Early January 2018 #ColoradoRiver streamflow forecast is sobering #COriver

Westwide SNOTEL basin-filled map January 4, 2018 via the NRCS.

From The Las Vegas Review Journal (Henry Brean):

The National Weather Service’s Colorado Basin River Forecast Center predicts the river will flow at about 54 percent of its average volume during the key runoff period from April to July…

There’s still plenty of time for conditions to improve. The river basin tends to accumulate much of its snowpack in January, February and March…

Lake Mead ended 2017 almost 2 feet higher than a year ago, as use of Colorado River water by Nevada, Arizona and California hit its lowest level since 1992.

According to preliminary accounting figures from the U.S. Bureau of Reclamation, the three states consumed a combined 6.7 million acre-feet from the river last year, driven by wet conditions in California and widening efforts to curb use in Arizona…

That left enough water in Lake Mead to keep it more than 7 feet above the trigger point for a federal shortage declaration, which would mean mandatory cuts for river users in Nevada and Arizona.

Colorado River author and expert John Fleck said the reduction in consumption is impressive considering the population in the areas served by the river has grown by about 7 million people since 1992.

“It’s a sign that we are succeeding in using less water in the Lower Colorado Basin,” said Fleck, director of the Water Resources Program at the University of New Mexico in Albuquerque. “It’s critical that we’re learning to do this, because this isn’t enough. … We’re going to have to do it more.”

[…]

California was able to cut its river use by more than 440,000 acre-feet last year, in large part because of huge snow accumulations in the Sierra Nevada mountains that helped refill the state’s drought-depleted reservoirs and above-average precipitation elsewhere that reduced water demand.

Fleck said Arizona cut its river use by almost 360,000 acre-feet mostly to stave off more substantial, mandatory cuts in the future. “Arizona has been cranking down their use to try to avoid a shortage,” he said.

Nevada used about 239,000 acre-feet of its 300,000 acre-foot allocation in 2017, an increase of roughly 2 percent over the previous year.

Bronson Mack, spokesman for the Southern Nevada Water Authority, water use in the valley went up slightly last year because of increased economic activity and the addition of more water customers. Hotter, drier weather also may have played a part, he said…

Federal projections released last month called for Lake Mead to finish 2018 roughly 4 feet lower than it is now but still safely out of shortage territory. In light of Wednesday’s ugly — but early — river forecast, the projections for the lake are almost certain to get worse.

From Slate (Geoff Fox):

What is it about snow that makes it so tough to pin down?

Though temperatures at ground level are important, the critical numbers for assessing snowfall are much higher up in the atmosphere. We’re looking for ice crystal growth, which happens when the air is wet enough and cold enough—sometimes down to -20° Fahrenheit, though the biggest snow growth happens at somewhat warmer temperatures.

The ice crystals start small, but as they collide they grow, until finally they’re large enough and heavy enough to fall to Earth. Snow is water plus air—air being very important. It’s the fluff factor, the reason an inch of water can be 5 inches of snow or 30 inches or something in between. The snow liquid ratio, or SLR, is different for every storm (high SLRs are good for skiing, bad for snowballs). And that’s what we’re trying to predict—how much liquid is going to produce how much snow.

Most snowstorms are driven by low pressure systems hundreds of miles across. Around the low, warm air rises and cools. That causes water vapor in the air to condense and form clouds. Liquid droplets come next until gravity and temperature begin to dominate. For those who live in snow belts there’s a second method to produce snow, the lake effect. Assessing these two methods of snow production should allow you to get a good idea of how much snow to expect, but often your final estimate is really the combination of two estimates.

The process is very exacting, intricate even. When temperatures are cold enough and the wind properly aligned through the atmosphere, lake effect snow produces narrow bands of intense snow that are extremely hard to predict. For example, I drove from Buffalo to Erie, PA one winter’s day. Downtown Buffalo had flurries, but as I headed into the “Southtowns,” conditions became dicey. The snow rate was a few inches an hour. And then, a few miles later along Lake Erie’s shore, the snow stopped, clouds parted, and the sun came out. My trip back saw the exact same conditions in the exact same places. Nothing had moved.

Marquette, Michigan is a good example of how this makes forecasting more difficult. Not only does Marquette get your run-of-the-mill winter storms, they also get lake effect snow. Lake effect there has an SLR in the 30 to 40:1 range, meaning that one inch of liquid equals 30-40” of snow. The larger storms that pass through are 10 to 15:1. Figuring out how this hybrid storm is going to combine includes a lot of room for error. Luckily, Marquette averages around 17 feet of snow per year—lots of time to practice.

So we forecast the amount of water, then how that water will act as it drops. Most of the time the atmosphere warms as the flakes fall…but not always. What starts in the clouds as snow can fall as sleet, rain, freezing rain or even grauple (snow flakes pocked with rime ice). The form it falls in obviously changes how much snow ends up on the ground.

When and how you measure snow affects the final total, too. Officially it’s measured off the ground on a ‘snow board,’ usually a large piece of plywood. Snowflakes fill gaps in the snow pile as they fall. Measuring every hour, without giving the snow time to settle will give a higher amount than measuring every six.

Over the years forecasts have improved. There are fewer busts. One reason we’ve gotten better is through improved computer modeling: We can now look at the atmosphere a little more finely. The grid points and time steps are closer together. The mathematical integration of physics is better honed.

From KOAA.com (Bill Folsom):

There are multiple important mountain locations contributing to southern Colorado’s water supply. Right now snowpack on Pikes Peak is just less than 30% of what’s considered normal for this time of year. Sources closer to the continental divide are better, but also very low. The Arkansas river basin at about 50% and the Upper Colorado River basin just above 65%.

The numbers are benchmarks at about half way into the snowpack season. It is typical to get heavier water dense snowstorms in January, February and March, but some long term forecasters are seeing some indicators of a storm track with lower snow totals.

A major buffer preventing immediate concern is the current level of our reservoirs. A couple of very wet years before now, have filled reservoirs. For Colorado Springs, storage is enough for three years.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s