This is very scary:
https://fivethirtyeight.com/features...lection-night/
Pennsylvania has a lot of postal votes and there could be a large discrepancy between votes on the day and the postal votes counted afterwards. So on the night, Trump could be ahead and if this is the deciding state then there is going to be an almighty argument about how long they can count postal votes before shutting it down and declaring a winner. The Supreme Court upheld a decision giving them three days to count them but that can be appealed.
So if Trump is ahead the Republicans could go right to the Supreme Court - which is right leaning - to try and get them to shut down the count ASAP.
Again at the moment, the current evidence is the court will allow votes postmarked before election day to be counted. That's what other states do with postal votes and what we do in the U.K. Flordia usually takes time to fully finish their vote do a few others.