I pretty mirror what everyone else has said here. Dedicated summers that go on once the early mornings are consistently over 45 degrees. Sometimes this is March, sometimes its May just depends on the year. I don't let my calendar determine when I change tires.
The truth is most all season tires still have decreased traction below 45 degrees, so even if it isn't snow or ice you are talking about, just cold pavement on a standard all season tire can increase you stopping distance.
Remember with AWD you should have no trouble getting going, its stopping that is the true test of the performance of a tire in winter conditions.
As far as studs, horrible idea in this area. Studs actually lower the amount of contact that the rubber on your tires makes against the road on non-iced / snow packed roads. Meaning, when it rains (i forget, with these 95 degree temps does it rain here?) hydroplaning becomes a much greater danger on studded ice/snow tires.
Long and short, if you want to drive your car all days of the year, make sure you have the proper equipment. If you want one solution that will get you by 90% of the time, get an all season tire. Just realize you will pay for that with about 1-2 weeks of a parked car, or curbed wheels and or scuffed bumpers if you are dumb enough to drive when it gets nasty.