#Snowpack news: #ElNiño fails to deliver good snow in S. #Colorado

Click on a thumbnail graphic to view a gallery of snowpack data from the Natural Resources Conservation Service.

From the Fort Collins Coloradoan (Jacy Marmaduke):

Fort Collins received a whopping 67 inches of snow between October and March, more than 20 inches above the seasonal average and the third-most snow the city has seen during that period in a quarter century.

Yet snowpack in the South Platte River Basin — the mountainous area that feeds Northern Colorado’s rivers, reservoirs and farmland — was mostly unfazed by the storms that have secured the 2015-2016 season places in the Fort Collins record books.

As of April 2, South Platte River Basin snowpack was 107 percent of average. The Poudre River Basin snowpack sat at 106 percent of average and the Upper Colorado River Basin snowpack was 104 percent. Not too shabby, but those aren’t numbers we’ll tell our grandkids.

What gives?

The answer starts with an “El” and ends with a “Nino.”

More specifically, an El Nino strong enough to land the nickname “Bruce Lee” among forecasters didn’t impact Colorado quite as much as some people predicted it would. More on that later.

Fort Collins receives on average 47 inches of snow annually, so a few big snows can have a huge impact. Here’s proof: Between October and March, Fort Collins received nine storms that yielded more than 2 inches. The three biggest storms — 13.7 inches on Feb. 1-2, 13.4 inches on March 23, and 7.4 inches on Dec. 15 — accounted for more than half of the total snowfall.

Same goes for precipitation from snow and rain. Fort Collins received 8.18 inches between October and March, close to double the average of 4.61 inches during the last 25 years. A few especially wet snows made all the difference.

Experts measure snowpack using a metric called “snow water equivalent.” For example, Rocky Mountain National Park’s Bear Lake on April 1 bore a 53-inch blanket of snow that contained 17.5 inches of water.

So it’s not just mountain snowpack but the moisture in the snow.

Enter El Nino — or so forecasters thought. The irregular warming of the equatorial Pacific Ocean was supposed to bring storms into the southern and southwestern U.S., making for cooler, wetter winters in the South and warmer, drier winters in the North.

Instead, explained National Weather Service meteorologist and resident El Nino aficionado Mike Baker, the Pacific jet stream that carries storms into the U.S. from the west set up further north than expected. That resulted in the Pacific Northwest getting straight-up clobbered with rain and snow.

“They were getting pelted with flooding rains, gale force winds,” Baker said. “That’s not what you usually see with a strong El Nino.”

At times, the jet stream wiggled southward, and the Pacific Northwest’s problem shifted to the Southeast.

But alas, the jet stream never really got comfortable in the south and southwest, where it would have had a greater impact on Colorado’s storms…

As for why that happened, climatologists have a few theories.

Perhaps chief among them is a phenomenon called the Pacific Decadal Oscillation, which causes warming of the Pacific Ocean farther north of the equator, all the way up to Alaska. It only happens every 20 years or so.

Baker theorized in a September Coloradoan article that the combination of the PCD and the historically strong El Nino could produce unstable winter weather. That’s because the jet stream that brings storms settles on a temperature gradient, where the Pacific shifts from warmer to colder.

“It likes to latch onto the warm water,” Baker said. “Well, we had warm water everywhere. Maybe it was confused.”

[…]

Baker emphasized the jury’s still out on what exactly happened this winter. But for mountain snowpack, the story is far from over.

Snowpack generally peaks around the end of April, said Sarah Smith, water resources engineer with Northern Water.

Northern Water, which supplies water to Fort Collins and other parts of northeastern Colorado, keeps a close eye on snowpack because it impacts their water users. Local snow is great for soil moisture in municipalities and farmland, but mountain snowpack plays a bigger role in reservoir and river levels, Smith said. Horsetooth Reservoir, a primary water source for Fort Collins fed by the Colorado River, held 116 percent of average levels on April 1.

Although local officials say they’ll focus on water conservation no matter what, Smith noted spring rains and wet snows in particular can boost runoff and curb demand for supplemental water from reservoirs. So snowfall and rain in April and May — Fort Collins sees an average of 6.2 inches of snow in April and 0.7 inches in May — will be ones to watch.

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