Squalls or Tornadoes?
On the 24th September a very active cold front worked east across the UK in the early morning. As it did so there were some very strong winds reportedfrom various places, as well as a lot of localised damage, especially to trees which were still in full leaf. It seemed that whilst some of the damage was caused by strong winds (or straight line winds -SLW- as meteorologists tend to call them) there were also some tornadoes involved but the difficulty can be sorting out what damage was done by the SLW and which were actually caused by a tornado.
Something similar happened on the 4th October 2004 as well, when a system with some impressive line convection developed, again during the early morning hours. The convection was associated with a marked veering of the wind from south-south-west to west and some very strong gusts on a cold front. You can see the synoptic set up on the surface chart, as the cold front developed a few hours before the event just here http://www.wetterzentrale.de/archive/2004/brack/bracka20041004.gif
In particular, Wainfleet (Lincs) reported a 68mph wind gust, Mumbles (near Swansea, S Wales) and Wittering, Lincs (well inland) also reported 65mph wind gusts. On the surface these events seemed to be simply a case of rather severe gust on the cold front which went through. However, reports of rather more substantial damage that came in after the event made it seem likely that some tornadic activity was also involved.
There were reports of fairly localised damage from a number of locations around the country. Media coverage was given to a 'tornado' that occurred at Cawood in North Yorks (south of York) and there was an eye witness report of damage in the Esk Valley, about 18 miles west of Whitby with further reports from nearby Glaisdale, about 11 miles west of Whitby. In Kingston upon Hull (Lincs) at about 8.20am many large branches and some trees fell, closing roads; also at Bilton nearby, an Asda store had part of it's roof taken off. At Huxley, a few miles south east of Chester, substantial tree damage was reported with suggestions of a tornado having occurred being reported in the press here too. There was some notable damage further south also, including reports from Paulton, north of Midsomer Norton (Somerset) of damage.
The problem is that squall lines in the UK, often bring a threat of tornadic activity embedded within the front, due to the sudden change in wind direction and strength, which can initiate rotation. However the skill is to determine from reports how the reported damage was iniated when only the evidence from the damage can determine the existence of possible tornadic activity.
Tornadic events tend to produce a tight swathe of damage, often only a few tens of metres wide and the trees can look 'mangled' and twisted, the trunk broken above ground level, rather than being uprooted as straight line winds tend to do. Tornadic winds will cause tiles to be removed from the lee side of a roof, rather than the windward side as classic strong wind events do, with debris being catapulted as projectiles, and embedded in the ground or wood. After the event site investigation will determine the nature of the wind. You may well see someone from the 'TORnado and storm Research Organisation (TORRO) out in the area afterwards, trying to find out more by carrying out a visual inspection. For more about their organisation see the website at http://www.torro.org.uk/
