Let me preface this by saying that you're asking a San Antonio girl what 15degreees is like.
<pondering that for a moment>
All righty, then! ...
How cold will it get in the car? Will the sun be shining? If so, tuck the machine somewhere out of the direct sunlight. If the sun isn't going to be out, then I wouldn't leave it in the car. Keep the machine above 32degrees if at all possible.
Just pack it with you ... it's a portable ... <grin>