with weak mid-level
rotation. Multicells and bowing structures are possible, with a small
chance of brief supercell-type structures. With reasonably low LCLs and
0-1km shear of 15-20 knots, isolated tornadoes are also possible. If
cells become more organised, a small area may be upgraded to a WATCH."
Periodically checking the Netweather radar, my eye was drawn to a
multicell line that was moving up through Wales (below) - the brightest
colours indicate the heaviest rainfall rates. I set off up the mountain
road once again at 1200 for a brief but eventful intercept....
Here, the anvil cirrus dominates the skyline - it stretched from left
to right as far as the eye could see. I continued on uphill....
By the time I reached the top of the pass, the multicell line was
coming up the southern part of the Plynlimon massif, the summit visible
As it got close, I headed a little down the northern side of the pass
again and watched as the northern part of the line surged forwards over
Machynlleth, obscuring the view like a curtain slowly being drawn. A
few flashes of lightning were visible through the murk....
a little more, I got to a section where the road follows an exposed
ridge, and suddenly the wind hit: it was as if a high-velocity hosepipe
was being jetted at my windward windows and the vehicle rocked
alarmingly. I got in behind a banked section for a bit of shelter and
took this through the windscreen: the small trees were bending almost
ninety degrees at times!
These were some of the strongest straight-line winds I have been in.
Organised multicells like this can certainly generate pretty severe
High cloud from the anvil was streaming away northwestwards and I
headed back to Machynlleth to get past it. This was the view from the
golf-course, with lots of small mammatus on the anvil underside
indicative of the strong turbulence associated with the system:
In this satellite-image, taken at 1354BST, the anvil outflow can be
seen streaming away from the line of storms (the solid white "blobs").
By mid-afternoon the outflow covered almost all of Wales, stifling
convection until it cleared in the early evening and further storms
trundled across East Wales, some quite violent - but out of my range.
The low pressure system that brought the thundery showers remains out
in the mid-Atlantic but is slowly drifting north-east, so it will
continue to dominate our weather for the rest of the week with further
showers continuing to improve the groundwater situation for gardeners.
I don't think anybody is likely to complain about that - well if they do I think they'll find
themselves being ignored!
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