California and National Drought Summary for May 4, 2021
May 6, 2021 - This U.S. Drought Monitor (USDM) week saw an active weather pattern with severe weather observed across portions of the central and southern Plains, Texas, mid-South, Midwest, and the Northeast. In Texas, 7-day rainfall accumulations ranged from 2 to 10+ inches leading to significant improvement in drought-related conditions across the state. Likewise, areas of northeastern Colorado and portions of the central Plains received much-needed rainfall (2-to-4-inch accumulations) leading to improvements on the map. Out West, 83% of the region is currently in moderate-to-exceptional drought with the most severe conditions centered on the Four Corners states, California, and Nevada. In California, conditions deteriorated on this week’s map in response to a combination of factors including back-to-back dry water years, above-normal temperatures, below-normal snowpack, and drought impacts (agricultural, ecosystem health, water supply, recreation).
On this week’s map, areas of the region—including western and northern New York, and southern portions of New Hampshire and Vermont—saw improvements in areas of Moderate Drought (D1) in response to precipitation (~2 inches) during the past week. Likewise, this week’s rainfall led to reduction of areas of Abnormally Dry (D0) in Maine, Massachusetts, and Rhode Island. For the week, average temperatures were above normal across the southern part of the region with the greatest positive anomalies (6 to 8 deg F) observed in southern Pennsylvania, southern New Jersey, and Delaware. Further to the north, slightly cooler-than-normal temperatures (1 to 4 deg F) were observed across much of New York, as well as northern portions of New Hampshire and Vermont. According to the U.S. Department of Agriculture (USDA) for the week ending on May 2, the percentage of topsoil moisture in Maine rated short to very short was 61%, whereas all other states in the region ranged from 0 to 29% short to very short. However, the NASA Crop-CASMA (Crop Condition and Soil Moisture Analytics), a remotely sensed geospatial soil moisture and vegetation index mapping application, is showing negative soil moisture anomalies (May 1) across parts of eastern Pennsylvania where 7-day average streamflow percentiles are ranging from the 1st to the 24th percentile.
During the past week, scattered precipitation was observed across the region with rainfall accumulations generally ranging from 1 inch to 3+ inches in northern Alabama, whereas other states in the region—including Georgia, South Carolina, and Florida—logged lesser accumulations. However, after the 8:00 a.m. ET USDM data cutoff on Tuesday (May 4), a severe storm outbreak impacted portions of Alabama and Georgia with areas of very heavy rainfall, hail, flash flooding, high winds, and power outages. On this week’s map, rainfall during the past week helped to alleviate short-term precipitation deficits, which led to reductions in areas of Abnormally Dry (D0) in Georgia and South Carolina, as well as slight improvements in an area of Moderate Drought (D1) in southwestern Florida. According to the USDA, the percentage of topsoil moisture in South Carolina rated short to very short was 73%, whereas both Florida and North Carolina were 38% short to very short. Across most of the region, 7-day average streamflows were running at normal levels or above, with the exception of areas of eastern North Carolina where flows have dipped below normal. Average temperatures for the week were above normal across the entire region with the greatest anomalies observed in eastern portions of Virginia and North Carolina where temperatures were 6 to 10 deg F above normal.
On this week’s map, widespread improvements in areas of drought were made across Texas (and southern and eastern Oklahoma) in response to significant precipitation accumulations (ranging from 2 to 10+ inches) with areas along the Texas Gulf Coast and the Hill Country receiving the heaviest accumulations. The slow-moving front that entered the region last week brought severe storms with frequent lighting, tornados, and softball-sized hail that caused extensive property damage with damage estimates expected to exceed $3 billion. This week’s rainfall significantly improved soil moisture levels across much of Texas, but negative soil moisture anomalies remained across the Trans-Pecos and the Texas Panhandle regions according to the NASA Crop-CASMA. According to Water Data for Texas (May 4), monitored water supply reservoirs are currently 83.6% full, with most of the reservoirs in the eastern half of the state ~80% to 100% full and reservoirs in the western half of the state generally <50% full. Average temperatures for the week were below normal (2 to 10 deg F) in the Trans-Pecos, Edwards Plateau, and southern High Plains regions of Texas, whereas the rest of the region was above normal with the greatest anomalies (5 to 15+ deg F) observed in eastern Texas.
