For me, it's when ambient temps start to stay consistently above 45° F (not just the high temps but the low temps). And its not just about impending "snow"...but when temps are cold...the rubber compound hardens on my summer tires so if its cold and raining, or just cold...they don't do a good job when stopping or handling (turning)...as well as acceleration.
The temps listed by the OP seem to be edging upward where their summer or all season tires could handle the ambient temps and weather conditions.
