Calculate Ranked Probability Score
Tidy up your math

cumulative = cumulative + (sum(predictions.iloc[x, 1:i]) - sum(obsvec[1:i])) ** 2


can be

cumulative += (sum(predictions.iloc[x, 1:i]) - sum(obsvec[1:i])) ** 2


and 

RPS[x] = (1/(ncat-1)) * cumulative


should be

RPS[x] = cumulative / (ncat-1)


Make a main method

This is a very small script, but still benefits from pulling your global code into a main.

PEP8

By convention, method names (i.e. RPS) should be lowercase.

That's all I see for now.