in my question on a load forecast model using temperature data as covariates I was advised to use regression splines. This really seems to be a/the solution.
Now I face the following problem: if I calibrate my model on winter data (for technical reasons calibration can not be done on a daily basis, rather every second month) and slowly spring arises I will have temperature data outside of the calibration set.
Are there good techniques to make the regression spline fit robust for values outside of the calibration range? Any experiences or references? Thanks!