Hot--temps could range from somewhere in the 80s to over 100. At least it's not as humid as WDW. But, S CA does tend to cool off at night. You'll likely want a sweatshirt or light jacket unless it has been really hot (upper 90s-100s) at night.
Last year it pretty much stayed in the 90's. Just be prepared for the dry heat, stay hydrated. The evnings do cool off and the crowds drop off as you go towards the late evening.
June: Average High 80.2F (26.8C), Average Low 60.5F (15.8C) July: Average High 84.4F (29.1C), Average Low 63.7F (17.6C) August: Average High 86.8F (30.4C), Average Low 64.2F (17.9C)
On occasion, it does reach 100 degrees F (37.8C) at DLR. But overall it is more typical for the high temperatures to be in the low to mid-80s. If you follow good touring strategies you will not experience these temperatures.
Good summer touring strategies are described here: