Yes, that's the correct way to specify the extrapolation behavior, but you don't seem to use it afterwards; your last two lines refer to Uinterp and Vinterp, rather than Uextrap and Vextrap. To get the desired behavior for OOB indices, you must index into the result of extrapolate(...).
If you want to avoid this, you can of course do both interpolation and extrapolation in one step: itp = extrapolate(interpolate(...), NaN) itp[-3.5] # NaN // T
