California and National Drought Summary for May 22, 2018
May 24, 2018 - The active weather pattern persisted across most of the nation, though unfavorably dry, hot weather lingered over parts of the South and Southwest. During the 7-day period ending Tuesday morning, areas of heavy to excessive rainfall provided widespread drought relief across the central and southern Atlantic Coast States and from Texas northward into Montana and the Dakotas. Conversely, short-term dryness intensified along the central Gulf Coast, while worsening drought conditions were noted in portions of Arizona and Oregon. Likewise, short-term dryness continued to develop in parts of New England. Please note the wet weather pattern continued through the week; any rain that fell after 12z Tuesday (8 a.m., EDT) will be incorporated into the following week’s drought assessment.
Generally dry conditions in northern portions of the region contrasted with additional heavy rain farther south. Short-term dryness continued to develop over southern portions of Vermont and New Hampshire, with some locales reporting locally less than 50 percent-of-normal rainfall over the past 60 days; with sub-par rainfall again this past week, D0 (Abnormal Dryness) was added to capture the driest areas over the past three months. Across the southern third of the region, widespread heavy to excessive rainfall (4-8 inches, locally more) erased lingering D0 across central and southern Maryland (over 7 inches reported in Leonardtown, MD).
An influx of tropical moisture brought widespread heavy to excessive rain to much of the region, resulting in considerable reductions in drought coverage and intensity. In Virginia, rainfall totals topping 8 inches led to 2-category reductions to drought along the I-95 corridor, especially southwest of Richmond. While heavy rain was reported in the western Carolinas, the lingering Moderate Drought (D1) and Abnormal Dryness (D0) areas received more modest amounts of rain (1-4 inches), resulting in a 1-category reduction. In Georgia, rainfall totals of 3 to 8 inches netted 2-category drought reductions in the northeast (nearly 6 inches in Waynesboro) and locales to the west and southeast of Waycross (7.7 inches near Nahunta, GA). The story was similar over central and southern Florida, where 3 to 9 inches of rain led to widespread 2-category drought reductions west of Miami (Hollywood, FL, reported nearly 9 inches) and southeast of Orlando, where weekly totals approached 5 inches. Modest reductions in D0 and D1 were made in central and southern Alabama where weekly rainfall totaled 2 to 4 inches and 60-day precipitation was now near normal. Conversely, D0 was expanded in northern portions of the state around Piedmont, AL, where 60-day rainfall was at or below 70 percent of normal and streamflows were likewise rapidly declining.
Intensifying dryness from northeastern Texas to the central Gulf Coast contrasted with heavy rain and much-needed drought relief in western portions of the region. Despite isolated downpours (locally more than 4 inches) providing localized relief from Abnormal Dryness (D0) in coastal portions of Louisiana and Mississippi, most locales reported sub-par rainfall for the week. Over the past 30 days, many of the newly-expanded D0 areas have reported less than 25 percent-of-normal rainfall (locally less than 10 percent) with deficits over the same period totaling 2.5 to 5 inches. In northeastern Texas, locales from Dallas eastward have reported similar shortfalls over the past 30 days, with 60-day precipitation totaling less than 70 percent of normal. Meanwhile, 7-day rainfall totaled 1 to 3 inches (locally more) from central Texas northward into western Oklahoma, resulting in widespread reductions to this week’s drought intensity and coverage. Notable amounts included: 4.92 inches near Bertram, TX (north of Austin); 4.76 inches in Crowell, TX (west of Wichita Falls); 3.35 inches in Willow, OK (north of Altus). The rain was not only heavy, but also frequent. Childress, TX, saw measurable rainfall for 8 consecutive days from May 13 – 20; this ties the all-time record dating back to 1948. Despite the beneficial rain, Severe (D2) to Exceptional (D4) Drought persisted over much of the southern High Plains, where 6-month precipitation has totaled locally less than 25 percent of normal.
Conditions were largely mixed over the region during the 7-day monitoring period, with generally dry weather in the north contrasting with additional rain in southern portions of the region. Moderate to heavy rainfall (1-4 inches, locally more) led to widespread reductions in Abnormal Dryness (D0) in central Missouri, with a report of 5.52 inches in Appleton City (north of El Dorado Springs). Similarly, moderate to heavy showers (1-2 inches) netted a modest reduction to D0 in southeastern Iowa. Conversely, rain largely bypassed locales from northeastern Missouri into west-central Illinois, with D0 expanded to capture areas reporting half of normal rainfall over the past 60 days. Farther north, the recent tendency toward unfavorable dryness continued, with D0 expanded across northern Wisconsin and environs to capture locales which have reported less than 50 percent-of-normal rainfall over the past 60 days. Nevertheless, rain amounts topping an inch in and around Red Lake in northern Minnesota led to small reductions to Moderate Drought (D1) and Abnormal Dryness (D0).
