In the TV Universe, it rained on St. Patrick's Day in New York City. At least it rained during the evening; no idea if the parade was a soaker as that wasn't seen.
At least, that's how it was shown in the first new episode of 'How I Met Your Mother' since the end of the writers' strike ("No Tomorrow"). There were no other new TV series to contradict that weather report, so that's how it is recorded for Toobworld on March 17, 2008. (And there's no getting around which St. Paddy's day it was - Ted announces the date as he tells the story to his kids.)
This is just another example of the differences between Toobworld and the real world. It never rained in New York City this past Monday, not even during the parade (which was broadcast, but doesn't have as much validity for inclusion in the TV Universe).