Hello
Hi Lilly, most people might find it a little brisk in early December, but being from Alaska, you might not find it too unbearable. You can get lucky with the weather often times, but don't count on it. We have had a turn of the weather the last couple of days. Just keep an eye on it. It's a great place to sun, if you have never been. It lies within the boundaries of McGregor Park so check the website for hours of operation.
I hope you enjoy your visit here to Austin. I live about 5 miles from Hippie and in Dec the weather can be warm or it could be cold. I am not going to make a prediction or even check the Farmers Almanac. All I can say is that usually January in Texas sucks! I have caught some nice sun on the rocks at Hippie in all months but if I were a betting man ( or woman in your case) I would bet the weather would be a bit cool for a Texans blood but maybe not an Alaskans. Good luck!