"Stephen Davenport" wrote in message
...

But they are not saying that there will be significant rain with 95%
probability. They are saying that there is low probability (5% if you like)
of a heavy shower or thunderstorm at any location but that if that
probability should manifest the impact from heavy rain could be significant.
==========================

That's not how I read it. The rain column is headed 'Precip. (%)' which I
take to mean the probability of anything more than a few drops of rain and
is a figure that stands on its own irrespective of whether any warning is in
force. Yesterday evening this column was saying 5%, which seems plain
enough to me - no rain was expected. It's not saying that there's 5% chance
of the warned circumstance happening.

(This is just a quibble about a detail of course - the storms did
eventually move through, though nothing too noteworthy for the time of
year - but it seems to me just the sort of blatant logical inconsistency
that brings the warning system into disrepute.)

I suspect that your other point, ie that the warning and the forecast are
generated by two different processes is the explanation. The progress of the
rain and storms was, in the event, perhaps somewhat slower than looked
possible when the warnings were generated?

JGD