On this week’s map, some minor improvements were made in areas of Moderate Drought (D1) and Abnormally Dry (D0) in portions of Ohio, Indiana, and Wisconsin, where areas of isolated heavy rainfall (2 to 3 inches) were observed this week. Elsewhere in the region, mounting precipitation deficits (2 to 4+ inches) during the past 60 days led to an expansion of areas of Moderate Drought (D1) in northern Illinois, as well as new areas of Abnormally Dry (D0) in southern Iowa and northwestern Missouri where conditions have been dry during the past 30 days. According to NASA Crop-CASMA, very dry soils (compared with historical averages for the date) are being observed in northeastern Illinois in Boone, DeKalb, Kane, and McHenry Counties. Average temperatures for the week were above normal (2 to 12+ deg F) across the region with the greatest departures observed in northern Illinois, eastern Iowa, northern Missouri, and southern Wisconsin.
On this week’s map, areas of the region—including Kansas, Nebraska, Colorado, and Wyoming—saw improvements, including a reduction in areas of Severe Drought (D2) in southwestern Nebraska and northwestern Kansas as well as in areas of Moderate Drought (D1) in northeastern Colorado and southeastern Wyoming. In northeastern Colorado, 2 to 4+ inches of rainfall were observed during the past week, which provided a timely boost in soil moisture conditions for recently planted crops. Elsewhere, areas of Extreme Drought (D3) expanded in northern South Dakota and southern North Dakota. In northwestern South Dakota, the town of Lemmon saw its driest January through April period on record with only 0.71 inches of precipitation observed. The South Dakota State Extension and the North Dakota State Climate Office are both reporting drought-related impacts in their respective states, including poor water quality for livestock and dry stock ponds. In western North Dakota, dry conditions and strong winds have been exacerbating fire-related conditions as firefighters are battling two wildfires in the Dakota Prairie Grasslands. Average temperatures for the week were above normal across the region with positive temperature anomalies ranging from 2 to 9 deg F above normal.
On this week’s maps, areas of drought expanded across California, Oregon, and Washington following a very dry April. In California, areas of Extreme Drought (D3) expanded across the northern and central Sierra Nevada, as well as in areas of the San Joaquin Valley where water deliveries have been severely reduced due to the poor snowpack conditions across the Sierra (59% of normal on April 1 statewide) and below normal reservoir conditions. For the Water Year (since October 1), precipitation across most of California has been much below normal (bottom 10th percentile) with some locations—including areas of southeastern California, and the greater Bay Area—experiencing record or near-record dryness. In Marin County, the Marin Water District declared a water shortage emergency on April 20 in response to Marin’s total reservoir storage level dipping to 50% of capacity, whereas average storage for the date (May 4) is normally 90% of capacity. California’s two largest reservoirs, Lake Shasta and Lake Oroville, were at 50% and 42% of normal, respectively, on May 4. Across the region, statewide reservoir storage levels were below normal in Arizona, California, Colorado, Idaho, Nevada, New Mexico, and Washington according to the NRCS on April 1. On the Colorado River system, the U.S. Bureau of Reclamation (May 5) is reporting Lake Mead at 38% of capacity while upstream Lake Powell is 35% full. In Oregon, drought-related conditions continue to deteriorate in western Oregon after a dry April. On the map, areas of D1 to D4 expanded in Oregon this week in response to a rapid decline of the mountain snowpack across the Cascades in addition to anomalously dry soils and well-below-normal streamflow levels. For the week, average temperatures were above normal (2 to 10 deg F) across most of the West, with the exception of areas of southeastern Arizona and southern New Mexico where temperatures were 2 to 9 deg F below normal.
Alaska, Hawaii, and Puerto Rico
On this week’s map, no changes were made in Alaska. In the Hawaiian Islands, generally dry conditions prevailed across the island chain during the past week. On the map, an area of Severe Drought (D2) was introduced in the lower South Kohala District where rangelands are in poor condition. On the windward side of Maui, areas of Abnormally Dry (D0) were removed from the map because of improving streamflow conditions and recent rainfall, whereas D0 was added on Kauai and Niihau because of downward trending streamflow conditions and below-normal rainfall. In Puerto Rico, rainfall was light-to-moderate (generally <2 inch) across the southeastern part of the island while areas in the western half received some higher totals (2 to 6 inches).
Patchy moderate rains brought improvement to a few areas in the U.S.-Affiliated Pacific Islands April 28 - May 4, 2021, but most locations remained essentially unchanged from last week. This continues the pattern of the past several weeks, where areas of rain bring improvement to relatively small areas, resulting in a longer-term trend toward gradual, periodic improvement in the regions of ongoing dryness and drought.
Weather features included a near-equatorial surface trough that persisted across Palau and western Yap State; trade-wind convergence combined with a series of surface troughs across central to eastern Federated States of Micronesia (FSM) and the Marshall Islands (RMI); and weak circulations that migrated across southern parts of the FSM and Palau. The surface features in the east interacted with upper-level divergence at times to enhance precipitation. A frontal zone/shear line moved across the Marianas early in the week, but otherwise a dry trade-wind pattern dominated this region. South of the equator, a ridge of high pressure persisted across the Samoan region with low-level moisture feeding showers over the mountains.
