I have a strange plant that showed up in my garden about 3 weeks ago - it is upright, about 1 foot tall and the leaves are similar to watermelon plant leaves, rather like a rounded oak leaf shape. The stems are quite spiny and it has just recently sprouted bright yellow blossoms. I have no picture available, but I was hoping that you would have an idea from the description what it may be.
Weeds which sprout when the weather warms include Prickly lettuce, (Lactuca serriola). Check out this weed page from University California IPM Weed Gallery: http://www.ipm.ucdavis.edu/PMG/WEEDS/prickly_lettuce.html
There are links to some similar plants at the bottom of the prickly lettuce page. Without more description or a photo, I'm not sure what else fits the description. Happy to guess again if you provide more clues.