"Chef!"

Now I am confused. I sometimes refer to www.metcheck.com. Their 7-day
forecast predicts rainfall to the nearest mm, windspeed to +/- 1 mph,
pressure to the nearest mb, cloud cover to +/-1%.

That particular website has been discussed endlessly in the past.

It is very cleverly put together, neat and presentable (full marks for that)

But it has fooled a lot of people into thinking that because it purports to
be so accurate, then if must be extremely trustworthy. Don't be taken in.
It's a bit like the conversion of units that seems to give an amazing degree
of precision.

For example, a cloud base might be reported as being 3,000 feet - clearly
accurate to no better that +/- 100 feet.
(two significant figures). That is then converted to 914 metres and suddenly
we have an accuracy (?) of one part in 914.
It is a rule of thumb (commonsense actually) that you cannot get more
precision in an answer than the level of precision that was put into the
calculation

Look around your supermarket and you will see conversions from a round
number of eg pounds into an amazingly precise number of grams. I drive my
wife mad (also a science graduate) by forever pointing this out:
"Yes I know, you have said that all before many times. But does it really
matter?" It does to me!

So, don't be fooled into believing that metcheck is capable of such
precision. It's a brave effort, but interpret the output with caution.

Jack