It is not accurate for us. We always have a real cold winter every few years and zone 8 plants would all die. About an hour away they grow some palm trees but they won't survive with out pampering/protection in our area. This the the 'global warming' projected map.

I agree. Seems like it is politically motivated. Or maybe plant sales motivated - because if all your plants die in a cold winter you have to go buy more?

I think one of the problems is that it relies on 30 yr averages. In recent years, the weather has been more extreme with a much warmer winter than average followed by a much colder winter. The average of those two may tell you a plant can survive but the actual data rules it out.

I always considered the usda hardiness zones as sort of an average guideline that wasn't micro-climate sensitive.
Living in the west I have relied on the Sunset Western Garden book for specific detailing of the very many micro-climates in the West.
When I was living on the east coast I depended on the local university and or the ag extension for specific hardiness zones.

I think my usda zone is now 9b. which sounds about right. Prior I think my Northern California zone was just plain 9.
The coldest it has been so far this year is 27 degrees F. We usually get one or two of those a winter . On the average it is high 30's, low 40's during the coldest months of late Dec, Jan and early Feb.
Though last night it was a balmy 50 F and today it reached 69F.

Yes, it's averages - they have never said a plant not hardy below the average range for a zone would be fully hardy in that zone. Here if you figure a plant needs to be about 10 degrees hardier than the low end of our average that works out most of the time.

That said, the new zone finder, if I worked it right puts my girlfriend's apparently much colder garden in the same 8b that it puts me in. I do have to wonder about that. Plants I've had for years freeze out at her place after only few to several years. And her snowfalls are about triple mine. I could see her being 8a, after all the numbers are worked, but 8b? Really?

Suddenly I'm z6! Very exciting.