Denver 1/2 Marathon??

I agree with jneugeba that you can't count on a Colorado weather report being accurate beyond just a few days in advance, but based on last year's miserable conditions at Denver Marathon (rain, cold, sleet, wind), I think you do need to be prepared for anything.  Last year many runners were underdressed and felt like they never warmed up. You may want to consider doing some training runs with your different apparel options.  Your first layer closest to your skin should be breathable, like a poly/technical fabric shirt.  The next layer may be a more difficult choice.  Ideally, you would wear something you won't want to peel off in the middle of the race, although some people will wear a long sleeve cotton tee.  If you do plan to have the option of removing a layer mid-race, but if you want that item back you may want to plant a friend along the course to give it to.   If the weather brings rain, you'll want to be prepared with an outer shell layer that breathes and repels water.   ANd part of your decision will depend on your competitve goals as well - If you're looking for a PR or to compete, "lighter & sleeker" is the theme.  If your goal is to finish the race, comfort trumps speed factors. 

Another good strategy is to be very well warmed up so you've broken a sweat before the start, not a few miles into the race.  Not only will it help you run a more consistent race, but you'll have a better idea of what to wear.  Take a couple early mornings and test a warm up routine along with your clothing options.  This weekend might be a great time since I've heard it's supposed to get colder.  That will also help you determine what to wear.  I think Denver Marathon has a gear bag check in at the start.  Check your race packet for details, but if they do, you can prepare your clothing options for any conditions and then make your final decisions at the start.

Good Luck!

I'm running the half, too.  I've started keeping an eye on the long-range forecast but I won't make any final apparel decisions until the day before, most likely.  If it DOES turn out to be in the low to mid 40s at the start as they're currently predicting, I think I'll be wearing shorts with a mid- or lightweight long sleeve tech shirt.  Maybe lightweight cheapo gloves (so I can just toss them once my hands warm up later).  As the others have said, this all depends on your own comfort level, race goals, and what you've been wearing in training up to race day.

The race DOES have gear check (according to the website), so as the last poster said, you can always bring a few different options with you and check whatever you decide not to wear.

I don't want to repeat what anyone else has said, but wanted to offer this up for a suggestion.  If I don't have any old old clothes I'm looking to donate, I'll hit the thrift store and buy a cheap-o LS shirt or pants to wear to the start and then toss to the side when I get hot.  I did that for the Elephant Rock Century - it was chilly but I didn't want to carry a shirt for 100 miles, so I bought a LS shirt at Goodwill for $2.  After 20 miles I left it at an aid station. 

I'm running one of the relay legs, so hope to see you there! Good luck!

So far, Weather.com is calling for 51 degrees and sunny at 7am tomorrow - I don't think we'll have to worry too much about being cold!  (knock on wood)

Best of luck to everyone running the 1/2, full, and relay tomorrow!

If you're still up and reading...I'm wearing my favorite running skirt, running tank and ls shirt over the tank.  This is very similar weather to the 12 mile training run I did a few weeks ago, so I'm thinking I'm good.

Good luck...I'll be the one finishing the 1/2 as the full marys cross the line!!