On 25 Aug, 13:26, "Martin Rowley"
wrote:
"Martin Rowley" *wrote in message ...

... one of those situations when the surface charts doesn't really
do justice to the situation; there is a marked upper trough (colder
air throughout troposphere in this case, relative to either side)
swinging from west to east, being hustled along by events further
west and relaxing (i.e. lessening the dynamic / vorticity forcing),
but still enough of a force hereabouts to give short-lived moderate,
locally heavy showers - they're not lasting long (15 mins at most).

http://www.wzkarten.de/pics/Rtavn001.png

should improve as the trough moves on and we get descent to its
rear. On the GFS timing, the axis of the trough should be passing
overhead here ~midday GMT.

... and just about on time, we duly had a complete change in
'skyscape' with Cu now much shallower - not capped as the air is still
unstable, but I suspect that the mid-level moisture has changed and
won't sustain the taller convective towers, at least hereabouts -
obviously still perky over the spine of Devon & Cornwall & over Wales.
Also, scrappy elements of layer Ac which indicates some medium-level
descent/stabilisation going on.

However, the trough can't be as straightforward as that image posted
(ex GFS) as looking carefully at the cloud elements I can see that the
cloud winds aren't following the simple shape indicated. Going back to
the 00Z plots of actual upper winds at 700 and 500hPa, it's clear that
the trough was being distorted by the developments further out in the
Atlantic (ex-Bill), and Valentia winds in particular didn't verify too
well against background: a common problem with such developments. Even
the model 700hPa trough isn't anything like as symmetrical as the 500
hPa for example.

... in major forecasting centres (such as Exeter), they will overlay
water vapour imagery in particular against other imagery and model
fields - modern NWP suites can produce 'pseudo WV' fields - and such
errors are quickly picked out and can be allowed for.

However we seem to be poorly off for real-time WV imagery - I can't
find a loop of such - does anyone know of a source?

Martin.

--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N * Long: 01.88W
NGR: SU 082 023

Down here the trough seemed to clear eastwards around 08:00, but then
another line of stratocumulus/embedded cumulus around 10:30, with a
few spots of rain. Since then good sunny periods, with the cumulus
mainly in it's normal 'spinal' location

After the coolest night since 9th (Min 11.5C) the temperature was
initially very slow to rise, but has risen steadily since 11:00 to
reach 19.7C currently, making it the warmest day here since 20th
(yesterday 19.1 max). Noticeably cooler inland under the cloudy strip.

19C and still fine out on Scilly. www.scillyman.co.uk/Web_Cam.html /
www.scillyman.co.uk/Web_Cam.html In fact it sill looks pretty calm out
there.

On a slightly different topic, maximum temperatures on Scilly &
coastal west Cornwall, seem to have been relatively high this year
compared with inland parts of Devon/Cornwall. I can't see that this is
due to the SST anomalies, so I can only put it down to the
differential between the sunshine on the coast, and that inland, being
greater than normal this summer. (It may be I'm being influenced by
the cold conditions in the far SW last year, when average maximum
temperatures in August were easily the lowest I've recorded - since
1991).

If anyone has access to maximum temperature data for St Mary's (I've
got Penzance!) and for a scattering of other locations in Cornwall/
Devon, for July/Aug for say the last 3 or 4 years, it would allow me
to compare the differentials. I hope to get data for St Martins.

Well, as there's still no sign of the cloud steaming ahead of 'Bill',
and the breeze is only of Force 3 or so, I'm off to Lamorna to do a
bit of snorkelling before it all gets stirred up.

Graham
Penzance

www.turnstone-cottage.co.uk/recent.html