U.S. Drought Monitor Update for July 31, 2018

According to the July 31, 2018, U.S. Drought Monitor, moderate to exceptional drought covers 29.0% of the United States, an increase from last week’s 27.4%. However, extreme and exceptional drought—the worst categories—contracted slightly to cover 7.1% of the nation, down from 7.2% last week.

A strong high pressure ridge maintained its grip over the western contiguous United States, and the Bermuda High dominated the eastern North Atlantic during this U.S. Drought Monitor week. Weather systems and upper-level lows from the Pacific rode over the western ridge then plunged southward into an upper-level trough over the eastern Lower 48, becoming trapped between the two high pressure systems. Monsoon showers developed over parts of the Southwest, but the upper-level ridge kept weather conditions warmer than normal and mostly drier than normal across most of the West, stoking many large wildfires.

Surface low pressure systems and Canadian cold fronts swept across the contiguous United States east of the Rockies beneath the upper-level trough. The lows and fronts triggered showers and thunderstorms as they moved south, leaving above-normal precipitation across parts of the central to southern Plains in their wake. Moisture from the Gulf of Mexico and Atlantic was funneled across the Southeast and up the East Coast by a southerly flow on the eastern end of the upper-level trough and western edge of the Bermuda High. Above-normal precipitation and locally heavy rain were the result along the Southeast Coast and in the Mid-Atlantic to Northeast states. Temperatures were cooler than normal from the northern and central Plains to the Southeast behind the Canadian cold fronts, and the dry Canadian air masses brought below-normal precipitation to the northern Plains, Midwest, and interior Deep South.

This complex weather pattern produced many areas of changing drought conditions. Drought and abnormal dryness expanded in parts of the West, southern Plains, Mid-Mississippi Valley, Deep South, and Great Lakes, but contracted under monsoon rains in parts of the Southwest, under thunderstorms in parts of the central to southern Plains, and beneath heavy precipitation in parts of the Northeast.

Abnormal dryness and drought are currently affecting over 128 million people across the United States—about 41.2% of the country’s population.