As you stand over your suitcase and wonder what to put in, remember what Erin said at our orientation "León gets chilly, León gets hot, it rains a lot." :)
With that said, the weather is typically cool in the morning - warm (and sometimes very sunny) throughout the day and the temperatures will fall in the evening. You'll want to bring a light jacket and/or fleece with you. Also, your clothes WILL BE washed (either by you or for you) and the more you bring with you now, the less you will have room for upon your return to the U.S. Choose your clothing and accessories carefully - if you don't think you'll "really" wear or use an item, don't bring it. Be sure to include a backpack and a collapsible duffle bag for new purchases as well. An umbrella might be useful, but these are reasonably priced and readily available in Spain if you're already worried about space and/or luggage weight.