To confirm what happened to UA870 on 4th Jan (I was on it):

Departed 3 hours late to try to avoid predicted storms, rescheduled to arrive around 13:30. At 12:00 storms too severe so started to reroute to LAX. At 13:00 storms had cleared so reverted to SFO. Landed 13:45. At gate 14:30. Doors opened 15:15. Baggage delivery started 17:05 (we were apparently the 5th 747 to arrive in 40 mins and only one baggage crew!).

We were trying to connect to LHR. Queue for flight connections desks looked 90 mins + and getting much longer. Quick check on-line showed UA had rebooked us to the FRA flight that had been rescheduled for 11am next day, with arrival into LHR 33 hours late. Luckily tried calling M+ (just general member) and after some delay was rebooked to the 19:30 BA flight to LHR. They hadn't released the UA-only ticket requirement right, so had to run to find a UA ticket agent at SFO - much quicker at domestic, int'l queues and anger levels looked pretty bad - and they managed to fix it up for me.

The severe weather wasn't UA's fault, but there wasn't much back-up to fix things up. However, good end result as far as I was concerned.