Skiing and "Conditional Probability"

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Skiing and "Conditional Probability"

Postby jamesdeluxe » Sun May 02, 2010 6:45 am

I'm fascinated by the way that as a rule, we base our destination trip plans on a "conditional probability" logic pattern that basically says, "Region A is known for its statistically consistent snow conditions, therefore, I have a better chance of good snow when I go there in two months. Therefore, I'm going to choose Region A over Region B or C, which have a less stellar record." Makes sense, but anecdotally (as I look over my ski-day lists over the past ten years), it's never worked out that way for me.

To give two recent examples, I went to New Mexico last season, which was having a below average season (and is one of the trickier western locales for snow), and scored beautiful conditions. I went to Quebec two separate times this season, which was also having a lackluster snow year, and had nonstop great conditions. So my question is, how much of your decision-making process for destination trips is based on conditional probability?

Take a look at this NY Times article and see how much/how little we can apply to skiing:
http://opinionator.blogs.nytimes.com/20 ... n&emc=tya1

Do you consult detailed records like the ones on Tony's site before booking a trip more than a week in advance or just let it fly and see what happens? I do a little of both, but have had such good luck over the past few years in rolling the dice, I sometimes wonder what the point is.
Last edited by jamesdeluxe on Sun May 02, 2010 2:35 pm, edited 1 time in total.
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Re: Skiing and "Conditional Probability"

Postby riverc0il » Sun May 02, 2010 7:54 am

Indeed, this is tricky as long term probability of good conditions has no effect on the actual result of the next immediate roll of the dice but is simply the long term data.

That said, I think I would still go with the probability approach in certain situations. If you are booking in advanced and conditions are really important to you, I can not see why you would not do so. Especially if you book early often as in a long enough time line, it seems like odds would eventually favor the location with the better odds.

Though this all just goes to prove that booking in advanced is the worst way to arrange skiing trips. As the closer to the ski day you can make your trip, the better your outcome is likely to be.

I played against the odds myself this year going without a Jay pass. I won big time as I was able to follow the storms and only skied Jay twice, and both were not powder days. If this was a 400" season up there, I might have been kicking myself. But ultimately, I decided that moving around and playing the field was more important than being tied down to one place due to reliability of powder. So I think that is my answer to the question. Though who knows what type of song I would have been singing if it was a big year for Jay. Past experience certainly has more hold over most people than statistics...
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Re: Skiing and "Conditional Probability"

Postby Tony Crocker » Sun May 02, 2010 5:53 pm

riverc0il wrote:Though this all just goes to prove that booking in advanced is the worst way to arrange skiing trips. As the closer to the ski day you can make your trip, the better your outcome is likely to be.

The "conditional probability" is the whole point of my snow conditions analysis, but the above statement is absolutely correct. My track record at Mt. Baldy is a perfect illustration as I usually make that call the night before, and sometimes change my mind when I get up in the morning. Despite my growing list of good days there I think it's idiotic for anyone to plan skiing Mt. Baldy as much as a week in advance given the high variability of conditions and significant percentage of when it's not so good. Thus my skepticism about the "deals" that require advance reservations. This experience combined with the reports I read here lead to my oft-expressed opinion that the Easterners should have a similar attitude.

Once you travel out of easy drive distance then the conditional probability becomes important. The first question is "conditional probability of what?"
I think there are three answers:
1) Probability of all terrain open or for some, the terrain they want to ski. Usually an early season question.
2) Probability of pleasing surface conditions. Reasonable means packed powder to a lot of people. Spring conditions are not a deterrent to many, but some places ski a lot better than others with transitional snow. Usually a late season question, but not if rain is part of the winter weather picture.
3) Probability of fresh snow.

I'll deal with #3 first, because the answer is straightforward and not what people want to hear. Historical probability of 6 inches or more in one day is barely over 20% at the snowiest places and given streakiness of weather most places have a probability of NO 6+ inches days over a winter week in the the range of 50%. In the best case scenario of Alta it's still 25%, along with another 25% of just one day between 6 and 12 inches. So if powder is your criterion of success of a destination trip you will be unhappy a lot of the time. If you're doing better than that consider yourself lucky and don't expect that it will continue. The backcountry types can do better, but they still need to be aware of historical weather patterns in terms of how long the powder can last if it hasn't snowed for awhile.

