You may actually have one of a few problems.
The problem that comes with biking in the snow/ice is not only moisture, but salt, and other road debris/grime that can get into your chain or derailleur. Leaving your bike outside, covered not, is a guaranteed way to corrode your parts as the moisture in the air gets into your frame, cable housings, bearings, any exposed metal etc.
You mention that your freewheel is skipping. If I'm not mistaken, I think you mean that your chain skips between gears on the rear cassette. If this is the case, you have a few things you can do. The first is rather simple, make sure that your shift cables are "in-tune". There are a few resources online that will help you adjust the cable so it is properly tensioned and you aren't hopping around gears while riding. If they do happen to be in tune, make sure they're properly lubed. A drop of chain lube in the cable housing should do just fine.
Another issue you may have is that your chain has been degraded by the weather you've been riding/storing your bike in. It is very possible that your chain isn't adequately lubed, has rusted, or water/grime may have seeped into the inside of your chain. A good chain cleaning may just solve your problem. There are resources online for that as well. A method I hear highly effective is using a wax-based lubricant as so previously mentioned.
You can also dip your chain (cleaned thoroughly) in a bath of paraffin wax, and this does a great job at creating a physical barrier for the moisture/road grime/salt. It's a very old school method of chain lubing, but some people swear by it.
Another common problem you may have is a "stiff link" in your chain. This is where the chain will not bend past a certain angle because it's either been bent, damaged somehow, or dirty. This can easily be solved by bending the chain at the link forward, back, side to side a few times, with care not to overdo it. And then of course, lubing it.
You might also have a loose cassette, if this is the case, you can take it to your local bike shop for a quick tighten up, unless of course you have the tool yourself.
In short, check that your cables are lubed/tuned. Check that your chain is in good condition (clean, lubed, undamaged), and if all the above fails, or you don't feel like going through the trouble, a good shop will be able to get everything in working order for you.