(Last Updated On: August 9, 2010)
Anyone trying to play in Colorado’s mountains in the last couple of weeks has probably been a bit frustrated by the weather. A shift in the flow to the south and southwest, carrying copious amounts of tropical moisture, has created a classic wet monsoonal weather pattern. In some cases, these heavy rains have even caused flooding and landslides, as seen here on the Durango-Silverton train route:
A recent internet discussion finally confirmed a few things for me which I had long wondered about. NWS weather forecasts mention a probability % of rain or snow on most days, but what does that mean? As it turns out, a 70% chance of rain means that if you had 10 nearby weather stations, 7 of them would record measurable precipitation, and 3 would not. It does not mean that there is a 70% chance of rain, and a 30% chance of no rain. Here is what Stan of the Pueblo NWS office had to say: Part of the confusion is due to (IMO) an unfortunate policy of the NWS to mix Probability with Spatial characteristics in defining what constitutes a POP. For example, usually in the first 12-24 hours, for thunderstorms, the 3% or 60% refers to a spatial distribution So, if there is a 30% chance of rain for the Sangres for example, that means that 30% of that total area will see measurable rain, the rest will stay dry. (In contrast, a probability forecast of 30% would suggest that 3 out of 10 times its going to rain over the whole area. Confusing, if you ask me. Regardless, either way there is a 30% chance if you are in that area of seeing rain. What bugs me is when I see statements to the effect of “Well, the forecast called for 30% chance of rain, but it poured all night, so the forecast was wrong” THAT is wrong. First off, that person is not looking at a map of rain gauges, or a radar image of accumulated precipitation, so he/she has no clue what percentage of the area got rainfall. Rainfall in rugged terrain is extremely variable, it can be pouring in one location but totally dry a mile away. Of course, the complainer never recalls the 7 out of 10 times that it was dry when the forecast was for 30%–he/she only recalls the time it poured. If, like me, you were forced to actually look at the hard data for verification, you would see that the forecast, over time, is statistically quite good, better than you might expect. In short, it’s not something to ignore.
Another interesting tidbit from Stan: As an experienced climber (not much of late due to a bad back) I am fully aware of how radically different weather conditions can be in the alpine zone. Weather forecasting is largely a matter of applying personal experience to scientific data, so someone with the personal experience of relying on forecasts for ventures above 12000 feet is going to have a better understanding of the rules that apply there. Since we (NWS) are a team of forecasters (10+ per office) and offices (4 in Colorado, numerous surrounding) the forecast is a collaborative effort and sometimes it goes the way of the “lowest common denominator”. In other words, if from ones personal experience one knows that the chance of it raining in the mountains is 80+ percent, unfortunately if the other forecasters don’t have that experience and think it will be lower, to maintain a consistent product sometimes the compromise results in a ‘watered’ down forecast (pun intended). That is probably why some of you in the thread i referenced note that POP (Probability Of Precipitation) forecasts are sometimes underdone for the summer monsoon season–the same is true during orographic (terrain-driven) snow events in the mountains. I can’t tell you how many times Ive been up at 10K+ feet in the Winter, and it’s dumping like mad, chains required, when the official (sometimes mine!) forecast calls for 20 percent chance of snow or some similar smallish number.
This quote confirmed some of my other observations as a keen weather observer (at least in the winter, when my mind constantly thinks SNOW!). It can be quite informative to look at two nearby forecasts, each forecast coming from a different office (i.e. Grand Junction, Pueblo, Boulder, as Colorado examples) Even if they are just a mile apart and similar in elevation and aspect, the forecast are often quite dissimilar. Sounds to me that different forecasts can easily be chalked up to differences of opinion in the forecast offices.
Hopefully that clears a few things up for people, as there is certainly a bit of confusion on the subject. Weather forecasts for the Crested Butte area can always be found on the right sidebar here on 14erskiers. The weather for the next week looks good, by the way, so get out there!
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