#2 is the most predictable, being driven mainly by altitude and exposure in the western destination resorts. If you do your homework and a late season trip turns out crappy that's just bad luck.

Given the volatility of snowfall it's hard to eliminate the risk of #1 before February and at Christmas or earlier the list of resorts with low risk is quite short. I've gotten on a plane to ski before Christmas exactly once, and that trip was booked 2-3 weeks ahead after a big November.

With regard to James' New Mexico trip it was February 8-13, which was a reasonable plan. If it had been January 8-13 I would have considered it a dumb plan, at least for Taos.

I try to emphasize the volatility measures in my snow stats. With respect to snowfall it's going to take a very large number of ski days for anyone's personal experience to "settle into" the long term probabilities at any area. So basing your destination plans on your buddy's last ski trip is not a good idea IMHO without a little research to back it up.

The bottom line is that using "conditional probabilities" works quite well in avoiding disastrous trips but helps only a little in maximizing peak experiences. Boosting the latter is most enhanced by where you live and how flexible your schedule is.
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Re: Skiing and "Conditional Probability"

Postby Tony Crocker » Mon May 03, 2010 5:42 pm

Reading James' reference on the formal definition of conditional probability leads to a slightly different question. If you're making a decision on January 1 where you're going in February or March, how do you use the recent November/December snowfall information?

The good news is that issue #1 of snow coverage will be clarified completely for the areas with a good start (like Whistler this season), so it's most sensible to take advantage of the extra info and choose one of those. For the areas that are not quite there yet, the best assumption is average going forward. I've done a bunch of correlations and prior month snowfall has negligible predictive value for ensuing months. For the areas lagging it's often not to hard to figure out by how much in terms of time. In the Taos example if what's open at New Year's is what's usually open mid-December you might want to push the expectation of full operation from beginning to mid-February. For Front Range Colorado the lag in terms of open terrain was in the 3-4 week range in December/January this season. Full operation was attained in late February and the skiing was quite normal for the rest of the season.

A severe snow shortfall can result in terrain losing cover prematurely. But only in very worst case scenarios does this ever happen in the major western destinations when the typical vacationer is considering a trip (early April at the latest). The altitude/exposure considerations are independent of snowfall once coverage is adequate.

The powder issue is also independent. But if there's an inadequate base on the steeps and you get lucky with a dump just as you arrive you still may not get to ski the terrain you want. This issue is relevant to some who try to push the envelope and book LCC for early December. Typically the first major dump of the season requires careful snow stabilization work by patrol, so there can be a several day lag between the storm and when the terrain is opened.
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Re: Skiing and "Conditional Probability"

Postby Marc_C » Mon May 03, 2010 10:41 pm

Tony Crocker wrote:The powder issue is also independent. But if there's an inadequate base on the steeps and you get lucky with a dump just as you arrive you still may not get to ski the terrain you want. This issue is relevant to some who try to push the envelope and book LCC for early December. Typically the first major dump of the season requires careful snow stabilization work by patrol, so there can be a several day lag between the storm and when the terrain is opened.

Um, basically, no.
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Re: Skiing and "Conditional Probability"

Postby Admin » Mon May 03, 2010 11:17 pm

Marc_C wrote:
Tony Crocker wrote:The powder issue is also independent. But if there's an inadequate base on the steeps and you get lucky with a dump just as you arrive you still may not get to ski the terrain you want. This issue is relevant to some who try to push the envelope and book LCC for early December. Typically the first major dump of the season requires careful snow stabilization work by patrol, so there can be a several day lag between the storm and when the terrain is opened.

Um, basically, no.


:lol: I was going to try to ignore that blatant misstatement... :roll:
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Re: Skiing and "Conditional Probability"

Postby Staley » Tue May 04, 2010 3:53 pm

Tony's correct that conditional probability applies choosing mid-late season trip based on early season snow. Many people book trips this way, since the average person does have to plan their trips in advance, but doesn't need to book as early as August for a March trip.

