A weather system moving in the upper-level westerly flow brought swaths of precipitation to the Pacific Northwest, Great Plains, and Midwest early in this U.S. Drought Monitor (USDM) week. Another upper-level weather system stalled out later in the week as it moved across the contiguous United States (CONUS), generating violent weather and locally heavy precipitation, especially along and east of the Mississippi River. Precipitation was below normal across much of the Southwest, southern Plains, and coastal Southeast. Weekly temperatures averaged below-normal in the northern states and in the West, but above normal from the Plains to the Southeast.
The week was drier than normal across much of Puerto Rico, with streamflow continuing to be lower than normal at many river gauges. D0 was expanded to reflect additional below-normal streamflow gauges in the western interior region. In Hawaii, D2 on Molokai was deleted, leaving D1 in its place, due to improved Kualapuu Reservoir levels. Water levels on the reservoir have risen to pre-drought levels, but a mandatory 20% cutback in water use remained in place as a precaution for now. This week marks the first time since June 2008 where D2 has been completely removed from the State of Hawaii. Leeward areas of the Big Island, Maui, and Oahu have been dry in recent weeks; D0 was introduced over west Oahu and expanded into the lower leeward elevations of west Maui. D0 was expanded on the Big Island to reflect USDA/FSA reports that, despite having a decent wet season, pastures in the South Kohala and leeward North Kohala Districts have already started to dry out because of the strong trade winds, and coffee growers indicated dry conditions over the Kona slopes. Like last week, Alaska was mostly drier than normal this week, but streams were running above average (except in the panhandle D0 area), so no change was made to the depiction.
An inch or more of precipitation fell across most of the mid- to upper-Mississippi Valley and Ohio Valley, with locally 3 to 5 inches reported. D1 was deleted and D0 shrank in Wisconsin, D0-D1 were trimmed in Minnesota, and D0 was reduced in Illinois. In Iowa, the above-normal rainfall helped recharge topsoil moisture but subsoil deficits remained. D2 was deleted in eastern Iowa and D0-D1 contracted in many parts of the state. The USDA reported that soil moisture and crop conditions improved in most districts of Missouri, except the southwest district, with the percentage of the state having topsoil moisture short or very short dropping to 20%. D0 was trimmed in parts of eastern and southeastern Missouri, but D1 expanded in southwest Missouri to reflect dryness at the 30-120 day time scales.
Precipitation ranged from a few tenths of an inch in the western portions of New York and Pennsylvania to 1 to 2 inches in southern New England. Even though the week was drier than normal over the New England D0 areas, reassessment of short- and long-term precipitation, streamflow, and soil moisture conditions resulted in their removal this week. Long-term dryness resulted in reduced streamflows and growing soil moisture deficits, which supports the expansion of D0 southward along the central to southern Appalachian spine. The 0.5-2.0 inches of rain that fell this week prevented expansion of D0 along the Appalachians, but it was not enough to remove the existing D0 in northern West Virginia and western Maryland.
Bands of 2-4 inches of rain fell across parts of the Dakotas, Nebraska, and Kansas this week, but the western portions of Nebraska and Kansas largely missed out on the precipitation. The precipitation from this week's slow-moving upper low replenished topsoil moisture, but subsoil moisture was slow to respond, limiting any improvement. USDA observations improved only slightly, with 53% of Nebraska and 67% of Kansas topsoil still rated short or very short of moisture as of April 27. D0 was pulled back in South Dakota, a D0 hole was added to south central Nebraska, D1 was contracted in eastern Nebraska, and D2 was pulled back in south central Nebraska and north central Kansas. But deterioration occurred in the Nebraska panhandle, where D0 expanded, and in western and southern Kansas, where D2-D3 expanded.
