hey Sam, greetings from the colonies.
I'm an avid skier myself and also I work in the meteorology field, though I'm not a meteorologist or a climatologist myself. I do work with a lot of them though and that’s what my degree is in so I know enough to make me dangerous. I also have skied Whistler just about every year for the past 15 years.
The effects of El Niño are not totally predictable. And there are a lot of other factors at work that will drive a given winter’s snow patterns. Also, the severity of the El Niño condition plays a role. In general, El Niño makes for very wet conditions in Central California and drier (and warmer) conditions in the Pacific Northwest where Whistler is. Obviously, warm and dry are bad things for a ski resort. I understand that so far the El Niño condition is somewhat mild, unlike the extreme event we had in 1997-98.
The last big El Niño was in 1997-98 when I lived in San Francisco. It seemed to rain almost every day that winter and we had flooding and mudslides everywhere. Lake Tahoe and the Tahoe resorts got buried with snow. I don’t recall how much snow Whistler got or didn’t get that year.
The following year (1998-99) was a La Niña year (the opposite condition from an El Niño). That was the winter that Mount Baker in Washington State set the all-time, worldwide snow record for a season (1140-inches). Not for a ski resort, but for any place on earth in recorded history! They got like 330” of snow in February alone, that’s a daily average of over 11 inches. I also remembered that Tahoe got a ton of snow that year too.
I also seem to remember that the late 90s were pretty lean snow years in Colorado. In fact up until these past couple of years snowfall has been less there.
With that being said, I’m in the process now of booking my Whistler trip for late February and I have faith there will be snow. I’ve never been there when there wasn’t snow. Except for one day in 1993 (on Super Bowl Sunday), I’ve had good weather. Even if it’s wet and rainy at the base, it will usually be snowing up the mountain. And even if the bottom of the mountain lacks snow or is wet, downloading from mid-station is easy and there’s so much terrain above mid-station, that it doesn’t matter.
I think Whistler offers so much more than Mammoth and is worth the extra hassle for me to get to. I can drive to Mammoth but I rarely do. If I were you and if it’s powder you’re after, I’d consider Tahoe. If this El Niño does develop further it could be another deep season there. I’m curious why you’d consider Mammoth, where there is just the one resort, instead of Tahoe where there are many…plus casinos, night-life, restaurants, etc.
Not sure this helps and I’m not trying to talk you out of Whistler. My un-expert guess is that the El Niño will be mild and there’s no telling where the snow will be deepest. Plus, it’s sort of a crapshoot when you plan way ahead, you could pick a dry week with hard pack in an otherwise huge powder year or a great snow week in what turns out to be a lean snow year.