Snowpack Comeback

At the start of December 2019, snow levels in the Cascade Mountains were looking pretty bleak. Then came January, and with it a veritable dumping of snow on many of our region’s mountains. What accounts for this remarkable comeback? We’ll disentangle the details in this post.

Let’s start with this winter’s bleak, low-snow beginnings.

At the start of December, accumulated snow, or snowpack, in the lowlands of Washington, the southern Cascades, and nearly all of Oregon showed less than 10 millimeters (less-than half an inch) of snow water equivalent.

Snow water equivalent (SWE) is a measure of the amount of water you would get if you melted a given amount of snowpack.

Less-than 10 mm, as you might image, isn’t much. In fact, it’s an abnormally low number for the month of December. We show this in the map below, which was created using the Climate Toolbox, a CIRC-associated project.

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Figure 1: This map displays percentiles of snow water equivalent (or the amount of liquid water you would get if you melted a given amount of snow) for the calendar date December 1, 2019. (Image Credit: The Climate Mapper Tool—The Climate Toolbox.)

The map displays percentiles of SWE for the calendar date December 1st, 2019.

The SWE percentile ranking works by comparing December 1st, 2019 SWE levels to SWE levels for every December 1st that occurred from 1981 to 2010. Once ranked, we get a percentile number that is then color-coded. For example, the color orange—which can be seen very clearly in Idaho—denotes SWE between the 5th and 10th percentile. Idaho, in other words, had much lower snowpack this December 1st when compared to past years. However, this isn’t as low as the snow got this past December. Note the black areas on the map. These denote where snowpack typically can be found on December 1st but where very little could be found this December 1st. In fact, the blacked out regions denote where snowpack numbers were lower than 10 mm. Snow, in other words, was very likely a no-show at many of our region’s lower elevations.

Now let’s compare our December numbers to a still more recent date to see how our snowpack has changed.

Consider the SWE map for January 27th, 2020, displayed below. This map looks totally different! The black areas are mostly gone. In fact, some regions—including Washington’s northern Cascades and southern Olympics, as well as Oregon’s southern Cascades—have high (greens and blues) snowpack numbers. In plain English, our snow rebounded, and it did so over the course of just about a month.

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Figure 2: Percentiles of snow water equivalent (or the amount of liquid water you would get if you melted a given amount of snow) for the calendar date January 27, 2020. (Image Credit: The Climate Mapper Tool—The Climate Toolbox.)

So, how has this winter’s SWE numbers recovered so quickly?

The answer is storms and more storms.

From the end of December through the beginning of January, the Pacific Northwest was pummeled by a series of strong precipitation events. What we largely experienced as steady rain in our lowlands dumped feet of snow on our mountains. Our region even saw substantial snow along the coast of Washington’s Olympic Peninsula and even some snow in our lowlands, including Seattle. (Seriously, it was something. Check out these amazing photos from the snow along the Strait of Juan de Fuca on the Olympic Peninsula.)

If we examine the 30-day period from December 30th to January 28th, we can clearly see how much precipitation-pummeling occurred across our region. The map below shows us that precipitation over this 30-day period was upwards of 50% above normal.

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Figure 3: Precipitation anomalies for the 30 day period between December 30, 2019 and January 28, 2020. (Image Credit: The Climate Mapper Tool—The Climate Toolbox.)

Much of this precipitation turned to snow due to lower temperatures. If we look at temperature anomalies for late December to late January on the map below, we can clearly see that our region’s mountains were colder than usual over the 30-day period.

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Figure 4: Daily mean temperature anomalies for the 30 day period between December 30, 2019 and January 28, 2020. (Image Credit: The Climate Mapper Tool—The Climate Toolbox.)

Combine higher-than-usual precipitation with colder-than-usual temperatures and you get Prime Snow-Dumping Conditions (PSDC). Okay, that’s not an official term. Just seeing if you’re paying attention.

Given just how abysmal SWE was looking in December, this is the underdog SWE story we had hoped for.

We talk about snow in terms of SWE—as opposed to something a little more relatable like snow depth, which depends on snow density—because water resource managers want to know how much water will ultimately flow into reservoirs when the snowpack melts in the spring and summer. With a healthy snowpack, we are much more likely to be able to fill our reservoirs for the summer months.

However, while much of our region experienced a clear snowpack comeback during the afore-mentioned 30-day pummeling of precipitation, the pummeling hasn’t quite fixed the snowpack problem everywhere.  Snowpack in the interior Rocky Mountains, central Idaho, and western Montana remains low, and low snow in these regions could have a big impact on water reserves for agriculture come summer.

The good news is we’re still in the middle of the snow accumulation season. So, there’s still time left to accumulate that snow. Here the key date is April 1st.

April 1st is the date typically used by water managers as the end of the snowpack accumulation season, meaning after April 1st, snow stops accumulating and starts melting. (Although, of course, the exact date of peak snowpack may occur earlier or later than April 1st depending on multiple factors, including elevation and year-to-year variability.)

April 1st is also generally considered a good indicator of water resources for the summer season. Years with low April 1st snowpack tend to be followed by summers with low flows in rivers and streams. Years with low April 1st snowpack can also see low water levels in area reservoirs come the summer months.

So, what are the SWE forecasts projected for this coming April 1st? Let’s figure this out using the new seasonal forecasts feature built into the Climate Toolbox’s Climate Mapper Tool.

Our forecasts project that come April 1st, 2020,  snowpack is likely to be about normal (30th–70th percentile range) in the northern and southern parts of the Pacific Northwest. This is shown in white on the map below. However, other parts of our region are likely to see some pretty low SWE numbers, hovering around the 10th–30th percentile. These low SWE areas include those same regions in central Idaho and the eastern Cascades that currently show low snow levels.

We’ll keep you posted as we get closer to April 1st. In the meantime, here’s hoping the precipitation continues (however much it might make this researcher’s bicycle commute to the University of Washington a grumpy, soggy slog).

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Figure 5: Projected snow water equivalent for April 1, 2020. Forecast made on January 8, 2020. (Image Credit: The Climate Mapper Tool—The Climate Toolbox.)

ochyrdobw.pngCIRC team member Oriana Chegwidden is a research scientist and PhD student in Civil and Environmental Engineering at the University of Washington. Oriana is investigating the future hydrology of the Pacific Northwest. Oriana’s current work for CIRC includes running a hydrologic model for the Climate Toolbox.

 


About Northwest Climate Currents:

This post is part of an ongoing series we are calling Northwest Climate Currents. Northwest Climate Currents uses the Climate Toolbox (formerly the Northwest Climate Toolbox) and the data it collects to help us understand and prepare for our region’s climate events.

The Climate Toolbox is a suite of free online applications designed by CIRC researchers and intended to help foresters, farmers, and water managers respond to and prepare for climate variability and change and related impacts.

CIRC team members working on the Climate Toolbox include, John Abatzoglou, Katherine Hegewisch, Oriana Chegwidden, and Bart Nijssen.

Acknowledgements: The Climate Toolbox was funded in part through the NOAA Regional Integrated Sciences and Assessments (RISA) program and National Integrated Drought Information System (NIDIS).


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Featured Image: Percentiles of snow water equivalent (or the amount of liquid water you would get if you melted a given amount of snow) for the calendar date January 27, 2020. (Image Credit: The Climate Mapper Tool—The Climate Toolbox.)


OSU_icon_gears_Black Resource: The Climate Toolbox


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