Bands of 1+ inches of rain fell over parts of Oklahoma, with heavier amounts falling in northern Arkansas. But the southern parts of the South received little to no rainfall, with extensive dust storms developing across parts of the southern Plains. In Louisiana, increasing soil moisture deficits, deteriorating crop conditions, and impacts of the dryness on trees, which were losing leaves, prompted the expansion of D0 across southern Louisiana and introduction of D1 along the coast. All of the drought categories expanded across parts of Texas, resulting in D4 covering virtually all of the Texas panhandle, and D3-D4 expanded in northern Oklahoma. 1+ inches of rain resulted in the shrinkage of D0 in eastern Oklahoma, while similar rainfall amounts in southwest Oklahoma had virtually no effect on alleviating long-term deficits. April 27 U.S. Department of Agriculture (USDA) reports indicated that 78% of Texas and 72% of Oklahoma topsoil was short or very short of moisture. The Oklahoma State research station in Goodwell reported widespread crop loss and destruction, with the abandonment of all dryland winter wheat in Cimarron, Texas and southern Beaver counties. D0 was trimmed in parts of Arkansas that received 1+ inches of rain, but it expanded in southwest and northwest Arkansas which missed out on the recent rains and which showed moisture deficits out to 120 days.
Convection associated with the upper-level systems dropped 3-5 inches of rain in bands over parts of Mississippi, Alabama, Tennessee, and Kentucky, with widespread 1+ inches of rain as far east as western North Carolina. The D1 in northern Mississippi was deleted and the D0 from northern Mississippi to central Tennessee was trimmed back where the heaviest rains fell. The D0 in northern Mississippi was labeled SL to reflect long-term dryness but also lingering soil moisture and crop impacts.
Widespread 2+ inches of precipitation fell along coastal Washington and Oregon, with locally 5+ inches of precipitation. Along the Sierra in California, in the northern Rockies, and in parts of northern Nevada and Utah, 1-3 inches of precipitation was reported. Half an inch to 2 inches of precipitation occurred over the Colorado Rockies, and 0.5-1.0 inch was reported in other parts of the West. But the precipitation was largely hit or miss, with other areas receiving only a few tenths of an inch, and much of southern California, Arizona, and New Mexico getting no precipitation. Improvement in the drought depiction occurred in the Northwest, where D0-D2 was pulled back in northeast Washington and parts of Oregon, but D3 expanded in parts of southwest and southeast Oregon. D2-D4 expanded across parts of New Mexico to reflect both short-term precipitation deficits as well as lingering long-term deficits, some stretching over the course of 4 years. D0-D3 expanded in northeast Colorado to reflect dryness at the 30 day to 6 month timescales. D2 was added to Duchesne Co. in northeast Utah and D1-D2 expanded in southeast Utah and southwest Colorado to reflect dryness at 120 days. In the Four Corners area, the town of Monticello, Utah has major water supply problems. With their reservoir at a critically low level, groundwater wells were drilled for supplemental supply. Even with conservation, they have at most one season of water supply. If snowpack is low again next year, they will have very little water available for municipal supply.
As a slow-moving upper-level weather system gradually exits the CONUS, it will leave behind an inch or more of precipitation along the East Coast, with 2-4 inches possible in parts of the Northeast and eastern Gulf of Mexico Coast. In addition, the NWS HPC 7-Day Quantitative Precipitation Forecast (QPF) calls for 1-2 inches of precipitation along the northern tier states, but no precipitation across the Southwest and southern Plains to Lower Mississippi Valley. Temperatures for May 1-6 should be below normal in the central CONUS as the upper low and surface cold front migrate eastward, and above normal in the West. Colder-than-normal air slides into the northern states during May 3-8.
The 6-10 day and 8-14 day outlooks indicate that a change in the upper-level circulation pattern, consisting of a trough over western North America and a ridge over the east, is predicted for May 6-14, bringing warmer-than-normal temperatures for Alaska and the southern Plains to Northeast, and below-normal temperatures for the West to northern Great Lakes. Precipitation is expected to be above normal for much of Alaska and the CONUS, except for the immediate West Coast of the CONUS, Southeast, and extreme Southwest.
Richard Heim, NOAA, NCEI
Richard Heim, NOAA, NCEI