calculated using NCL, I tried R but it was difficult for me because I have to write everything, but NCL you just use the prescribed scripts. Again, severity cannot be calculated you will have the range of indices and you can categorize the severity based on the agreed thresholds

Philbert Modest Luhunga thanks for your answer

I will grateful to you if provide me the script

Using rstudio and PDSI pakages you should plot it. for different Time scale. It is one of the easyiest methods.

watch this video

https://www.youtube.com/watch?v=HgKrF26PtaA