Hi all, I am using the ECMWF SEAS5 long range forecast from the CDS to get a precipitation forecast. I am calculating the monthly rainfall in mm/month by multiplying the value by the number of seconds in each months, then by 1000.

I want to calculate the monthly historic average that is used in the** **forecast. So for example, if in a month I get a figure of -20mm for the **Total precipitation anomalous rate of accumulation **forecast, that means 20mm less than normal in the month. I want to know the normal value.

Here is how I calculated it: If for the same location, I get a value of 150mm from the **Total precipitation **forecast, is it valid to just calculate the "normal" conditions by doing -20 + 150mm = **130mm**? I just wanted to check this is a valid way of doing it because I don't see any product that gives me that value directly.

Assuming the above is correct, I have a further question - how is this normal value calculated? I assume it'a a monthly average from the past x years based upon a hindcast or something like that? Any further info you can provide would be great.

Thanks!