Weeds pop up quickly, with no effort, and can be really frustrating. Identifying them can also be tricky. We see Prickly Lettuce (Lactuca serriola) in Northern Colorado start to germinate in fall and grow over the winter and early months this year! The first picture is an immature plant and the second picture is a maturing plant. The more it matures the pricklier it gets so if you plan to pull them, do it when they are young before they get the prickles.
For more information I found this website to be a great resource. http://www.oardc.ohio-state.edu/weedguide/singlerecord.asp?id=1010