The monsoon...

[vandy]

Don't trust weather reports.

There's no reason not to trust them, as long as you understand what they are saying.

I understand exactly what they are saying.

Fair enough. Your original post seemed to be implying that because it didn't much on your trip, that forecasts weren't worth people's time, which I definitely disagree with. I think a better way to put it would be to say "don't let a forecast keep you from going." I think the NWS forecasts have two useful applications for hiking. If the probability of precip is say 20%, then I may decide to leave an hour or two later than if it's 50%. Also, if my plans are flexible, I may choose a summit in the Sangres instead of the Sawatch if the chance of rain is much lower to the south.

But yeah, I wouldn't completely bail on a day trip unless the forecast was completely ugly. Also, as someone who likes to take photos, inclement weather is sometimes a reason to go, as long as I'm set on not necessarily making a summit.

[jdorje]

A 50% chance of rain does not mean that for your mountain there are 50/50 odds of rain. It means that for the specified forecast area, there is a 50% chance rain will occur somewhere in that area.

I don't think that's true. Per your link:

PoP = C x A where "C" = the confidence that precipitation will occur somewhere in the forecast area, and where "A" = the percent of the area that will receive measureable precipitation, if it occurs at all.

So if there's a 50% chance of rain somewhere in the area, and if it rains it will cover 50% of the area, that'd come out to 25%. (The same logic has been explained on this forecast by people who are in the forecasting field.) But it's not really especially useful since if it rains over 50% of the area, that's going to be the highest 50%. While if there is a 50% chance of rain at all, that's the number you are interested in.

Also, at least with the NWS forecasts, they will typically give you a time frame. Forecasts for morning thunderstorms over the mountains are rare.

Well, it's better than just a timeframe: look on the lower right of the page and there's a graph showing the hour-by-hour chance. However, the math on this chance doesn't really work out; if there's a 50% chance of storms for a day then it's usually about a 6 hour period where the hourly chance is 50%. But when the storm happens, it's just an hour long. So if it were actually a 50% chance of rain each hour there would be almost a 100% chance of rain for the day. My point is simply that this hourly graph isn't actually a chance per hour, but is a chance per day that the rain will happen in that hour. It's a pretty useful graph, but the most useful part during the monsoon is, as Bill said, just looking to see when the "afternoon spike" happens to know if you need to be off the peaks by 3pm, 1pm, 11am, or 7am.

One other interesting thing is we usually think of rain coming in the mountains in thunderstorms, but obviously this isn't always the case. Morning rain very often is just rain. Even in the afternoon, the last couple of weeks have often been rain with no thunder. Depending on your route, you might not care about rain or about cloud cover. The hourly graphs also include numbers for cloud cover and for thunder chance.

[vandy]

So if there's a 50% chance of rain somewhere in the area, and if it rains it will cover 50% of the area, that'd come out to 25%. (The same logic has been explained on this forecast by people who are in the forecasting field.) But it's not really especially useful since if it rains over 50% of the area, that's going to be the highest 50%. While if there is a 50% chance of rain at all, that's the number you are interested in.

Well, yes, that's the logic that goes into formulating the number. But the simplest interpretation is the one I gave (and the one NWS tells you to apply). From that same page: "In either event, the correct way to interpret the forecast is: there is a 40 percent chance that rain will occur at any given point in the area."

Well, it's better than just a timeframe: look on the lower right of the page and there's a graph showing the hour-by-hour chance [...] One other interesting thing is we usually think of rain coming in the mountains in thunderstorms, but obviously this isn't always the case. Morning rain very often is just rain. Even in the afternoon, the last couple of weeks have often been rain with no thunder. Depending on your route, you might not care about rain or about cloud cover. The hourly graphs also include numbers for cloud cover and for thunder chance.

I find the hourly weather graph to be a very useful tool over the plains, but somewhat less useful in the mountains. However, it will certainly give you an idea as to when moisture is moving in or convection is expected to develop. And yeah, morning rain is often not convective. You just don't typically have the vigorous updrafts necessary to build clouds for lightning at that point in the day.

Obviously the best thing to do is to gain experience paying attention to the weather during hikes. Then you build an awareness as to when a sky looks threatening and when it doesn't. I have a meteorology degree, but experience almost always trumps theoretical understanding.

[PaliKona]

Does anyone remember a late August as wet as this?

[thebeave7]

Yeah, last year. I got hammered by snow storms several times late last August.

Eric

[Scott P]

Does anyone remember a late August as wet as this?

Looking at the actual weather data for various stations, it appears that at least at least in some stations in the Colorado mountains this is the wettest August since 2006. Monsoon moisture is so hit and miss though that it is extremely hard to generalize entire areas. Some mountain areas in Colorado actually have well below average precip for August so far.

[San Juan Ron]

Does anyone remember a late August as wet as this?

In the San Juan's. the monsoon always runs into September and last year was much worse than this year (as an example). SJ Ron

[Rarefied]

Monsoon moisture is so hit and miss though that it is extremely hard to generalize entire areas. Some mountain areas in Colorado actually have well below average precip for August so far.

^^This. Per the records I’ve kept for many years, this year's local monsoon moisture is running 33% below the long term average and 60% below the record year of 2006. Meanwhile, I would guess moisture is substantially above average this season for, say, east of Denver. Like real estate, location x3.

R

[PaliKona]

Is this monsoon showing signs of drying out for Sept , per the norm, in the Front Range and Summit/Eagle? Cannot believe how wet it's been.

[onebyone]

Maybe we're just returning to a more normal trend. Except for the last couple of years, we had quite a few dry years for awhile. I kind of remember that back in the 1990s, it would cloud up and threaten to rain practically every afternoon. Then we hit the drought which I think changed our perception of what is normal.

[DArcyS]

I'm not so certain this last round of precipitation was caused from the monsoon. The monsoon involves moisture coming from the south, which implies warm air. As evidenced by snow on the highest peaks, I think part of last week's precipitation was caused by a cold northern air mass and something different than the monsoon.

I also wondered if the jet stream for last week's precipitation was similar to the jet stream pattern that hit the state last year in mid-September. Obviously the results were quite different in terms of the amount of precipitation, but I remember reading an article discussing how the jet stream pattern that created that week-long event of flooding was becoming more frequent.

I'd be interested to hear from anybody who's an expert in meteorology.

[Rarefied]

I'm not so certain this last round of precipitation was caused from the monsoon. The monsoon involves moisture coming from the south, which implies warm air. As evidenced by snow on the highest peaks, I think part of last week's precipitation was caused by a cold northern air mass and something different than the monsoon.

Nice job picking up on the source not being part of the monsoon pattern. Veteran KCNC weatherman, Ed Greene, explained that the latest heavy rains came from the jet stream pulling in remnants of tropical depression/hurricane Marie in the eastern Pacific.

R