Questions:
You apparently want trailing 5, 10 and 20 year "averages" for 2018, and then assign those results to every year from 1999-2018, yes? Otherwise why are the 5 and 10 year "averages" constant over 20 years?
Also, I put "averages" in quotes because you also apparently want maxtemp90 in each average. So the "five-year average" would have an N of 6. And your illustrative attempt for five-year-average includes 2013 through 2018 (6 obs, not 5), so your "five year average" really has an N of 7. What is you actual intent?
Could you put your sample data in the form of a complete data step?
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