Or that spring in the valley means spring 3000' higher. A hiker in New York died two years ago while poorly dressed. I was there within a week of her, and at the trailheads there was as much grass as snow. 500' higher, and it was full on winter. She died, so we can't ask why she was wearing what she had, but getting into conditions she didn't think would be there seems likely.
This is definitely one of those things that screws people up. I was doing a hike years back that I had done many times before and ended up getting hailed on hard. It was a beautiful day out and only sprinkled rain in the town we had only really hiked 15 minutes away from. That was like July, the mountains are unpredictable. The hike itself lead to a beautiful lake but it was the third time before I was able to make it the whole way because we kept hiking up to winter. Even that time we couldn’t walk around the lake because it was still snowy so we jumped in and went back down. The good thing about that hike though is there is also a waterfall or look out to consider as the destination. It also helps that I lived in the town for the summer so I never felt a need to push through bad conditions. People ask all the time what to wear for hikes here and my answer has never changed, wear what your comfortable in now and then bring a pack filled with clothing for every possibility. That’s why all the expensive lightweight clothing is so worth the money. and at the trailheads there was as much grass as snow.
I'm also a proponent of packing for what's possible. I've carried extra winter layers up and back down every time and, I think, never worn them. I've done the same with rain gear, tops and bottoms, since they also make good wind blocks. The idea I follow is that the mountains will still be there tomorrow, next month, next year. Better to be safe and come back another time.