Parts of northern California and the Sierra Nevada and the western parts of Oregon and Washington received half an inch to over 2 inches of precipitation this week, with locally over 4 inches in Oregon and Washington. Parts of the Rocky Mountains received half an inch to 2 inches. While beneficial, the precipitation was not enough to overcome deficits that have built up over much of 2020. An exception was contraction of abnormal dryness in a couple spots in Washington. Moderate drought retreated in central Montana, reflecting moisture which has fallen in recent weeks. Severe to extreme drought were expanded in parts of northeast California, northwest Nevada, and adjacent Oregon where precipitation this week was light and deficits over the last several months continued to mount.
Southern portions of the West region received little to no precipitation this week. Moderate to severe drought expanded in the San Joaquin Valley and abnormal dryness expanded in southern California. The failure of the summer monsoon resulted in record dryness to the Southwest states, and record heat over warm season increased evapotranspiration, resulting in record SPEI values over the last 3 to 6 to 9 months. The SPEI values were not only record, they exceeded previous records by huge margins. Extreme to exceptional drought expanded to reflect this prolonged dryness in Arizona, New Mexico, and Nevada. According to USDA reports, topsoil moisture was short to very short across 81% of New Mexico, 75% of California and Utah, 58% of Montana, 55% of Idaho, and 40% of Nevada and Oregon. Parts of interior southern California, the southern Great Basin, and the Desert Southwest have had a seven-month dry streak. According to National Weather Service records, the last measurable precipitation in Bishop, California occurred on April 17, 2020, and the last measurable precipitation in Las Vegas, Nevada occurred on April 20, 2020.