I believe I would wait till warmer weather before I release any butterfly. I would hate to see such a beauty die from the wrong temp or weather condition from my lack of knowledge. To be safe is better than being sorry, is what I always say

I know this is a few days old, and I didn't really have an answer, but was just going thru some old posts and saw where MissSherry said: "I see adult sulphurs flying on sunny winter days every year" in the link below.
I knew I had seen it somewhere!

Here is a link that might be useful: sulphur thread

Well, I ended up releasing her on the 27th because I noticed she had already managed to lose a leg while she was in the mesh reptarium, not sure how that happened, I just saw the little leg on the floor of the cage, and I was afraid that maybe they are just too delicate to be enlcosed in those mesh cages for long. SO I fed her some sugar water (which was not easy, lol) and then put the reptarium outside and unzipped it. She basked for a few minutes and then took off, and flew really high and then out of sight, and i didn't see her again. I got home from the store earlier today and there was a cloudless sulphur flying around in the yard, not sure if it was her, but even if it wasn't, I was releived to know that there is a chance she survived since this one seemed to be doing fine, and it is in the low 50's today, and night temps have been in the 30's and 40's for the past week. I just saw your post fighting8r, nice to have some more reassurance that she probably did fine, even in the low temps! Thanks!

Well now I have one and it is not expected to get much above 60 the next few days. It eclosed yesterday and I did take it out in the sun, along with a gulf fritillary from the day before, and they both just sat, as it was quite cold yesterday as well. Anyway, my question is - how did you get the sulphur to drink?? You are right the proboscis is tucked away much more than the others, never noticed this since I've never had to keep one...

Actually, I found out a little trick by accident, lol. I tried to feed her one day, and had no luck getting her to unroll her proboscis, and since they are so feisty/hyper and so tiny/delicate I stopped trying and gave up. Well the next day, I noticed she was perched vertically on the side of the mesh cage and she hadn't moved in a while so i did the little "blow" test to see if she moved and happened to notice that right when I did it, she started unrolling her proboscis, so i did it once more and she unrolled it again. So I hurried and made some more sugar water and i got two toothpicks, I wrapped cotton around one toothpick and soaked it in the sugar water, I kept the other toothpick in my other hand and then I picked her up (wings folded, in between my index and middle finger) and I turned her to where she was facing me and lightly blew into her "face??" and as soon as her proboscis started to come out from that little groove, I hurried and put the toothpick inside the little loop and tried to finish unrolling her proboscis, and ran into another issue, lol, their proboscis is soo long, that when you try to unroll it with the toothpick, it gets wrapped around it too tight to finish unrolling, like it gets stuck, so I ended up putting the toothpick underneath the little loop and was able to get it unrolled without any problem and as soon as i got the tip onto the soaked cotton, she left it there and started drinking right away. I held her the whole time, they are so fast, I was afraid I would never catch her if she took off. The cloudless sulphurs seem to be very very delicate compared to the gsts, I noticed ANOTHER leg in the reptarium when I was cleaning it out the other day, long after she had left, and I barely handled her at all while she was indoors, in fact I think I only picked her up twice. The GST i had for three weeks (due to bad weather that prevented me from releasing her) never lost a single leg, or anything else, and I picked her up at least once everyday for 3 weeks to hand feed her. I don't know how the sulphur lost two legs in less than 3 days, she seemed to get around fine in the mesh cage, and anytime I saw her zooming around or getting anxious, I darkened the room and she calmed down almost immediately, so idk. Apparently, she was able to get around just fine even though she had lost a leg (or two) because she didn't seem to have any trouble whatsoever climbing or walking, and her balance didn't seem to be off at all. Just thought I would warn you about he leg issue if you are keeping your CS in a mesh enclosure, not sure if this is a common problem or not.

Hmm well I'll keep an eye on the legs, and thanks for the tip(s) on the proboscis. The forecast tells me I might need it. I'm sure you'll be hearing how it goes.

Well my orange barred sulphur took off today, a bit chilly but when I took the enclosure out for some sunshine, it started flying around like mad, and of course when I let it out it went high into the trees and sat in the sun, before disappearing. The gulf frit is not interested in taking flight in the mid-fifties weather.