Probably not applicable to where you are, but I froze my butt off once sleeping along a stream bed. There was a weather station on top of the ridge just a couple miles away 1000 feet up. I was watching it for a month before the trip and it never dropped below 40. Predictions for that weekend were similar. I took my gear that was good to freezing, but it dropped into the low 20's at night. Woke up to a layer of ice on the stream. I never took into consideration that down in the valley it gets much less sun. Here in California sun is everything.

I heard you can buy a cheap lightweight down blanket at Costco if you are member. That might be the perfect thing for this trip.