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Playing the percentages NOT for snow

Colorado 14er peak questions and conditions should be posted here. 14er Trip Reports
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Playing the percentages NOT for snow

Postby Dex » Thu Mar 21, 2013 6:05 am

Assuming you don't want to hike a 14er in snow; above what percentage for snow don't you go?

For Yale today - snow 80%, tomorrow chance of snow 40%
http://14ers.com/php14ers/weather.php?ltype=1#

The hourly max is 40-50% for Buena Vista,
http://www.weather.com/weather/hourbyhour/graph/Buena+Vista+CO+USCO0046:1:US?pagenum=2&nextbeginIndex=6

I got up early today to go for Yale - looked outside, cloudy and couldn't see the sky, checked the weather sites and went back to bed.
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Re: Playing the percentages NOT for snow

Postby susanjoypaul » Thu Mar 21, 2013 6:25 am

I generally expect it to snow if the forecast is for 40%. That said, it all depends on where you get your forecast. Nate Silver's book The Signal and the Noise is fascinating in its explanation of how different weather stations/channels capture data for - and then report - their weather forecasts.

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Re: Playing the percentages NOT for snow

Postby TomPierce » Thu Mar 21, 2013 7:22 am

I use the NOAA (online but also linked to this site) and it has been pretty accurate. Besides the number (and I'll usually reconsider like Susan in the 40%-ish range, depending on the climb/hike) I'd also watch the trend. If I plan to climb Saturday, and it's at 20% the Weds before, then 30% the next day, 40% the next...you get the idea. But if it also says it'll be clearing by 11am, or the amount of snow is minimal, I might still go for it.
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Re: Playing the percentages NOT for snow

Postby Jim Davies » Thu Mar 21, 2013 7:58 am

Isn't it interesting how the weather always changes at 11 am (or noon DST)? :wink:

For skiing purposes, I find the first digit of the percentage gives the approximate number of inches a resort will report, so 10% means 1 inch, 50% means 5 inches, etc. I'm sure that's not the way NOAA figures it, but it seems to work out that way within a factor of two or so on a lot of days.
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Re: Playing the percentages NOT for snow

Postby Dex » Thu Mar 21, 2013 9:09 am

I'm glad I stayed in bed - the mts are not visible from BV and flurries are in the air.
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Re: Playing the percentages NOT for snow

Postby dsunwall » Thu Mar 21, 2013 2:24 pm

I don't believe the percentages are intended to indicate how much snow, just the chance it will snow.

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Re: Playing the percentages NOT for snow

Postby Jim Davies » Thu Mar 21, 2013 5:36 pm

True, but I'm just observing the way it seems to work out in practice. 40% is a near certainty of measurable snow in my experience, which doesn't make sense but there it is.
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Re: Playing the percentages NOT for snow

Postby Dancesatmoonrise » Thu Mar 21, 2013 6:51 pm

Jim Davies wrote:True, but I'm just observing the way it seems to work out in practice. 40% is a near certainty of measurable snow in my experience, which doesn't make sense but there it is.


I think it has to do with the strength of the disturbance. In other words, related to the amount of moisture available. It seems to work that way pictographically off the GFS precip maps, anyway. For example, when there's a few isolated areas over or near Colorado on the GFS for the period in question, the NWS will indicated something like 10-30%. If there's a strong, widespread disturbance headed our way, pops go up (prob of precip) along with the amount of moisture anticipated. Using the pictographic models can thus be helpful to sort out when and when not to go. I'm probably making this more complicated than it is. It's a visual thing off the GFS precip models.

To answer the OP question:
1. It differs between winter and summer.
2. In winter, for high alpine travel (and ridges,) the 500 millibar wind plots are often far more important than the precip forecast. Think of it like this: a few inches of gentle snowfall on a winter day can be beautiful; 80 mph gusts on a clear day can be challenging.
3. For winter, note that NOAA does not often put in winds until 24-48 hours ahead, even though they may have been in the GFS for a week.
4. In summer, here's a method that seems to work, regarding the NWS pops: 0-10% = you're good to go, just keep an eye to the sky. 20% pops = t-storms possible, but less likely than 50%. 30% pops = t-storms 50% likely. 40% pops = greater than 50% chance of serious t-storms. 60-100% = most often totally socked in, where violent t-storms often seem not to happen.

Winter also seems to have more "inertia" to it than summer. Like the concept Tom alluded to, beware of a beautiful winter's day forecast sandwiched in between two lousy periods. The window will often evaporate before the day arrives and it will be crappy all the way through.

With summer, there are very often from two to six periods of high pressure dominance over the great basin every summer, giving all but relative impunity for high alpine travel. The trick is to spot these coming and coordinate serious alpine projects with such pleasant weather events.

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Re: Playing the percentages NOT for snow

Postby TravelingMatt » Thu Mar 21, 2013 7:22 pm

Jim Davies wrote:True, but I'm just observing the way it seems to work out in practice. 40% is a near certainty of measurable snow in my experience, which doesn't make sense but there it is.


I agree with this. Maybe this is just living in Summit County. It works out something like:

10-20% Expect a dusting
30-40% Expect an inch or two
50-60% Expect 2-3 inches
70%+ A few inches to a dump

We were listed at 20% snow today, but we're at five inches and counting.
So pleas'd at first the towering Alps we try,
Mount o'er the vales, and seem to tread the sky,
Th' increasing prospects tire our wand'ring eyes,
Hills peep o'er hills, and Alps on Alps arise!
-- Alexander Pope

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