Boy, I went up Mt. Diablo on Weds. evening and the day turned kind of nasty! We started up the Northgate around 4:45 - a quick call to the info line confirmed it - 42 degrees and 18 mph winds at the summit! We ended up going a bit past the junction - we did 2,400 feet of climbing, but in the end decided it was just too nasty.
Here's what it looked like on the way up: