Regulars will have noticed a lack of recent updates to my weather-blog: however I have been far from idle. The weather of 2021 was rather ordinary, 2022 likewise (so far) and as I have said before, the emphasis from now on with this blog will be on exceptionally interesting events, which means there will be some years with little or nothing, others with several pages.......

So I wrote just over 2 months age and just like buses, two notable weather events occurred within 48 hours in the Dyfi Valley!

Wednesday April 6th dawned unremarkably but I knew an unstable airflow was in place and shortly after 0900 an ominous feature was visible on the radar (below, L):

because the line looked to be interesting and would affect us in the Dyfi Valley. Here's the radar sequence 0930-1000: the system was heading our way at approximately 40 mph.

Although the line appears to be losing strength, it is also maturing and bowing out in places - this happens when parts of the line surge forward relative to others. Even stronger winds occur as a consequence, around such "bowing segments". Another thing that can complicate matters is if waves run along the line: it will tend to break up into a series of areas of strong rainfall separated by areas with little rainfall, forming what we call a line-echo wave pattern. Such features are associated with major turbulence, wind shear and high vorticity - the tendency for some of the air to be rotating.

Here's the radar for 0900 UTC (1000 BST). The bullseye is Machynlleth. Image credit: David Smart, UCL Hazard Centre. Original data: UKMO via CEDA. UK Open Government Licence V2.

In the radar image, the squall-line is visible close to and sub-parallel to the coast SW of Machynlleth (reds/pinks are the heaviest rain). To the north-west of Machynlleth is another area of heavy rain, over the hills north of the Dyfi estuary. Note the bowing sections on the squall-line and the narrow break in heavier precipitation west of Machynlleth. This break is close to the Dyfi Estuary and is potentially significant.

Reports of torrential rain started coming in as this system made landfall along the Cardigan Bay coast and then it reached Cwrt a bit after 1000 BST. High winds, poor visibility and rain getting atomised off surfaces - all usual squall-line stuff. At Borth Lifeboat Station, winds increased from 15 to 45 knots (thanks to Shem Llewellyn for this info) and at the Dyfi Osprey Project, one of the ospreys got blown off its nest. But only a mile away from me, at Gogarth Hall farm, it turned out something a lot more serious had happened.

The next day was dryer and I went over to the spot where a tornado had been reported. This was the sight that greeted me:



Being the Site Investigation Coordinator for TORRO for this part of Wales is a voluntary position that I have held for many years. Site investigation involves mapping reported tornado tracks and carefully documenting the types of damage done. Through such forensic studies, it can firstly be established whether or not the cause was in fact a tornado and secondly, if so, how strong the tornado was. Tornadoes have long been ranked in this manner, beginning in 1971 in the USA. Since tornadoes are uncommon in Wales, the position is not that time-consuming - except when something like this happens. I parked up and got to work. First thing was to determine the extent of the track and that could likely be done by walking along the main road's grass verges, since the damage-path ran in parallel to it, by and large.

The start of its track was evidently near the base of a steep hillside:



Here, over around a hundred metres across its path, a number of trees had been brought down by the tornado, blocking the road up to the farm (in the photo they have been cut up to reallow road access). Continuing along the main road, I had several side-views of the damage-track:





This was clearly a powerful tornado - I knew it was a tornado through experience but would later confirm that through detailed scientific observation.

You could actually see where the tornado crossed the main A493 road; this is damage to the line of trees on its other (southern) side:





Beyond here the tornado seemes to have petered out quickly: small debris was scattered in the pastures but no heavy stuff, whereas the remains of the hedge were also full of lofted tree-branches that had crossed the road in mid-air.

So I had a track: a wider look-around and binoculars helped me confirm that there was no other evident damage (apart from occasional trees and branches brought down by the usual winter gales). This area would therefore be the focus of the invesigation.

Next day the light was a lot better for photography so I set aside the afternoon to complete the work. I had contacted the landowners, Ron and Deilwen Breese, and arranged a visit, for which I was very grateful at such a difficult time. There is a potential conflict here because from the perspective of science, the sooner a tornado track is examined, the better: ideally, we need to see the ground as the tornado left it. But the human perspective is also important because tornadoes are so extreme that they leave the people affected by them shaken and distressed by what amounts to an immense sensory overload. Tornadoes, by their very nature, represent a major departure from any sense of normality and TORRO site investigators have to bear this in mind at all times.

What I would be looking for within the track amounted to small details that might seem trivial at first sight, but are in fact critical for confirming the existence of a tornado - as opposed to strong gusts of wind. OK so we know the hardwood trees damaged by the event have withstood many gales, facing into the prevailing SW wind, but had been wiped out in moments in this instance. But science is all about good data.

The next image is one that has circulated in the media:



This is looking straight down the tornado track, i.e to the north-east.

