Willing to change your SITE table slightly?
I think I have an answer:
wind_dir_1_from wind_dir_1_to wind_dir_2_from wind_dir_2_to
15 (NW) 19 (NE) 9 (S) 11 (SW)
That is, if the "to" is smaller than the "from", then add 16 to the "to".
And the added 16 means that you use the ALT, else you ignore ALT.
Still thinking about it, though.