"Freddie" wrote in message
...
Graham Easterling Wrote in message:

Haha i guessed this post would be about point probability
forecasts before I read it!! :-)

At 08:00 this morning we had a shower which dropped 3.4mm in 10mins. We
are surrounded by showers, with little gap inbetween. The radar
demonstrates this. The MetO Fax chart for 06:00 shows a trough overhead,
there is clearly deep convection but . . .

Chance of rain for Penzance @ 08:00 & 09:00 still shown as 5% (it's now
09:20 by the way - so it's happened). If anyone involved could explain
that, I'd be interested.
It's an automated forecast with no manual input. I would hazard a
guess that the model wasn't forecasting showers at the grid point
nearest you at that particular time. Why it wasn't forecasting
them is the question that needs an answer...


Because it is a sea point perhaps, and in any case the test for instability
is static in time and does not advect showers, for example. Graham does not
say what the "probability of precipitation" was? I suspect that was non-zero
but not high enough to produce a symbol. The POP is derived for convection
by using a sequence of "what if" tests, e.g. if temperature was a bit
higher, for example. It is quite complex now and is not a matter of
interpolation etc. Also "nearest gridpoint" takes into account wind
direction, thus the actual nearest is given far less weight if its wind is
blowing away from the site. An "intelligent interpolation" is used from
surrounding points. Cornwall being a peninsula presents all sorts of
problems, however.

Will
--
" Some sects believe that the world was created 5000 years ago. Another sect
believes that it was created in 1910 "
http://www.lyneside.demon.co.uk/Hayt...antage_Pro.htm
Will Hand (Haytor, Devon, 1017 feet asl)
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