The upper-level circulation over the contiguous U.S. (CONUS) during this U.S. Drought Monitor (USDM) week consisted of a ridge over the West Coast and trough over the north central states. Strong closed lows rotated within this upper-level flow, bounding from the Southwest to the Northeast in their trek across the country. The ridge blocked Pacific moisture from entering the country and kept the West Coast warmer than normal for the week. The trough funneled cold and dry Canadian air into the Plains. The cold fronts moved south and east, bringing colder-than-normal air to much of the CONUS from the Great Basin to the Appalachians and beyond to the Mid-Atlantic Coast. Only parts of the Southeast and Northeast averaged warmer than normal for the week. The fronts and their surface lows brought rain and snow to the northern and central Rockies as they bumped up against the mountains, with precipitation amounts meeting or exceeding weekly normals in parts of the northern to central Rockies and High Plains. The fronts and surface lows spread above-normal rainfall across Oklahoma and northern Texas to Florida, with amounts along the northern Gulf Coast exceeding 5 inches in places. They also brought rain to the Northeast, with some areas nearing or exceeding weekly normals. But much of the West, northern Plains to Ohio Valley, Southeast, and Maine were drier than normal for the week. Streamflow was well below normal all along the West Coast and in the interior West, in North Dakota and northwest South Dakota, parts of southern Texas, the southern and eastern Great Lakes, and parts of the Northeast. Soils continued to dry out in the West and Southeast, according to U.S. Department of Agriculture (USDA) reports. Satellite and station reports, as well as modeled indices, showed dry soils across the West, northern and southern Plains, the southern and eastern Great Lakes, and into the Northeast, as well as parts of the Southeast. Groundwater observations from wells and estimated from satellite measurements revealed dry conditions across the West, northern and southern Plains, Northeast, and parts of the Southeast. The cumulative effect of lack of precipitation and drying soils has stressed vegetation, as seen in such indices as the Vegetative Health Index and VegDRI, as well as field reports.
D0 was expanded along the southern coast of Puerto Rico and into eastern sections due to below-normal precipitation for the last week to 4 months, low streamflows, and drying soils.
Dry and very warm conditions continued to affect the USVI this week. St. Croix had no rain at the Henry E. Rohlsen airport and the Christainset 4.1 ESE CoCoRaHs station this week, while only 0.11 inch of rain was reported at the Christiansted 4.11.6 E station. The dry conditions were accompanied by very warm temperatures, with daily maximum temperatures of 87 deg F or higher. On April 14, the maximum temperature of 89 deg F at the airport tied the daily record last set in 1993. Groundwater levels at the Adventure 28 well in St. Croix dropped to 22.63 feet below land surface as of April 17. This was the greatest depth since November 2016. Meanwhile, the month-to-date rainfall total at the airport of 0.56 inch was only 50% of normal precipitation. Similarly, the airport’s year-to-date rainfall total was 3.85 inches, which is only 59.4% of normal. SPI values at the 1 (-0.53) and 3 (-0.53) months are consistent with abnormally dry conditions, while the 6 (-1.28) and 12 (-1.08) month periods are indicative of moderate drought. Due to the little rain and groundwater impacts, the drought classification was changed to severe drought.
St. Thomas and St. John also continued in severe drought this week as they received no rain. The month-to-date rainfall total at St. Thomas airport was 1.52 inches or 105.6% of normal rainfall. However, the year-to-date rainfall of 4.01 inches was 60.1% of normal. SPI values for the 1 (0.03) and 6-month (-0.37) period was indicative of drought free conditions. On the other hand, SPI values for 3 (-0.69) and 12 month (-0.79) periods were indicative of abnormally dry conditions. The Grade School 3 Well had groundwater at 13.51 feet below land surface (as of April 17), which is also the greatest depth since August 2017. St. John’s month-to-date rainfall total at the Cruz Bay 1.6 E CoCoRaHs station was 0.57 inch and 1.18 inches at the Windswept Beach. SPI values for the 3 (-0.68), 6 (-0.55), and 12 (-0.62) months were indicative of abnormally dry conditions. Groundwater at the Susannaberg DPW 3 Well was 15.79 feet below surface.