Satellite-based estimates of 7-day precipitation (QPE) showed a band of precipitation (the South Pacific Convergence Zone, or SPCZ) stretching southeastward from Indonesia to south and east of the Samoan Islands, and another broken band of precipitation stretching eastward across Micronesia to the Date Line and beyond. The satellite QPE depicted 1 to more than 4 inches of precipitation associated with the SPCZ south and west of the Samoan Islands with little to no rain detected by this product over American Samoa. Areas of 1 to 2 inches of rain were evident over Palau and parts of Yap State, with a continuous band of 1 to locally 4 inches across central to eastern FSM and the RMI. Patchy areas of 1 to 2 inches of frontal rains were detected in an arc bisecting the Marianas.
Moderate drought (D1) continues over Saipan, Rota, and Guam in the Northern Mariana Islands. These locations recorded several tenths of an inch to nearly 2 inches of rain last week, resulting in totals of generally 2 to 5 inches for April 1 - May 4. Some orographically-favored parts of western Guam recorded as much as 8 inches during this period. Because of this variable rainfall pattern, areas receiving the heavier rains are in better condition than D1 affecting most locations.
Across the Federated States of Micronesia, only the southernmost islands are affected by dryness. Even there, over 20 inches of rain pelted Kapingamarangi in April, and a few tenths more fell during the first few days of May. Conditions are trending toward slow improvement there, though the assessment of long-term moderate drought is unchanged from last week.
Drought continues in Kwajalein and Wotje in the Marshall Islands, but conditions improved a bit. Almost 4 inches of rain fell on Kwajalein last week, bringing their total since April 1 to over 9 inches. As a result, the previous D2 was improved to D1. Rainfall has not been as generous at Wotje, but April rainfall of over 3.5 inches is closer to normal than previous months had been, with only 1.2 inches falling for February and March combined. Still, given the sizeable longer-term moisture shortages, the classification of extreme drought (D3) is unchanged from last week.
Palau remained free of dryness, with another 2.5 to 4.5 inches of rain falling this past week. American Samoa was drier than it had been the prior several weeks, but remained free of dryness in the Drought Monitor
Most of the Virgin Islands recorded 0.5 to 1.5 inches of rain, the highest totals in four weeks. But given the large multi-month deficits that have accumulated, the damp week did little to change the severe drought (D2) that is entrenched across the islands. Moderate rains have been a little more widespread and a little heavier on parts of St. John, but not enough to change the assessment as a whole. The University of the Virgin Islands' also reported close to normal rainfall for April. But as of May 4, less than 7 inches of rain has accumulated on St. Thomas since December 1, 2021 - about 61 percent of normal. St. Croix has been wetter, but rainfall is typically higher there. Since October 1, Rohlsen Airport on St. Croix reported 62 percent of normal rainfall, which is a deficit of over 8 inches.
The NWS WPC 7-Day Quantitative Precipitation Forecast (QPF) calls for moderate-to-heavy liquid accumulations ranging from 2 to 4+ inches across the mid-South and lower Midwest while portions of the Plains, Northeast, Mid-Atlantic, and the Southeast are expected to receive <1-inch accumulations. In the Intermountain West and Pacific Northwest, light precipitation (<1-inch accumulations) is forecasted for areas of the central and northern Rockies, and portions of the Cascades. The CPC 6-10-day Outlook calls for a moderate-to-high probability of above-normal temperatures in the Far West, Southwest, Great Basin, and Florida while a high probability of below-normal temperatures is forecasted across most of the Eastern Tier. In terms of precipitation, there is a moderate probability of above-normal precipitation across areas of the central and southern Plains, as well as the southeastern tier of the U.S. Below-normal precipitation is expected across the Pacific Northwest, Great Plains, and areas of the Intermountain West.
David Simeral, Western Regional Climate Center
Richard Tinker, NOAA/NWS/NCEP/CPC
D0 Abnormally Dry—used for areas showing dryness but not yet in drought, or for areas recovering from drought.
Drought Intensity Categories
D1 Moderate Drought
D2 Severe Drought
D3 Extreme Drought
D4 Exceptional Drought
Drought or Dryness Types
S Short-term, typically less than 6 months (agriculture, grasslands)
L Long-term, typically more than 6 months (hydrology, ecology)
SL Area contains both short- and long-term impacts
Source: National Drought Mitigation Center