Spring
2005 PART 2:
April Showers (and a well-defined convergence-line)
18th April 2005
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During the third week of April, at
last the classic April situation of sunshine and heavy
showers was forecast. Monday the 18th looked just
possible for something to fire over the Cambrian
Mountains although the setup was a bit marginal. So off I
went, keen for a break from the desk apart from anything
else....
...pausing to get a last (probably) shot of snow
on Plynlimon for this Spring, anyhow. Grey clouds
massing behind....
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Onwards towards Llanidloes
more intense convective weather could be seen so
I continued south towards Rhyader. Here's a cell
just to the north of Rhayader...
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And a more menacing one to the south. Going east,
the signs that these cells were displaying
combined with the only modest rain that was
falling from them suggested one thing - that they
were outflow-dominated (i.e. all the
"whoomph" was pouring out of them and
nothing was going back in - so that as they moved
further NE they were decaying away). After
calling a friend to get some data I decided to
move SW to hit them where they were developing,
which seemed to be over the Cambrian
Mountains....
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.....here at Newbridge-on-Wye more solid-looking
storm-clouds are visible over the mountains,
forming a backdrop to the swollen, muddy waters
of the River Wye.
The only way to reach this area, one of the
remotest in Wales, would be to continue to Beulah
and take the single-track mountain road through
Abergwesyn and over the mountains to Tregaron,
and hope I might get closer....
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...which was not apparently going to be the case!
The very low cloudbase was still miles away to
the NW. The only possibility now was going to be
to drop down into Tregaron and head N towards
Devil's Bridge, as it was becoming apparent that
the cells were initiating at the western edge of
the Cambrian Mountains. A pity as I love the
wilderness up here, although today's peace was
broken by the two A-10 "tankbusters"
that buzzed my jeep a couple of times! Onwards,
anyway....
So down along winding roads through dense
forestry, across the fields in the valley below
and on through Tregaron, up to Pontrhydfendigaid,
turn off through Ffair-rhos and ahead is a long
tall deck of cloud. Stretching way out west and
disappearing into a blackness to the east, this
was it at last. I know this area quite well so
was able to position myself right under its
centre in no time...
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This was taken from the old lead-mines above
Ysbyty Ystwyth. It shows the bank of convective
cloud stretching well out SW into the distance.
To its SE are scattered cumulus-clouds, but to
the NW the skies are clear. The steering flow was
a SWerly. What was fixing the convection along
this line alone?
The answer was that it must have been a week
sea-breeze coming in from the Cardigan Bay coast,
not much more than 12 miles to the west. That
would be the clear-air zone. Meeting the
marginally unstable SWerly airflow would create a
convergence-zone - a place where two airflows
meet. Where these occur, air is forced upwards,
sometimes to the point where convection is
possible. This was what was occurring here,
resulting in this well-defined line of convective
cloud in an otherwise pretty much uneventful sky.
I waited a while as the convection overhead
continued and intensified. It got darker and odd
hailstones started to fall.....
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...which was when this was taken! This shows
rather better how a convergence-line acts as an
airmass boundary: the clear sea air to the R and
the cloudier, weakly convective air to the left.
Wanting a better look at what was going on I
pressed on, and hail started to fall more
regularly. Getting out from beneath the line, I
headed north and found a good vantage point not
far from Devil's Bridge....
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....but by now evening was well in progress and
what convection there was had weakened. This is
looking north across Cardigan Bay, towards the
Llyn Peninsula - the spine of which is marked by
a line of sizeable cumulus cloud, marking in turn
another sea-breeze convergence-zone.
Today had been mostly unremarkable for
photography but provided some interesting
information on storm initiation. The cells
developing along the convergence-zone running
over Ysbyty Ystwyth were then drifting NE in the
steering SW airflow. As they did so they ran up
over the Cambrian Mountains where they were
further intensified giving heavy downpours,
probably helped due to air being lifted into them
as it was forced up over the hills. To the east
of the hills, all such assistance ceased to exist
and here, as I had seen earlier, the cells
collapsed showing typical outflow features such
as mammatus and virga. An educational day! The
19th on the other hand would be tiring but quite
exciting!
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