Do your impression of a Californian accent.
Now do your impression of a Canadian accent.
Your impression of the Californian accent probably has a "laid-back" sound, with some vowels lower in the mouth and slightly farther back than the average: Bit might sound a bit like "bet," while bet sounds a bit like "bat." Meanwhile, some other vowels might be a tad more forward: Dude probably comes out more like "dewd" than like "dood," for instance.
If you're like many Americans, your impression of the Canadian accent will be quite different. The vowels probably sound "clean" and maybe a little "tight." Dude would definitely be like "dood," and you wouldn't have any "laid-back" sound in words like bit and bet.
If those are your general impressions, then you're probably more or less right about California, according to studies done by various linguists such as Penelope Eckert, Robert Hagiwara, and Charles Boberg.
And entirely wrong about Canada.
In fact, if you wanted to improve your Canadian accent, you would do better to just use your Californian accent. In some important ways it is closer to the real Canadian accent (and vice versa) than other American accents are — especially those right across the border from Canada in Buffalo and Detroit!
It's not that the Californian and Canadian accents are identical. In words such as night and house you may hear a difference (though the common idea among Americans that Canadians say "oot" for out is false — actually the out vowel is just a little higher in the mouth than the American version). But there are a lot of points of commonality.
Let's start with the words cot and caught. Depending on where you're from, you might say these words differently — maybe even very differently — or you might say them exactly the same. If you're from California or Canada, you'll say them the same: The distinction between the two vowel sounds has disappeared, and they've merged somewhere between the two. Some linguists think this is what is causing some of the other commonalities between the two accents — because a lot of unused space has opened in the mouth, other vowels spread out to allow for more distinction between them.
For one thing, the vowel in a word such as hat is farther back in the mouth in Canada and California. This is completely different from what you'll hear in places like Buffalo and Detroit. In fact, the way Californians and Canadians say hat is almost exactly the same as the way people from the big Great Lakes cities say hot.
On the other hand, before some letters such as n the sound Canadians and Californians make is actually higher: A word such as and will sound almost more like "end" — actually in the same direction as the "eeand" sound that you'll hear in Buffalo.
With the sound of words like bat being farther back, words like bet and test have moved closer to where bat would have been. If you're not from California or Canada, the Cali/Can bet can sound to you like "bat" and test like "tast." This is especially true among younger speakers — this trend in pronunciation is getting more pronounced (sorry). Meanwhile, in Buffalo and Detroit, the same two words will be closer to "but" and "tust."
Since bet has moved down, bit has some extra room, too. And the result is that bit can also be a bit lower, so it sounds closer to what bet would have sounded like… if it weren't itself lower in the mouth. But again, in both of these cases, it matters what comes after the vowel: Pink is much "tighter" in sound than pick, for instance.
Meanwhile, that "oo" sound you'll hear in dude is farther forward in the mouth for both Canadians and Californians than for many other accents — closer to "dewd," especially for younger speakers. This may be no surprise when it comes to the California accent, but many Americans doing a "Canadian" accent would say "dood" about as far toward the back of the mouth as they can make it. To check whether this is accurate, just ask a Canadian to say Vancouver. It will likely sound more like "vankewver." (If you don't know any Canadians, try watching videos of Vancouver TV news. Then, for fun, watch videos of L.A. or San Francisco news. You'll be surprised how similar they sound.)
Why are these two different places coming to have such similar features in their accents? Canadian weather and California weather are obviously very different. You're also unlikely to think of Canada and California as similar in too many other ways, although some stereotypes — such as granola-style nature-loving and liberal political trends — can be pretty similar. No one really knows whether any of those factors have any effect on the accents. But we can be pretty sure that the weather isn't a big factor, because the accents change dramatically as soon as you cross the Niagara River from Ontario to New York state.
What we do know is that if you get three people, one from Toronto, one from Buffalo, and one from Los Angeles, to stand next to each other and talk to you, you'll know which one is from Buffalo right away, but you might have trouble telling which one is from Toronto and which one is from L.A.
Dude. That's a bit of a surprise… eh?