Cooler temperatures and snow spread across parts of the northern Plains this week. Western and southern parts of the High Plains region received 0.5-1.5 inches of precipitation this week, while the Dakotas were mostly dry with less than 0.25 inch. Precipitation in Wyoming in recent weeks has improved several drought indicators, especially the Standardized Precipitation Index (SPI), resulting in significant contraction of the D3 area in the central part of the state. D0-D2 shrank in other parts, but D1-D2 expanded in western Wyoming. Colorado saw contraction of D1-D2 in the north. Half an inch to over an inch of precipitation in southwest South Dakota shrank D2, while D2 was removed from southeast South Dakota due to improving indicators. Even though much of Montana received welcome snow this week, in most areas it amounted to below-normal precipitation, so D1-D3 expanded. Abnormal dryness contracted in south central Montana where precipitation has been above normal. In North Dakota, the snow was enough to prevent further deterioration but not enough to reduce deficits. Parts of the state remain in a burn ban and are experiencing dry soils, poor pasture conditions, and drying ponds and dugouts, some of which were testing high in TDS and sulfates. Producers are selling or making plans to cull more livestock. Dust storms have been reported recently in North Dakota and Montana. USDA reports show 78% of North Dakota, 61% of Montana, 58% of South Dakota, 56% of Wyoming, and 49% of Colorado with topsoil moisture short or very short.
Precipitation brushed the edges of the Midwest region, with half an inch or more falling over parts of western Missouri, southwest Iowa, northern Minnesota, and Upper Michigan. Most of the region received less than 0.25 inch of precipitation for the week. D1 expanded in northeast Illinois, northern Ohio, and across much of Michigan. D0 expanded in parts of Illinois, Indiana, Michigan, and Ohio. But D0 and D1 contracted in northern Minnesota where recent precipitation has reduced drought indicators. USDA reports of short or very short topsoil moisture ranged from 9% in Illinois to 29% in Iowa. County extension field reports in east central Indiana included drought impacts on planting, pasture growth, and pond levels.
Half an inch or more of rain fell across much of the Northeast, from New York to Maine, with up to 2 inches locally across parts of New England. Half an inch was also noted across southern parts of West Virginia, Maryland, and Delaware. Less than half an inch fell in between across Pennsylvania, southern New Jersey, and northern Maryland and Delaware. The rain was enough to prevent further expansion of the moderate drought (D1) and abnormal dryness (D0) in the region, but not enough to warrant contraction. The rain did improve soil moisture conditions, dropping the percentage values of soils short to very short of moisture down to 65% in New Hampshire and in the 20s and 30s in other New England states. Still, local authorities in New Hampshire were monitoring water supplies and preparing to adopt water restrictions if dry conditions persist, while Massachusetts residents continued to conserve water.
No change was made to the depiction in Alaska or Hawaii. The week was drier than normal across both states, and warmer than normal in Alaska but cooler than normal in Hawaii, and streamflow was still normal in Hawaii.
Palau had over 12 inches of rain in just one week, surpassing the monthly threshold of 8 inches to meet most water needs. This resulted in continued drought free conditions for Palau.
Dry conditions were present across the Marianas this week, with Guam, Rota, and Saipan reporting less than half an inch of rain. Monthly precipitations for the last few months were also below the threshold of 4 inches to meet most water needs for all three locations. According to the NWS Tiyan, Guam forecast, the KBDI (an index used to determine forest fire potential) was 718. The maximum value for KBDI is 800, which represents very dry conditions. Due to the dry weather, the NWS doesn’t recommend outdoor burning since spot fires will ignite easily. Given this, Guam’s drought classification was degraded by one category this week. Rota’s (D0-S) and Saipan’s (D1-S) drought remained unchanged.
Across the Federated States of Micronesia, Yap, Kosrae, and Lukunor had the highest rainfall totals this week, with rainfall totals above 5 inches. Pohnpei, Pingelap, Nukuoro, Ulithi, and Woleai also had a wet week with rainfall totals surpassing the weekly threshold of 2 inches of rain. Fananu had a little less than 2 inches of rain this week. However, all of these locations continued to be drought free. Chuuk was the only location with little rain this week, receiving only 0.32 inch of rain. However, drought free conditions were unchanged this week since last month rainfall totals were over 18 inches. Kapingamarangi also had over 5 inches of rain this week, resulting in a month-to-date rainfall of 15.05 inches. According to the drought information statement produced by the NWS Tiyan, Guam, catchments are full and vegetation is starting to recover across the region. For this reason, Kapingamarangi's drought classification was improved by one category (from D2-L to D1-L).
This was a wet week for most of the Marshall Islands, with most locations receiving over 2 inches of rain. Ailinglaplap had the most rainfall at 5.61 inches. Meanwhile, Wotje had the least rainfall at 0.90 inch of rain. Wotje’s drought classification of extreme drought continued since the last few months have been very dry. Even though Kwajalein had a very wet week (3.90 inches), its drought classification of severe drought was unchanged due to local reports on yellowing vegetation and water shortages. There were no drought concerns in the other locations.
Drought free conditions persisted in Tutuila since rainfall totals across the three locations were close to or over 2 inches of rain.
