I gather that you live in Fresno, I lived there for many years too. There will be 2 types of weeds growing, one in cool weather, that will start to germinate after the first rains and second the warm weather types , those that will start to germinate in late Feb to May. Then, there is the ever so popular hard pan to contend with ( during the 60's the orchardists used dinamite, deep tilling rippers with D9's or backhoes to break up the hardpans prior to planting the orchard trees/ wine grapes)... varies from the soil surface to a few inches to a couple feet down. I have battled the stinging nettle for at least 10 years. The hordes of blackbirds numbering in flocks in the thausands late fall will roost on power lines or trees and spread the seeds in their excrement. From hoeing the seedlings, to rototilling the area and covering with black plastic, etc. then in desperation chemical warfare. They are still there. The best success that I have had in establishing a new lawn was first to water the area very well, then cover with black plastic for about a month. Then remove the plastic, spread several inches of horse manure ( or compost) and rototill as deep as possible that area at least 2 times to break up the hardpan, water well, then cover with black plastic for another month. Then, spread the grass seed, cover lightly with compost and keep moist for about 2 weeks, grass as well as some weeds will still germinate ( hand weed the weeds). Good lucck !