I'd expect it to be rather crowded since Halloween officially opens this friday. I remember last year it was so crowded that Saturday (opening weekend) that my bf & I left. Weather-wise: I'll be cool and partly cloudy.
I was just there today. Still light crowds. Weather was so refreshing after the heat of summer! Still saw lots of non-costumed maintenance people around painting and such. CM's seem more relaxed and willing to interact with guests.
This is good - I'll be there Saturday also (since last February) I don't mind the crowds - I'm going alone so rides aren't my priority (tho I'll go on a few) I'm there for ambiance, friends of mine in some shows, shopping and some of the food
On the last few times that I went, the crowds have been light to decent. On my last visit last Saturday, the crowds were moderate. The weather usually affects how busy Disneyland will be. The crowds on weekdays around this time of year are usually light, despite the good weather. In the past month, the weather has ranged from around the high 70's to low 80's. However, I just had a look at the weather from ABC 7.com, and they have just updated Friday and Saturday. Friday night says an 80% chance of rain, and on Saturday morning, they say there is a 60% chance of rain in the morning, and they say it should clear later on in the afternoon. When there is no rain, the crowds stay light.
We had light crowds yesterday although I think the majority of people were in line for Nemo - it was an hour wait 30 mins after opening. Except for that, it was easy to walk on practically everything all day. The weather was about 75 degrees and we were comfortable in our jeans and short sleeved shirts.
Hopefully whatever storm comes along will pass before you get there! Keep checking in with www.weather.com.