I won't argue about Vipera berus occupying the most northern latitude, but there are some pit vipers that are very cold tolerant, and that may be what you saw.

There is a Himalayan pit viper that has been observed basking in the sun while snow was on the ground. And there are other pit vipers that occur in montane regions that get very cold. I believe it was the pit vipers that crossed the landbridge from Asia into North America, then traveled down from there.
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Mark

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