Nationwide
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Several Pacific weather systems, in the form of shortwave troughs, moved in the jet stream flow across the contiguous U.S. (CONUS) during this U.S. Drought Monitor (USDM) week.
The weather systems brought rain or snow to parts of northern California and the Pacific Northwest, western Colorado, the northern Plains to Mid-Mississippi Valley, and Tennessee Valley to Northeast. Rain also fell across parts of Florida. The rest of the CONUS had little to no precipitation. Even where the precipitation fell, it was mostly below average for the week. Areas receiving above-normal rainfall included parts of the Sierra Nevada, southern Idaho, other scattered parts of the Pacific Northwest, strips across the central Plains to Ohio Valley and across New England, and parts of Hawaii.
Improvement in drought conditions occurred where precipitation was above average, while drought expanded or intensified in some areas where dryness continued. Temperatures were mostly warmer than normal across the CONUS, with anomalies 9 degrees or warmer from the Southwest to northern Plains. Parts of the Pacific Northwest and East Coast were near to colder than average.
SNOTEL observations of mountain snowpack showed increases in snow depth in the Sierra Nevada and parts of Oregon and Washington, and snow water equivalent (SWE) values were in the high percentiles in the Sierra Nevada and Pacific Northwest, but this is early in the snow season when the snowpack is just getting established. SWE values were in the low percentiles from Nevada eastward. Western reservoirs continued relatively low, especially in Oregon, California, Nevada, Utah, Colorado, Wyoming, and New Mexico. Mounting dryness was indicated in several drought indicators and indices.
The satellite-based Vegetation Health Index shows stressed vegetation across the California valleys and southern California, the Southwest, parts of the central Plains and Ohio Valley, and especially in southeastern New Mexico to western Texas. Where VegDRI is still in season, it shows drought across much of the West (especially the Southwest and West Texas) and parts of the Northeast. QuickDRI is still in season. It indicates very dry conditions across the West (except for a very small part of coastal southern California) to southern and central Plains, much of Texas, and parts of the Midwest and Northeast.
The KBDI shows significantly dry conditions in California, Nevada, Utah, Arizona, New Mexico, and Texas. Evapotranspiration (EDDI) for the last week has been high in the southern to central Plains, southern Alabama, and the Midwest to Northeast; at the 2- to 3-week timescales, across much of the CONUS from the Southwest to Northeast; and at longer time scales (1-3 months), in the Southwest to central Plains, and from the Ohio Valley and southern Great Lakes to Northeast. NIFC wildfire maps show large wildfires still burning in California and Colorado, several across Oklahoma, and some in other parts of the West, Kansas, Texas, Mississippi, the Florida panhandle, and the central Appalachians. USGS real-time groundwater level data show low groundwater at points across the West, in northern Indiana, southern Georgia, and parts of the Northeast.
NASA GRACE satellite-based groundwater estimates show low groundwater across most of the West to central and southern High Plains, most of New York to New England, much of Texas, and parts of North Dakota, Pennsylvania, Virginia, Alabama, and Florida. Soil moisture is dry across the West from California to the southern and central Rockies, in the southern and central High Plains, in North Dakota, across Nebraska and Iowa, across central Illinois to northern Indiana, parts of Pennsylvania and New York, and (for some indicators) most of New England and southern Alabama (CPC, NLDAS, and UCLA/VIC models; satellite-based AAFC/SMOS, GRACE, and NASA/SPoRT analyses).
The Standardized Precipitation Index (SPI) shows dry conditions in various places at different time scales. These include North Dakota to Minnesota, Wyoming, New England, and southern Texas to the Lower Mississippi Valley (at the 1-month time scale); California to the central and southern Rockies, much of the Great Plains, Iowa and Missouri to Indiana, parts of the Northeast, and southern AL (2 to 4 months); California to the central and southern Rockies, much of the Great Plains, Iowa, Indiana to Ohio and Michigan, most of Northeast (6 to 12 months); parts of Pacific Northwest (9 to 12 months); and the Southwest to southern and central High Plains, and parts of Pacific Northwest, Texas, Iowa, Indiana, and the Northeast (24 months).
When the desiccating effects of hot temperatures are included, the Standardized Precipitation Evapotranspiration Index (SPEI) shows more intense drought conditions over the SPI dry areas than indicated by the SPI.
The West
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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 built up over much of 2020.
An exception was the contraction of abnormal dryness in a couple of spots in Washington. Moderate drought retreated in central Montana, reflecting moisture that has fallen in recent weeks. Severe to extreme drought was expanded in parts of northeast California, northwest Nevada, and adjacent Oregon. This week’s precipitation 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 summer monsoon’s failure resulted in record dryness to the Southwest states. 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 the 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 percent of New Mexico, 75 percent of California and Utah, 58 percent of Montana, 55 percent of Idaho, and 40 percent 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.
The High Plains
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Half an inch of precipitation fell over parts of western Wyoming and the Colorado mountains, with a strip of half-inch precipitation bisecting Kansas. But for most of the region, it was a dry week with less than a quarter-inch falling or no precipitation.
Moderate and severe drought was trimmed slightly in western Wyoming, but the big story was the expansion of severe and extreme drought in parts of Kansas and Nebraska; moderate drought also expanded in eastern Kansas. This expansion was prompted by several indicators, including SPI at several time scales and multiple soil moisture tools.
According to USDA reports, half to two-thirds of the topsoil moisture was short to very short in all of the High Plains states except Colorado, where 87 percent of the topsoil moisture was short to very short. USDA statistics show that 43 percent of the winter wheat was in poor to very poor condition in Colorado, which is a jump of 11 percent compared to a week ago. In Nebraska and Kansas, the winter wheat statistics were 20 percent and 26 percent, respectively, poor to very poor.
The South
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Half an inch or more of rain fell across parts of Oklahoma and Arkansas, but other than that, the South region was dry. Abnormal dryness expanded in parts of all of the states in the South region. Moderate to exceptional drought was expanded across various parts of Texas; moderate drought was introduced in Arkansas and Mississippi and expanded in Louisiana, and moderate and severe drought expanded in Oklahoma. In the Oklahoma panhandle, moderate and severe drought retreated.
According to USDA reports, topsoil moisture was short or very short across 69 percent of Texas, 41 percent of Oklahoma, 31 percent of Mississippi, 24 percent of Louisiana, and 14 percent of Arkansas; 38 percent of the winter wheat was in poor to very poor condition across Texas. —UNL Drought Monitor
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