The overall trend toward improving conditions in the south contrasting with increasingly dry weather in the far north continued, though some northerly areas benefited from locally heavy rain. In southern Kansas, another week with moderate to locally heavy showers (1-3 inches, as high as 3.72 inches in Longton, KS) led to widespread reductions of drought intensity and coverage. Nevertheless, 6-month precipitation in the state’s lingering Extreme Drought (D3) was less than half of normal, while the Exceptional Drought (D4) in the state’s southwestern corner stood at less than one third of normal over the same time period. Moderate to heavy rainfall (locally more than 3 inches) in northeastern Colorado likewise trimmed the coverage of Abnormal Dryness (D0). In south-central Nebraska and north-central Kansas, heavy rain (2-3 inches; Phillipsburg, KS, reported 3.95 inches) yielded a corresponding reduction of D0. In southeastern Nebraska, increasingly dry conditions over the past 90 days (40-60 percent of normal) led to a modest increase of Moderate Drought (D1) southwest of Lincoln. Farther north, sharply wetter conditions between Bismarck, ND, and Aberdeen, SD, (6.17 inches in Java, SD) resulted in a considerable reduction of D0. Beneficial rain (1-2 inches) was also reported in northeastern Montana, where D0 was reduced accordingly. Meanwhile, D1 and D2 were increased somewhat in North Dakota from Bismarck to the Canadian border, where 60-day rainfall shortfalls (locally less than 30 percent of normal) have added to the region’s lingering long-term drought.
Outside of beneficial rain in eastern-most portions of the region, lackluster water-year precipitation and unusual warmth have led to increasing drought despite the cool wet season having drawn to a close. Beneficial rain was reported during the period in northeastern portions of Montana and Colorado, resulting in reductions of Abnormal Dryness (D0) and as well as Moderate (D1) and Severe (D2) Drought. However, the overarching theme in the West continued to be the ongoing and intensifying drought in the lower Four Corners as well as the interior Northwest. In the latter region, Moderate and Severe Drought (D1-D2) were expanded over much of Oregon’s Harney Basin to reflect a sub-par water year (50-75 percent-of-normal precipitation, or 10-25th percentile) as well as protracted dryness over the past 60 days (less than half of normal). Farther south, Extreme (D3) and Exceptional (D4) Drought were expanded over Arizona. The numbers from the Four Corners Region as a whole tell a dire story, with water-year precipitation totaling a meager 10 to 30 percent of normal in the hardest-hit areas; the Standardized Precipitation Index (SPI), which puts these values in a drought-intensity equivalent, are D3- and D4-equivalent over much of central and northeastern Arizona. A sub-par snowpack and above-normal temperatures have left little water available through snowmelt. Furthermore, the satellite-derived Vegetation Health Index (VHI) — which incorporates both vegetation greenness and thermal stress — shows extremely poor conditions over most of Arizona as well as neighboring portions of southern California and southern New Mexico. This region will be in need of a robust Southwest Monsoon beginning in early July to help offset the impacts brought on by this season’s Extreme to Exceptional Drought in the lower Four Corners Region.
Alaska, Hawaii, and Puerto Rico
Heavy rain significantly reduced Abnormal Dryness (D0) in southern Alaska, with amounts of 2 to 10 inches (locally more) eradicating dryness concerns on windward-facing islands and terrain along the southern and southeastern coast. On Hawaii, most of the islands remained free of dryness concerns; however, the short-term Standardized Precipitation Index (SPI, which puts rainfall into a drought-category equivalent) indicated developing dryness on parts of Oahu over the past 30 days. Puerto Rico remained free of Abnormal Dryness or drought.
An active pattern will continue, with two significant areas of wet weather over the next 5 days. Forecast data continues to show a tropical or subtropical system developing over the eastern Gulf of Mexico and lifting slowly northward over the Memorial Day holiday weekend; if this were to verify, the potential exists for another round of heavy to excessive rain (2-6 inches, possibly more) over the lower Southeast. Meanwhile, a pair of slow-moving storms system will produce moderate to heavy rain (1-4 inches) from the northern Rockies eastward across northern portions of the Plains and Upper Midwest. A trailing cold front will trigger showers over the western Corn Belt and Mississippi Valley. Despite the continuation of a generally active weather pattern, the Southwest will remain unfavorably dry. The NWS 6- to 10-day outlook for May 29 – June 2 calls for below-normal rainfall over the Northwest and from Texas and the southern High Plains into the Great Lakes and New England. In contrast, wetter-than-normal conditions are expected from the northern Great Basin into northern portions of the Rockies and Great Plains, with a second higher-likelihood area of above-normal rainfall over the southeastern quarter of the nation. Abnormal warmth is expected over most of the nation save for near-normal temperatures in the aforementioned rainy and cloudy Southeast.
Eric Luebehusen, U.S. Department of Agriculture
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 (e.g. agricultural, grasslands)
L Long-Term, typically greater than 6 months (e.g. hydrologic, ecologic)
Source: National Drought Mitigation Center