#Drought news (March 4, 2021): Historic drought deepens in the West as window for rain, snow closes — The Washington Post #runoff

From The Washington Post (Becky Bollinger and Andrew Freedman):

Water supply and wildfire concerns grow for the dry season

According to the U.S. Drought Monitor, which has published weekly maps since 2000, the 2020 drought is the worst, in terms of its geographical scope, in more than 20 years.

Almost 80 percent of the Western U.S. is in drought, with nearly 42 percent of the region in “extreme” or “exceptional” drought.

Much of the region experienced developing drought in the summer, following a warm and dry spring. Since then, conditions have deteriorated, and the precipitation deficits continue to build. At its maximum extent in January 2021, 47 percent of the West was in extreme drought or worse. Nearly a quarter of the area was in the worst drought category, an event with a probability frequency of once every 50 to 100 years.

February did bring an active weather pattern with it. The Pacific Northwest received more than 10 inches of precipitation last month. Much of the interior Rockies through Idaho, Montana, Wyoming, Utah, and Colorado received between 1 and 5 inches of moisture for the month. The Sierra Nevada in California received between 2 and 6 inches, much of that in the form of snow.

West Drought Monitor March 2, 2021.

However, despite the precipitation, some areas are still struggling. Blue outlines in the map below show where snowpack increased last month. The Southwest was much drier in February.

Where the purple outlines overlap on this map, these areas are above average for snowpack now. Outside of the purple outlines, snowpack is still largely below average. Areas outlined in orange experienced a decline in the percentage of average snowpack since the beginning of February.

February precipitation in the West. (National Weather Service) via The Washington Post

And red outlines show the areas where snowpack is extremely low compared to normal. The evidence is clear — February was beneficial for many, but it was not a drought buster, and drought continues to maintain its stranglehold on the West…

Across the interior Rockies, snowpack usually reaches its peak in late March/early April and begins its slow melt — adding water to the rivers and eventually filling the reservoirs. While 2020 snowpack peaked around the time we’d expect, it melted out too fast, thanks to anomalously warm temperatures and no new snowstorms.

Does how it melts make a difference? You bet! Check out this water supply forecast for Lake Powell from the Colorado Basin River Forecast Center.

Water supply forecast for Lake Powell. (Colorado Basin River Forecast Center) via The Washington Post

Forecasts in the blue shading started out a bit below average (the green line is the average supply into Lake Powell). With each passing month, that forecast got a bit lower. And what actually happened was at the very low end of what was forecast (the orange line is the observed supply into Lake Powell for 2020). The actual inflow into Lake Powell was 3.4 million acre feet below average…

Fast forward to a hot and dry summer. With the exception of a couple of isolated locations in the Pacific Northwest, Idaho, and Montana, most of the West experienced much above average temperatures and below average to record low precipitation for June-September last year.

In the Southwest, July-September typically ranks as the wettest time of year, which is largely a result of the North American Monsoon. Monsoon moisture in the late summer is key for replenishing soil moisture. Without an active monsoon, soils dry out just before the beginning of snow season. And unfortunately, that happened in the fall of 2020.

Soil moisture as of Sept. 2020. (NASA) via The Washington Post

Modeled soil moisture at the end of September shows the extremely dry soils in the West. As we entered the cold season, this soil moisture was “locked in.”

The high elevation ground freezes, and that is the state the soil moisture will be at when the thaw begins in the spring. Start the season with dry soils, and that is the first “bucket” that needs to be filled when the snow starts melting.

The Colorado Basin River Forecast Center estimates that much of the Colorado River Basin needs 10 inches or more of precipitation for soil saturation. Averaged over watershed basins, normal snowpack peaks around at around 20-25 inches.

But to get the snowpack needed and cover the soil moisture deficits, these basins would potentially need 120-150 percent of average snowfall for the season. Can we expect that much snowpack this season? Unfortunately, no.

Climate change is playing a significant role in influencing water supplies in the West, with early spring snowmelt, hotter and drier summers and warming winters all acting to exacerbate drought conditions…

This map shows all the stations in the west that measure snowpack. As of Feb. 28, stations colored orange and red have below average snowpack for this time of year. The Sierra Nevadas in California are well below average for snowpack. With a typical peak date of April 1, there is only one month left to add to that snow cover. And considering they receive almost no moisture during the warm season, this month is extra critical.

Snow water equivalents compared to average. (USDA)

For the mountain ranges throughout Utah, Wyoming, and Colorado, snowpack is also below average. For the Colorado Headwaters region, there are roughly 45 days until normal peak snowpack, but the likelihood of a normal snowpack is decreasing by the day…

Becky Bolinger is the assistant state climatologist for Colorado and a research scientist at Colorado State University.

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