As we’ve covered in previous posts, the peak snowpack in Colorado’s mountains generally looked pretty decent this year, with the amount of water stored in the snow peaking pretty close to the long-term average in most areas. However, in the southern mountains, it’s been another year where the melt has happened a lot faster than it typically has in the past.

As of early April (left image above), all basins in Colorado had above average snow water equivalent, as measured by the SNOTEL network. But a month later (right image), the picture is quite different. The northern basins still look good, with a string of April snowstorms adding to the snowpack there. But southern Colorado largely missed those storms, and warm, sunny conditions, assisted by layers of dust on snow, really accelerated the melt. Cooler conditions this week will slow down the melt a bit, and a storm this weekend will add some much-needed moisture. But once the snow itself gets warmer than 32°F, it’s hard to slow the melt too much. The Rio Grande basin now only has half of the snowpack it typically does on May 6.

The time series graph for the combined San Miguel, Dolores, Animas, and San Juan river basins in the southwest corner of Colorado illustrates this nicely:

The trace for 2024 reached essentially an average peak, and right on time: the peak was 18.1″ of SWE on April 2, compared to an average peak of 18.6″ on April 1. It also stayed near that peak for about another 10 days, but then the melting progressed extremely quickly. In fact, it was the largest 14-day loss of SWE before the end of April in this basin since the start of SNOTEL data in the 1980s.

Before going into those numbers, a quick note on snowpack melt rates. In absolute terms, the fastest melts come in years when there are big snowpacks that linger late into May or early June, like 2019. Eventually that snow can’t stand up to the summer sun, and SWE goes away at a very fast rate. But in years like 2024, what we’re interested in is the snow melting quickly, and early.

So here, we’ll look at the largest two-week declines in SWE prior to the end of April, and we see that the combined southwest river basins lost over 8″ of snowpack from April 12-26 this year. That much melt so early hasn’t been observed before. The Upper Rio Grande and Arkansas basins also saw their largest 14-day SWE declines prior to April 30.

If you look on the bright side, you can’t get rapid melts like this without a good snowpack to begin with. At least, unlike some really bad drought years, the water was there in the first place! But early melts have big implications for the timing of water availability. It means higher-than-normal streamflows in May, but then much lower streamflows later during the heat of summer, when the water is really needed, especially by those who don’t have access to water stored in reservoirs. And the overall water availability situation for this spring and summer isn’t looking great in southern Colorado, with the latest CBRFC forecast projecting only 90% of average flow into Blue Mesa Reservoir, 74% of average on the Animas, and 80% of average into Lake Powell.

And unfortunately, years like this have been getting more common, and that trend is expected to continue as the climate warms. These changes are addressed in detail in the water chapter of Climate Change in Colorado, so dive in to that for more details. But in general, the changes observed up to this point have been toward modest declines in peak snowpack, but robust trends toward earlier melting, and these changes have been most acute in southern Colorado. For the future, there is still considerable uncertainty about what will happen to winter precipitation: some climate projections show more winter snow, others less. But every one of them shows a shift toward earlier snowmelt, and earlier peak streamflow on the Colorado River, meaning changes to when and where our water supply is available. In other words, we might need to get used to the snowpack looking pretty good in the southern mountains in March, but being disappointed in the numbers when May comes around.