I've been to Aspen/Snowmass March 28 - April 1, 1998, which had 15 inches new snow and was mostly winter conditions. I was also there March 21-24, 2004, which was nearly all spring conditions during a 3 week heat wave. But my kids' spring breaks were around the first of April for over a decade, and in Colorado I can say that the latter trip was much more unusual than the former.
- slush @ base by mid-day?
Probably 50+%. But except at Buttermilk (which I've never skied) you're going to be at least 1/3 of the way up the hill nearly all day.
- Cirque + Hanging Valley still open?
90% These areas take a long time to get open, but once open almost never close due to excellent snow preservation. Cirque/Hanging Valley were open during the 2004 heat wave, but they closed at 2:45PM each day.
- winter snow conditions up top?
Despite the unseasonable hot weather in 2004 and 2007, I'd say 70% winter up top over the long run on April 1.
- some lifts finished for the season?
Buttermilk and Highlands close first weekend in April (April 6, 2008 as posted on their website), so I wouldn't do a destination trip any later than that.
I would think not. If Aspen keeps a trail open, they will likely maintain it.
Definitely. Average April snowfall at Gothic (30 miles south of Aspen if you're a bird) is 80% of what it gets in March.
I'm very high on these high altitude southwest destinations at this time of year. Very uncrowded; that's why they close so early, often with their maximum coverage and sometimes best conditions of the season. Telluride held up even better than Aspen in 2004, and we had a great spring break at Taos in 1993. Crested Butte was good too on my first two spring trips there, but unfortunately the 2007 heat wave was too much for the North Face in a below average snow year.