At the farmhouse (part of which is in the far left of the image) there was much roof damage. There's a lawn and garden in front of the south-facing gable end in the image. Broken slates and sections of guttering littered this area. So the slates had clearly been lofted from the roof and carried into the circulation. Then they were thrown back down to the ground in the opposite direction to that in which the tornado was travelling. This is  exactly as one would expect in a violently spinning column of air: strong evidence for a tornado indeed. But more was to come.

Tornadoes are often described as surgical in the damage they do, meaning that the sides to their paths are very sharply defined,  whereas strong straight-line gusts fan out chaotically after hitting the ground. Here's a neat bit of evidence for surgical damage:



It's a similar, but wider angle view to the previous image, looking down the tornado track, but taken from inside the field. The large softwood tree to the left has some slight damage to branches on its right-hand side. Conversely, to the right - and beginning only metres away, big old - and considerably stronger - hardwood trees have variously been snapped, uprooted and debranched. The RHS of the softwood tree thus marks the edge of the tornado - at this particular point.

The vast majority of tornadoes in the Northern hemisphere have an anticlockwise spin, just the same as Atlantic depressions. So we look for what we call cyclonic throw of heavy debris. Cyclonic debris-throw will manifest in a SW-NE tornado path as objects thrown to the N or NW, across the path.



In this image, the big hardwood tree mid-bottom R, with the fresh-looking rootball, had been thrown at an angle to the tornado track, to its north, by the winds in the tornado's leading edge. Cyclonic throw confirmed. Incidentally, the leading edge and southern flank winds tend to be the strongest in most UK tornadoes because they tend to travel in a SW-NE direction so are augmented by that ambient windfield. A 100 mph tornado will thus produce 130 mph winds on its SE flank in a south-westerly windfield of 30 mph speed. That means, as well as cyclonic throw, one finds a lot of objects displaced and thrown in a north-easterly direction, so solid evidence for cyclonic throw is important in distinguishing tornadic and straight-line winds.

There were plenty of other examples of cyclonic throw along the track, but on to the next bit of evidence: embedded projectiles.



Strong tornadoes have little trouble in hurling debris into the ground, which here is sheep-trodden pasture so pretty hard. You see such sights in straight-line wind-damage, where a tree has crashed to the deck and its weight has acted as the driving force. That is different to the image above, which is tens of metres from any tree: two moderately thick (40-50mm) fragments of branch have been driven straight into the turf through their airborne velocity alone. Again, sights like this were visible throughout the track. Uphill, areas of thin turf over bedrock were badly "bruised" or even gouged-out, where larger objects had bounced off them before being blown onwards.

So enough evidence there to say, "tornado" with confidence. But how strong was it? Let's take a look at the relevant criteria on the TORRO scale, which runs from T0 up to T10:

T2: Moderate Tornado. Winds 33-41 m/s (73-92 mph)

    Much damage to tiled roofs and chimneys. Ridge tiles missing.
    General damage to trees, some big branches twisted or snapped off, small trees uprooted.
   
That certainly applies to some of the peripheral damage to the farmhouse, which was on the NW flank of the tornado, where winds would be a bit slower.

T3:

Not to be outdone, as April 7th moved into April 8th, a thunderstorm moved in off the sea and afflicted the district south of the Dyfi Estuary with these hailstones, photograhed the following morning:





Thanks to Sarah Jenkinson for the images. All happening here; see if I can get this post finished before the forecast thundery showers arrive and I have to power down!

Finally, don't forget I write books when not investigating extreme weather and so here are the useful links

Wales - The Missing Years and The Making of Ynyslas are available online (price £9.99 and £7.50 respectively plus p&p) via the following links:

Coch-y-Bonddu Books in Machynlleth:

https://www.anglebooks.com/the-making-of-ynyslas-tales-from-an-area-the-size-of-wales-25-000-years-ago-to-the-present-day-by-john-s-mason-98895.html

https://www.anglebooks.com/wales-the-missing-years-tales-from-an-area-the-size-of-wales-4-567-to-700-million-years-ago-by-john-s-mason.html

Dyfi Osprey Project's new online shop (they also have Wales - The Missing Years but not in the online shop just yet):

https://www.dyfiospreyproject.com/store#!/The-Making-of-Ynyslas-%E2%80%93-John-Mason-SIGNED/p/405107688/category=60678072

Museum of Modern Art Cymru, Machynlleth (likewise, they also have Wales - The Missing Years but not in the online shop just yet):

https://moma.cymru/en/shop/products/the-making-of-ynyslas/

And at Aberdyfi, community art organisation Artworks has both:

https://artworksaberdyfi.co.uk/books/mason_john.html

If you are a bookseller and want to stock either title, then please contact me directly. Terms offered are identical for all sellers regardless of size: 35% discount off RRP for wholesale purchase.

For seasonal visitors to the area and locals, Ynyslas Visitor Centre, Cletwr at Tre'r-ddol and the independent bookshops in Machynlleth (Literary Cat, Penrallt Books), Tywyn (Clocktower Books, Talyllyn Railway shop), Llanidloes (Great Oak Books) and Aberystwyth (Ystwyth Books, Ceredigion Museum) also have stocks, on top of the outlets mentioned above.