My situation has always been different, as my family prefers to book vacations in the summer (as is often necessary for popular school breaks). This means I must depend upon simple probability, not conditional, when planning trips. In the past two years when I have completely planned the family's trips, I have based my decisions heavily upon Tony's historical data. Two years ago, I was planning a trip for the first week of April and settled upon Altabird as having the best chance of powder while being financially and logistically feasible. We were rewarded with about 10 feet of snow in our week.

This year, different spring breaks (college vs. high school) forced me to book a trip for Christmas week. Although the probability of powder is almost always the most important condition for me, I figured that I should also factor in the probability of all good terrain being open. For this reason, I chose Grand Targhee, whose Christmas snow record trumped all others. Of course, this happened to be Targhee's worst Christmas for quite some time.

The moral of the story is that for families like mine that must book trips far in advance, it's mostly luck. Looking at historical snow records can help to avoid catastrophes (like going to Taos for Christmas), but as Tony said, it cannot really guarantee success. More than anything, I feel that utilizing Tony's data allows me to be at peace with whatever decision I make. Even though everything is ultimately left to chance, I can rest assured that I have given my family the best probability of a fabulous ski trip.
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Re: Skiing and "Conditional Probability"

Postby Tony Crocker » Tue May 04, 2010 5:09 pm

admin wrote:I was going to try to ignore that blatant misstatement...

The delayed openings after big storms are due to the dump being on top of an unstable layer. This was the case in LCC in December 2009 and also December 2007. The instability being due to a prolonged dry spell with cold weather before those storms. So I misspoke about early season being the direct cause of the delay. But the requisite conditions (extended cold and dry before the dump) are somewhat more likely in November/December than later.

Staley wrote:I can rest assured that I have given my family the best probability of a fabulous ski trip.

Only a little bit. I think mainly you have given your family the least probability of a crappy trip. Targhee may not have been that great, but there's a long list of places that were much worse last Christmas.

Staley wrote:....book vacations in the summer (as is often necessary for popular school breaks)

Not necessary for spring breaks IMHO, which vary a lot by state and often even by school system within a state. I took something like 15 late March/early April spring break vacations while raising kids and booked most of them in January or February. The spread out spring breaks also prevent most of the obnoxious price gouging that occurs for Christmas week.
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Re: Skiing and "Conditional Probability"

Postby Admin » Tue May 04, 2010 5:14 pm

Tony Crocker wrote:
admin wrote:I was going to try to ignore that blatant misstatement...

The delayed openings after big storms are due to the dump being on top of an unstable layer. This was the case in LCC in December 2009 and also December 2007. The instability being due to a prolonged dry spell with cold weather before those storms. So I misspoke about early season being the direct cause of the delay. But the requisite conditions (extended cold and dry before the dump) are somewhat more likely in November/December than later.


Where you misspoke was:

Tony Crocker wrote:so there can be a several day lag between the storm and when the terrain is opened


Snow stabilization efforts take no longer early or late season, weak faceted layer or not. A bomb is a bomb is a bomb. A decision on whether to bomb it or let it settle doesn't vary by early or late season, facet layer or none. It is an odd mix of what's in the weather forecast, whether or not the whole thing's likely to come down, pent-up demand, etc., and those factors have nothing to do with early or late, other than the fact that demand is likely to be more intense early season. So if anything, terrain is likely to open more quickly early season, not the other way around.
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Re: Skiing and "Conditional Probability"

Postby Tony Crocker » Tue May 04, 2010 5:26 pm

admin wrote:A decision on whether to bomb it or let it settle doesn't vary by early or late season, facet layer or none.

Really? If the old base under the dump is only a foot or two with a minefield of rocks showing, wouldn't you be more inclined to let it settle first than bomb it right away and have it run to the ground?
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Re: Skiing and "Conditional Probability"

Postby Admin » Tue May 04, 2010 5:30 pm

Tony Crocker wrote:
admin wrote:A decision on whether to bomb it or let it settle doesn't vary by early or late season, facet layer or none.

