January is a great time to visit LA and Las Vegas. It's not so great
for the Grand Canyon.

Depending on where you are in the LA area, 70F to 80F days are common
in January. And the air tends to be cleaner... sometimes you can even
see the mountains! September tends to have some of the hottest days
of the year. In September, 90F to 100F highs are common and the
record high where I grew up (10 miles east of downtown LA) was 112 one
September.

BTW, Southern Califnrnia has more people than the whole of Australia.
LA County alone has roughly half the population of Australia.