'In case' in British and American English

There is a difference in meaning of ‘in case…+ verb’ in common U.S. and British usage. A non-native speaker could make a confusing mistake here.

I quote the Longman Dictionary of Contemporary English (1995 ed.), the best learners’ dictionary I know:

(just) in case - especially spoken a) as a way of being safe from something that might happen or might be true: Take an umbrella, in case in rains / I’m sure Harry will remember, but why not give him a ring just in case?b) AmE if: In case I’m late, start without me

If a non-native speaker said to me, ‘I will take an umbrella in case it is raining’, I would wonder what was meant: ‘I will take an umbrella in case it rains’ or ‘I will take an umbrella if it is raining’.

Geoffrey Pullum has an entry on just in case in Language Log. What he says is slightly different from what I say. He confirms the meaning, but he says most U.S. speakers use the British meaning (his alterations to his entry are in a different colour, which is useful). He says the usage meaning ‘if’ comes from those Americans trained in the formal sciences and philosophy.

This may well be true. Maybe the few Americans I’ve discussed it with have all had some science or philosophy education. And since I don’t hear Americans all that often - although I have been reading them almost daily on the Internet for over twelve years - I might conclude that a usage I find odd is standard. It’s very easy to generalize about the unfamiliar.