Anyone have a Space Bag?

I have two planned trips this summer and the first is coming up quickly.  It's still far enough out that I'm unsure exactly what the weather forecast will be, but the internet is full of helpful information like "temps fluctuate wildly," "often breezy," "average temp between 50 and 90 degrees."

How does one pack for that, exactly?

50 degrees means jeans and long sleeves.  90 degrees means dresses and sandals.  In between could be a crazy hodge-podge of layering.  Oh, and did I mention that I'm trying to do this with only my carry-on bag?   So far my must-take list includes 3 pair jeans, 3 dresses, 4 pair shoes, assorted short-sleeved tops, and then long sleeved things to layer over everything else.

I made this cardigan special for the trip.  Hope it's not too granny.  I wanted something warm that can be worn with both jeans and sundresses, because we're just climbing out of the Longest Winter Ever and I don't want to wear anything that resembles a coat, jacket, or windbreaker again until at least October.
 

Why is packing the hardest part of travel?  Gah!  The next trip should be easier.  Because Texas is always warm, isn't it?