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Topic - What does the MET Office forecast mean?

David Coley - on 02 Oct 2013
Hi, the Met Office forecasts split the day into 8 time slots and give a probability of precipitation in each slot. Often climbers would like to know not just the probability in a slot, but the probability over, say, an 8-hour period.

For example: A day's forecast is split into 8 time slots. If the probability of precipitation is 20% in each slot, I initially thought I should just simply add them up using:

P=1-(1-.2)^8

to give the probability (P) of rain in the whole 24hour period.

However I'm now thinking that rain can't always been seen as independent in each slot. For example a front comes through and dumps in the morning, then there is little chance of rain in the next time slot, however the timing of the front may have been unknown when the forecast was issued. This seems closer to a situation where there might be a near 100% chance of rain during the day (i.e. P=100%), but the chance in any slot is only P/8 = 12.5%, which most people would consider as indicating very little chance of rain, full stop.

Thanks for any help you can give. (I've sent an email to the MET Office asking much the same thing.)
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