Really? If the old base under the dump is only a foot or two, wouldn't you be more inclined to let it settle than bomb it and have it run to the ground?


See above:

Admin wrote:It is an odd mix of what's in the weather forecast, whether or not the whole thing's likely to come down, pent-up demand, etc.
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Re: Skiing and "Conditional Probability"

Postby Staley » Tue May 04, 2010 5:33 pm

Staley wrote:I can rest assured that I have given my family the best probability of a fabulous ski trip.

Tony Crocker wrote:Only a little bit. I think mainly you have given your family the least probability of a crappy trip. Targhee may not have been that great, but there's a long list of places that were much worse last Christmas.

I suppose this is true..it's just not how I like to think about it :wink:

Staley wrote:....book vacations in the summer (as is often necessary for popular school breaks)

Tony Crocker wrote:Not necessary for spring breaks IMHO, which vary a lot by state and often even by school system within a state. I took something like 15 late March/early April spring break vacations while raising kids and booked most of them in January or February. The spread out spring breaks also prevent most of the obnoxious price gouging that occurs for Christmas week.


This is more a function of my family. My parents have always liked to book any sort of trip far in advance. Airfare is usually a little bit cheaper, and for snow-sure resorts, I don't think booking in August is that much of a disadvantage compared to January for a late March trip. Of course, being able to book a trip 1-2 weeks out makes a huge difference. Looking back, I don't think booking a week or two out for a spring break trip is that unreasonable. As long as you aren't too picky about a certain resort or having perfect accommodations, I'm sure it's possible and would most likely lead to a higher probability of powder. Then again, powder didn't really become a priority to me until 3-4 years ago. Before then, I simply wasn't a good enough skier and skinny little rental sticks don't make powder so much fun.
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Re: Skiing and "Conditional Probability"

Postby Marc_C » Tue May 04, 2010 10:39 pm

Tony Crocker wrote:
admin wrote:A decision on whether to bomb it or let it settle doesn't vary by early or late season, facet layer or none.

Really? If the old base under the dump is only a foot or two with a minefield of rocks showing, wouldn't you be more inclined to let it settle first than bomb it right away and have it run to the ground?

No, once again, you're relying on deeply held convictions based in mythology. You keep seeking absolute rules in practically everything skiing when there aren't any, and often that seems to cause little parts of your brain to explode.
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Re: Skiing and "Conditional Probability"

Postby Tony Crocker » Wed May 05, 2010 11:32 am

Of course there are multiple considerations in snow stability control. ](*,) ](*,) ](*,) I would be surprised if a thin base underneath (which I suspect has a higher conditional probability of being weak) is not one of them.
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Re: Skiing and "Conditional Probability"

Postby Admin » Wed May 05, 2010 11:53 am

Tony Crocker wrote:Of course there are multiple considerations in snow stability control. ](*,) ](*,) ](*,) I would be surprised if a thin base underneath (which I suspect has a higher conditional probability of being weak) is not one of them.


Nobody ever said that a thin base beneath new snow isn't a factor in avalanche control. However, nobody ever said it is, either -- see above:

Admin wrote:It is an odd mix of what's in the weather forecast, whether or not the whole thing's likely to come down, pent-up demand, etc.


The biggest problem in this discussion is that you're now mixing things. Your first comment was that early-season snowpack requires longer to control -- it doesn't. Now you're saying that a thin base has a higher propensity to slide -- it doesn't. A thin snowpack in no way, shape or form has a higher conditional probability of being weak. It's a factor of crystal shape throughout the layers of the snowpack, not depth. If there's a facet layer beneath the new snow, then the probability of a slide ripping to the ground becomes a factor in control decisions. There's no apparent increase in probability that an early-season snowpack will have such a weak layer -- it can come anytime during the season. In fact, given that there are fewer layers in a thin snowpack I'd venture to guess that there's a decreased probability of a weak facet layer in a thin snowpack, not greater.
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