DROUGHT MONITOR



An upper-level ridge of high pressure dominated the western contiguous U.S. (CONUS) during this U.S. Drought Monitor (USDM) week. The ridge inhibited precipitation and kept temperatures warmer than normal across much of the West. Weekly mean temperatures were as much as 8 degrees above the long-term average from the Southwest to northern High Plains. Pacific fronts and weather systems rode over the top of the ridge, taking a northerly track which brought them across the drought-plagued northern Plains then into a trough over the eastern CONUS where they stalled out over the Southeast. Monsoon showers developed in the Southwest, bringing above-normal precipitation to some areas, and small but intense storms developed with the fronts as they moved across the northern and central Plains. But only a few of these storms brought above-normal precipitation to the Plains. Summertime convection and frontal lifting brought rain to parts of the southern Plains and areas east of the Mississippi River. The prolonged and intensifying drought ravaged crops and rangeland in the northern Plains, while soils continued to dry out across the West, Plains, and into the Mid-Atlantic region. Exceptional Drought (D4) returned to the USDM map this week as spots of D4 developed in the northern Plains where below-normal rain fell, and D0 expanded in parts of the Southwest where the monsoon precipitation was below normal. Persistent below-normal precipitation and enhanced evapotranspiration due to excessive heat expanded areas of drought and abnormal dryness in the central Plains to Midwest.

In the 2 days since the Tuesday morning cutoff time of this week’s USDM, additional frontal storms have moved across the northern and central Plains, and monsoon showers and thunderstorms have brought additional rain to parts of the Southwest. For July 20-24, 1 to locally 4 inches of rain is forecast for the Four Corners States and from the eastern Dakotas to Northeast, and half an inch to an inch is predicted for the central to northern Plains and most of the country along and east of the Mississippi River. Areas expecting little to no rain include much of the West from California to central Montana, most of Texas, and parts of the western Carolinas. Temperatures are forecast to be above normal for most of the CONUS. Little relief from the heat can be expected as above-normal temperatures are in the outlook for most of the CONUS and Alaska for July 25-August 2, with only the Northeast and parts of the coastal Northwest maybe having cooler-than-normal temperatures. Odds favor below-normal precipitation for coastal southern Alaska, the Pacific Northwest to northern Rockies, and most of the Plains into the Midwest. Above-normal monsoon precipitation is likely to continue for the Southwest, and Alaska is expected to be wetter than normal. The Northeast may start out drier than normal, then turn wetter than normal. In the Southeast, July 25-August 2 may begin

The Midwest had a patchwork of precipitation anomalies this week, with rainfall amounts ranging from less than a tenth of an inch to over 4 inches. Wet areas included parts of central Minnesota to Indiana and parts of northern Missouri, while especially dry areas included southern Minnesota to most of Iowa, southwest Missouri to northeast Illinois, and most of Kentucky. Cooler-than-normal temperatures dominated the Great Lakes, but temperatures have been hot during July from southern Minnesota to southwest Illinois. The temperatures have increased evapotranspiration, as seen in high ESI (Evaporative Stress Index) and EDDI (Evaporative Demand Drought Index) values, which has dried soils. A third of the topsoil (34%) and subsoil (33%) moisture was short to very short in Illinois, and 51% of the topsoil and 42% of the subsoil moisture was short to very short in Iowa, according to July 17 USDA statistics. In Iowa, 22% of the pasture and rangeland was in poor to very poor condition, up from 14% last week.

D0 was expanded in parts of Illinois and Missouri, and D0-D1 were expanded in parts of Minnesota. D1 was trimmed in southeast Iowa where over an inch of rain fell, but D0-D1 expanded in other parts of the state. As noted by the Iowa State Climatologist, the July 17 USDA/NASS crop/weather report shows a decline in all indicators. When comparing the soybean ratings for mid-July among the past 24 seasons only four years are rated worse than 2017 (2001, 2008, 2012 and 2013). For pasture/range conditions, six years among the past 24 have worse ratings than 2017 (1992, 2002, 2005-2007 and 2012). Soybeans are looking the worst owing to much of the later-planted crop being sown into very dry top soil. Stream flows continue to fall slowly in most areas with a few gage sites below the 10th percentile.