OregonDeadApril is one of the very best times to ski. Often it is when there is actually the most snow on the ground at places like Hood, and Bachelor. We ski 12 months a year in Oregon. I did a mid June trip to Squaw and Mammoth once and there was still a lot of terrain in so I’d say either of those are pretty solid and will for sure be open through April
Yeah, late April is usually prime at Squaw and Mammoth(afaik, I've never been but I hear great things about spring and they were open until mid-July last year) if it's a decent snow year. Not a lot of powder but usually plenty of slush and an awesome vibe at Squaw, they're almost always open into mid-May at the earliest.