Yeah Sunshine is so rad late season! Most hills in BC start getting "spring conditions" meaning melting snow and rain/snow in mid-late March. Well at Sunshine that usually happens in mid-May (or doesn't happen at all). In 7 years at Sunshine I've seen it rain 2 days. TWO! Spring dumps are typical. I'd say March is the best month.. there's usually mid-season conditions with ever run open and good pow to be had. Last year when the closed every run was open (well, the shut down Goats Eye a week early due to lack of business, as usual). January and February are inconsistent... sometimes snowy, usually dry spells.
That cold snap we mentioned normally hits between Christmas and Mid-January.
Early season is better on the coast. Sunshine rules for spring riding, the best I've yet to find.