Required code for 6 & 12 months lead time

i have code for drought index prediction by using monthly discharge data for a period of 50 years, now i need to improve my code to predict drought index for 6 & 12 months lead time for the same data

i have attatched below the code file of drought index prediction and my data, please help me improve my code to predict drought index for 6 & 12 months lead time for the same data