#Water year 2021 snow #drought conditions summary and impacts in the West — National Integrated Drought Information System #snowpack #runoff

From NIDIS:

DEWS Regions: California Nevada, Missouri River Basin, Pacific Northwest, Intermountain West

States: Alaska, Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, Oregon, Washington, Wyoming

Update Status: NIDIS and its partners release these updates every 4 weeks December through June. This update will be the last for the 2021 Water Year as snowpack and snowmelt are past peak values. Updates will resume in the 2022 Water Year.

Snow drought impacts have intensified as snow melted weeks early this spring.

Key Points

  • Remaining higher-elevation western snowpack is well below normal with the exception of Washington, parts of Montana, and the South Platte Basin that drains the Front Range of Colorado.
  • The 2020–2021 snow drought was initially caused primarily by a lack of precipitation and storminess, and intensified after April 1. A huge, and in some cases record breaking, decline in snow water equivalent percent of normal was observed throughout April due to warm and dry conditions.
  • Much of the western snow melted one to four weeks early, including three to four weeks early in the Sierra.
  • Low snowpack, rapid melt out, and poor runoff efficiency have led to significant water supply concerns going into summer of 2021. Click here to learn more about the impacts of snow drought.

Water Year 2021 Conditions Across the West

Current SWE: Western U.S. Via the NRCS. This USDA Natural Resources Conservation Service (NRCS) map shows Snow Telemetry (SNOTEL) snow water equivalent (SWE) basin values over the western U.S. as a percent of the NRCS 1981–2010 median. Only stations with at least 20 years of data are included in the station averages. The SWE percent of normal represents the current SWE found at selected SNOTEL sites in or near the basin compared to the average value for those sites on this day. This map is valid as of June 8, 2021. Source(s): USDA NRCS Last Updated – 06/08/21

Water Year 2021 Snow Drought Conditions Summary

Higher-elevation snowpack that remains throughout the West is well below normal with the exception of Washington, parts of Montana, and the South Platte Basin that drains the Front Range of Colorado. In the Sierra Nevada, snowpack is just about gone at SNOTEL elevations. For example, Mount Rose Ski Area, Nevada, on the northeast side of Lake Tahoe in the Carson Range, melted out on May 14, and the median melt out date was June 10. Melt out three to four weeks early was common throughout the Sierra. Throughout Utah, the Upper Colorado, and northern Rockies states of Idaho, Wyoming, and Montana, many sites melted out one to three weeks early.

Alaska mountain snowpack was generally typical for early June in the Chugach and Kenai mountains of south-central Alaska. A cool and wet spring has maintained higher-elevation snowpack in much of southeast Alaska except the far south, where precipitation and temperatures have been closer to normal.

Snow drought conditions developed early in the winter throughout California, the Great Basin, parts of Oregon, the Lower Colorado, and the Upper Colorado, with the most severe conditions initially in the Lower Colorado. Lack of precipitation and storminess was the initial driver of this snow drought, although temperatures throughout the West were above normal much of the winter and spring, particularly during the long dry spells. Strong storms in January brought some improvement to California, the Great Basin, and the Lower Colorado, but most basins still remained about 60%–80% of normal snow water equivalent (SWE) at the end of January. A series of atmospheric rivers in February brought a brief recovery from snow drought through heavy precipitation and snowfall to the mountains of the Pacific Northwest and Oregon into the northern Rockies. March was a very dry month except for parts of Utah, eastern Colorado, and Wyoming.

April 1 is a critical date for Western snowpack as it is often near peak SWE and, more importantly, is used as input for spring and summer water supply forecasts. Much of the West had below-normal SWE on April 1, although the severity of the snow drought was not nearly as bad as the benchmark 2015 snow drought when huge areas of the West were below 50% of normal SWE by April 1. Large river basin (HUC2) April 1 SWE percent of normal was at 75% for California, 77% for the Great Basin, 58% for the Lower Colorado, 87% for the Upper Colorado, 93% for the Rio Grande, 110% for the Pacific Northwest, and 90% for the Missouri. A huge, and in some cases record breaking, decline in SWE percent of normal was observed throughout April, and snow drought conditions intensified rapidly in just one month. By May 1, all of the HUC2 basins except for the Pacific Northwest and Missouri were between 15% and 61% of normal SWE. Temperatures soared to well above normal the first week of April, and dry conditions persisted throughout the whole month, leading to rapid snow melt rates. The dry and warm conditions allowed rapid melt to occur even in higher elevation basins like the Upper Colorado that typically have peak SWE several weeks after April 1.

The 2021 snow drought in the West highlights the need to track these events from the beginning of the snow accumulation season throughout the melt season. If April 1 or peak SWE was used as the snow drought indicator this year, the severity and downstream impacts that occurred throughout April would be lost. Low snowpack, rapid melt out, and poor runoff efficiency have led to significant water supply concerns going into the summer of 2021. The two biggest reservoirs in the U.S., Lake Powell and Lake Mead, are nearing historic lows, and a Colorado River water shortage is looming; many reservoirs throughout California are as low or lower for early June than during any of the 2012–2015 drought years. Click here to learn more about the impacts of snow drought on water management, ecosystems, recreation and tourism, and more.

April 2021 Precipitation: Western U.S.

This USDA NRCS map shows Snow Telemetry (SNOTEL) April precipitation basin values over the western U.S. as a percent of the NRCS 1981–2010 median. Only stations with at least 20 years of data are included in the station averages.
The precipitation percent of normal represents the current precipitation found at selected SNOTEL sites in or near the basin compared to the average value for those sites on this day. This map is valid as of April 30, 2021.

HUC2 River Basin Snow Water Equivalent

Large river basin (HUC2) snow water equivalent (SWE) percent of normal throughout the West for peak SWE, April 1, and May 1. SWE data are from the USDA NRCS SNOTEL network.

Maximum Temperature Departure from Average: April 1–8, 2021

Early April heatwave in the West depicted by April 1–8 maximum temperature anomalies. Temperature data are from gridMET, and the map was created using Climate Engine. For an interactive version of this map, please visit Climate Engine.

Reservoir Storage for the Western U.S.

Reservoir storage percent of capacity for the Western U.S. as of May 1 and June 1, 2021. Graph from the USDA.

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