I'm not sure where this idea that there is momentum going for Teachout is coming from. She's still way too unknown and going up against a sitting governor who is popular, especially among self-described liberals and Democrats. And I'm not sure why the more liberal candidate should carry Upstate New York, where she's even less known than she is in NYC. It usually works the other way around. Upstate is more conservative than NYC.
My best guess is that Teachout doesn't exceed 25% in a very low turnout primary where the party establishment - all backing Cuomo - dominates the vote.
I don't agree with Wulfric's point - he'd be correct if Teachout was just a generic protest vote upstate voters could use to vote against Cuomo, but since she's actually running a campaign I'd expect her to do much better than generic R in Democratic strongholds. But to address your point, it's fairly common for something like that to happen - see
FL Gov 2014. Rich overperformed (and actually won counties) in north Florida and the Panhandle, while underperforming in South Florida and I-4. Do the voters of Holmes County really prefer the liberal former state senator from South Florida with no money and no campaign north of I-4? Of course not, it's a way of voting against the Democratic establishment and their candidate. If voters upstate dislike the Cuomo administration they'll vote for the guy running against him, simple as that (and yeah, I realize the differences between FL and NY, especially the lack of the ancestral Southern Dem population like the Panhandle has, but still).
