According to this article, the US warned the Russians ahead of Syria missile strikes. It does not specify when exactly this occurred (a few minutes means nothing, 1-2 hours means that some troops/planes can be moved from the target area).
This article tells us about the Russian Growler anti-missile defense system that theoretically could be used against the tomahawks:
Russians have one of their self-proclaimed state of the art Growler anti-missile defense systems on Latakia Airbase on the Syrian coast. The Growler has reportedly been able to intercept targets at a range of 250 miles and at heights of up to 90,000 feet.
Question: what is the reason to make such a warning? From a military perspective, it does not make any sense. So, I suspect a political reason.