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Pont-Faen Flash Flood, 5th September 2008 discussion


There was a flash flooding event around the Battle Hill area of Powys (SO 0133).

This is a sparsely populated area however dozens of homes were flooded by runoff flowing down the hills through properties (not on a flood plain). 4" of rain was reported in around an hour between 1:30 - 2:30pm.

Several foot bridges have been destroyed, with an old road bridge at pont-faen being seriously damaged.

Please see this thread for further information



Today's analysis by Retron - Friday 17th October 2008

Here is a summary of the NWP output for noon (GMT) on Tuesday.
Issued 0501z, 17th October 2008

Analysis and evolution to T+120 via GFS
http://85.214.49.20/pics/Rtavn001.png
Westerlies cover the UK, with a weak ridge atop the British Isles. WSW'lies affect the UK tomorrow as a weak trough moves eastwards, followed by SSW'lies on Sunday. Monday sees strong southerlies, with a trough to the west.

T+120 synopsis
http://85.214.49.20/pics/Rtavn12014.png / http://85.214.49.20/pics/Rtavn1201.png
http://85.214.49.20/pics/Rukm1201.gif
http://85.214.49.20/pics/Recm1201.gif / http://www.meteociel.fr/modeles/nogapse_cartes.php
The jetstream chart shows a trough over the UK, with a broad westerly flow over the Atlantic. At the 500hPa level there's also a trough, with WNW'lies aloft. MetO shows an upper trough too, while ECM has an upper trough slightly further west. GEM also has an upper trough to the immediate west of the UK.
At the surface, GFS brings WSW'lies, with a ridge to the SW. MetO shows a weak ridge with NW'lies for England and Wales and westerlies elswhere, while ECM has a trough over eastern England with WNW'lies elsewhere. Much the same is true with GEM, which has stronger westerlies for all.

Evolution to T+168
Day 6 with ECM shows SW'lies for all with a trough over Northern Ireland and western Scotland. On day 7 strong to gale force SW'lies cover the UK, with a shallow low over the Western Isles.
On day 6 with GFS a trough lies to the west, with strong to gale force southerlies as a result. The trough moves eastwards on day 7, heralding lighter (but much coler) WSW'lies and SW'lies.

Looking further afield
On day 8 with ECM a deep low lies to the north, leading to strong to gale force SW'lies. A ridge covers the North Sea on day 9, with a mixture of southerlies and SSE'lies across the UK. Southerlies persist on day 10 as a low deepens near Iceland.
GFS brings low pressure over the North Sea on day 8, with NW'lies for all. Day 9 sees a ridge over England and Wales, with SSW'lies elsewhere. On day 10 there are further SSW'lies.

Ensemble analysis (http://91.121.94.83/modeles/gens/graphe_ens3.php?x=306&ext=1&y=141&run=18&ville=Londres)
The ensembles continue to show a zonal "sine wave" pattern.

In summary...
Low pressure will be close to the UK during the middle of next week, with strong winds and rain likely across the UK. The exact timing is hard to pin down, however, as the models differ greatly regarding cyclogenesis and track of the lows.


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