As someone who has lived in the S.F. area over 40 years and dealt with the effects of weather and operational delays into SFO hundreds of times, I wouldn't book a domestic on-line UA connection of only one hour -- even if, as is likely, your first flight will arrive into SFO Terminal 3 (or even the Int'l. terminal).

waver99, even if you can't a find a sympathetic agent who would waive any change or standby fee you'd incur if you're not a high-enough elite, it's a very small insurance policy to be on a 6 or 7 a.m. flight out of SAN. Take your favorite reading material and/or on-line media. There are plenty of good food and beverage outlets along your connecting path at SFO. Yes, I said a 6 or 7 am depature as those would have the best on-time arrival chances into SFO if there are any delay issues. And you're unlikely to know of the true extent of any delay issues with your first flight until boarding -- or even after boarding.

I can't stress enough the opinion of others up-thread that it's simply an unwise risk to take a midday short-haul flight to SFO on a one-hour connection time to SQ - whether or not there's checked baggage and even if you knew plane #1 would arrive at the gate next to plane #2!