I'll try this one:
"The Sentinel" (1977)
You can read the plot summary here, though I don't recall yawning:
Here's the trailer on YouTube:
There's an open mouth with a descending spider that has traumatized many people though I don't know if that could be mistaken as yawning:
"Deadly Blessing" (1981)
You could take a look at the box art, which shows a girl (a very young Sharon Stone) looking as if she's yawning. Here's the ultra-creepy spider/open mouth scene:
This bothered even me because of my phobia of spiders and my sleep problems.
EDIT: Oops! I don't think that last scene takes place in the city, so that knocks it out of the running I imagine.