Nationwide
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This U.S. Drought Monitor week saw an intensification of drought across parts of the western U.S. including California, Nevada, and Colorado, where precipitation has been below average since the beginning of the Water Year (Oct 1).
In California, statewide snow water content (SWE) is currently at 36 percent of the historical average for the date (Dec 7), and Water-Year-to-Date (WYTD) precipitation (statewide) is ranging from the bottom 10 percent to the bottom 33 percent with some areas in the Mojave Desert experiencing the driest on record for the period. According to NOAA’s National Centers for Environmental Information (NCEI), the six months from June to November 2020 were the hottest and driest on record for Arizona and California. Current snowpack conditions across the West are generally reflective of a La Niсa-like precipitation pattern with the mountain ranges in the Pacific Northwest and some areas of the northern Rockies observing near-normal to above-normal snowpack conditions.
Further south in the Four Corners states, basin-wide SWE is below average in nearly all drainage basins in the region. Elsewhere on this week’s map, areas of Texas, including the Panhandle and central Texas, saw some minor deterioration in conditions where both long and short-term precipitation deficits exist.
In the Northern Plains, unseasonably warm temperatures and dry conditions continued this week, leading to the intensification of drought conditions in North Dakota. Statewide precipitation for the September-November 2020 period ranked 3rd driest on record, according to NOAA NCEI.
The West
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During the past week, high pressure continued to dominate across much of the region, with virtually no precipitation observed except for some light precipitation (generally <1 inch) along with coastal Oregon and Washington.
On the map, below-normal WYTD precipitation across California led to deterioration across the Sierra Nevada, Central Coast, southern San Joaquin Valley, and areas of Southern California and the Mojave Desert. In California’s mountains, the current statewide SWE for the date (Dec 7) is 36 percent of normal. Considering the regional breakdown across the state, the current percentage of normal SWE is as follows: Northern Sierra/Trinity–40 percent, Central Sierra–44 percent, and Southern Sierra–17 percent. According to the California Department of Water Resources, the state’s three largest reservoirs were below historical averages for the date, with Shasta at 74 percent, Oroville at 60 percent, and Trinity at 79 percent. In the northern Great Basin, Extreme Drought areas (D2) expanded in northeastern Nevada, where snowpack conditions are below average across the Ruby Mountains, Independence Mountains, and Jarbidge Mountains.
Across the Nevada and Utah borders in southern Idaho, areas of Moderate Drought (D1) expanded where below-normal snowpack conditions are being observed in the Bear River Range and the Portneuf Range east of Pocatello, Idaho.
Across the mountain ranges of the Four Corners states, snowpack conditions are well below average across with the Little Colorado, Salt, Upper Gila, Rio Grande-Mimbres, Rio-Grande Elephant Butte, Upper Canadian, and Upper Pecos basins—all below ~50 percent of normal. According to the NRCS (Dec 1), statewide reservoir storage was below average in Colorado, Nevada, New Mexico, and Oregon, while above-average levels were observed in Arizona, Idaho, Utah, Washington, and Wyoming (data not available for California and Montana). Looking at the last 6-month period, the Southwest (Arizona, Colorado, New Mexico, Utah) and West (California, Nevada) climate regions both experienced their hottest and driest June-November period on record, according to NOAA NCEI.
The High Plains
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On this week’s map, areas of the region—including southwestern North Dakota and central Nebraska—saw modest expansion in areas of Severe Drought (D2) as well as expansion in areas of Moderate Drought (D1) in southwestern North Dakota in response to a combination of factors—short-term precipitation deficits, lack of seasonal snow cover, depleted soil moisture, and unseasonably warm temperatures (12 to 20 degrees above normal)—during the past week. In the eastern Plains of Colorado, areas of Extreme Drought (D3) and Exceptional Drought (D4) expanded where long-term precipitation deficits continued to mount.
The South
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On this week’s map, areas of drought intensified and expanded in the Panhandle and Central Texas, where both long and short-term precipitation shortfalls exist. In these areas, 6-month precipitation deficits ranged from 4 to 8+ inches.
Conversely, above-normal precipitation during the last 30-day period led to improvements on the map in areas of Moderate Drought (D1) and Abnormally Dry (D0) along the Coastal Plain region of Texas. According to the USDA for the week of November 29, 61 percent of topsoil moisture in Texas was rated short to very short, and 34 percent of the winter wheat crop was reported to be in poor to very poor condition. Elsewhere in the region, short-term dryness during the past 30-day period led to the expansion of Abnormally Dry (D0) areas across portions of Louisiana, Arkansas, Mississippi, and Tennessee. The week’s average temperatures were well below normal (4 to 10+ degrees) across the entire region. —UNL Drought Monitor
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