Heavy rains fell across frontal zones over Louisiana and southern Mississippi where up to 5 inches was reported for the week. Rainfall totals of 2 inches or more covered most of Louisiana into northeast Texas and parts of Oklahoma. Half an inch or more was widespread across Arkansas, eastern to central Oklahoma, and northern, eastern, and extreme southern Texas. Meanwhile, most of the Rio Grande Valley, west Texas, and western Oklahoma received little to no precipitation. Drought and abnormal dryness contracted in parts of Louisiana, Oklahoma, and Texas, while extreme (D3) to exceptional (D4) drought expanded in parts of west Texas. The rains improved soils across parts of Texas and Oklahoma, but USDA reports still have 60% of Texas and 26% of Oklahoma with topsoil moisture short or very short.
The extreme northern and southern ends of the Southeast received rain this week, with very little precipitation in between. Half an inch of precipitation was noted in Virginia with half an inch to 2 inches, and locally more, over Florida and extreme southern Alabama. The Florida rain contracted D0 in the central and southern parts of the state, while D1 in the south was reduced to better reflect improving drought indicators. D0 expanded in Alabama and Georgia and spread into the Carolinas as dryness this week added to deficits that have been growing for the last 3 months. Several in situ (ground) based and satellite-based indicators revealed drying soils, and field reports from South Carolina included cattlemen in Aiken having to feed hay now due to forage growth being limited, farmers having to begin irrigating crops (which is rare to non-existent for this time of year), and other farmers having to stop planting corn due to lack of soil moisture. According to USDA reports, the percent of the soils short or very short of moisture doubled in South Carolina this week to 55% of the state having dry soils, while the value in Georgia increased to 26%.
Just the higher mountains of northern Utah, northeast Nevada, and the Sierra Nevada in California received any precipitation this week, and that was only 0.25-1.5 inches, which was barely normal for this time of year. Most of the rest of the West received no precipitation. Low streamflow, dry soils, and precipitation deficits over the last 3 months or longer prompted expansion of abnormal dryness and drought all along the West Coast. In Washington, D0 expanded to the coast and D1 spread northeastward, with only above-normal snow water content (SWE) and water-year-to-date (WYTD) precipitation keeping the Olympic and Cascade Mountains free of abnormal dryness. The D1, and some D0, spread across northern Idaho and into northwest Montana. D1-D2 expanded in western and northeastern Oregon. D1-D3 expanded in California to better reflect the soil moisture, streamflow, and SPI indicators. Agricultural impacts along the southern California coast were especially severe. The sparse timing of rain that has occurred this season (end of December and end of January) has contributed to especially poor growth of the annual grasses that are needed for livestock feed. According to reports, the amount of forage on rangelands is low, with producers in Ventura County already shipping whole herds of cattle out of county because there is almost no forage. As the USDM week ended, California Governor Newsom declared a drought emergency in Mendocino and Sonoma counties. Other drought impacts around the region: Historically low water levels caused the closure of some launch ramps on Lake Powell. The Klamath Project will receive 33,000 acre-feet of water in 2021, about one-tenth the average amount, for the lowest allocation in the project’s history, due to drought and low inflows into Upper Klamath Lake. Dust storms have been reported recently in Oregon and blowing dust in eastern Washington. According to USDA reports, the percentage of topsoil moisture short or very short jumped this week to 65% of California and Oregon and 60% of Washington. The percentage increased slightly to 87% of New Mexico. April 21 reports of mountain SWE in California included 32% of normal in the North, 37% in the Central, and 16% in the South.
A weather system moved across the Northeast as this USDM week ended and the new week began. During April 22-27, the western ridge will break down, allowing a couple weather systems to move across the CONUS. One will move across the South and Southeast at mid-week, while another moves into the West as the USDM week ends. Much of the CONUS is expected to receive 0.10-0.25 inch of precipitation, with 1.0-2.0 inches in the mountains of the Coastal, Cascade, and Sierra Nevada ranges and northern Rockies. But the heaviest precipitation will be from eastern Texas to South Carolina, and southeast Kansas to southern Illinois, where 1.0-3.0 inches is forecast to fall. An inch or more is expected across a broad area from eastern portions of Texas, Oklahoma, and Kansas, across the Ohio Valley, to the Mid-Atlantic coast; south to the Gulf of Mexico coast; and across New England. Temperatures are expected to be cooler than normal in the east as the Canadian air masses exit the CONUS, then moderate to near to warmer than normal CONUS-wide. The outlook for April 28-May 1 warmer than normal in the East and cooler than normal in the Pacific Northwest. Odds favor wetter-than-normal conditions in the Pacific Northwest; along the Mississippi, Ohio, and Tennessee Valleys; and in the Great Lakes; with below-normal precipitation in most of the southern Plains. In Alaska, odds favor below-normal precipitation across the state except the panhandle, warmer-than-normal temperatures in the west, and cooler-than-normal temperatures in the east.
Richard Heim, NOAA, NCEI
Ahira Sanchez-Lugo, NOAA, NCEI