Southern California - How cold riding before 6am

I am riding the Pasadena Marathon bike ride. it starts at 5:45 am. I wonder how cold it is that early and how should I dress? Thanks.

Depends... you can probably get away with a long sleeve base layer or maybe just some arm warmers. It warms up fast up there though

I recommend the "classic" Weather Underground (http://classic.wunderground.com/cgi-bin/findweather/getForecast?query=Pasadena,%20California&wuSelect=WEATHER) website for Pasadena. Once you're there, off to the right there's a subheading for "5-Day Weather Forecast for ZIP Code 91101". Click on the "Hourly" link under the day in question, and there's a forecast for the temperature every three hours. You may need to interpolate.

I've used that website for different locations many times with pretty good results.

I generate a lot of heat while riding, and I have found that I can ride with just a jersey and shorts any time the temperature is 60° or higher. A lot of my riding friends use arm warmers, leg warmers or even a base layer until the temperature gets up to about 65°. As they say, YMMV.

Good luck on your ride!

Depends, sun usually comes out by that time and u can go short sleeve. 4am is when it gets cold and u need to wear something until the sun rises.

I ride at 4:30am every morning and Cycle shorts and a Jersey work great for anything over 12 degrees c. Feels a little cold to start, but when you're warmed up its perfect.