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.
