Weather has little effect on GPS having said that, ionospheric delays can play a factor especially when no WAAS corrections are available.
Also, we all so easily get used to the fact that GPS is normally so accurate that it brings us to the very same spot time after time but the truth is that a spot reading can easily be 30 feet off the mark or even more. To get an accurate reading, you have to average for substantial length of time. Now, if the original co-ordinates were also obtained using spot readings than, the error you had could be cumulative and really throw you off.
Think about t this way; if our cheap and merry receivers could be guaranteed to be always perfectly accurate, the surveyors and companies that sell $40.000 GPS systems would be out of business by now.
I was out with my GPS geocaching. The temperature was in the low thirties,cloudy sky and about 2 feet of snow on the ground. The GPS showed the location of the cache but it was not there. The guy that hid cache was with me and told me I was within 50 feet of it but wrong location. He also checked my GPS and came up with the same location as I did. I had the same latitude and longitude information load into my GPS as he had put on the web site. My question is, would my GPS be that far off or could it be weather condition?