Drier summer still possible in US Midwest Negative Pacific Decadal Oscillation could reduce precipitation by Drew Lerner O cean surface temperature monitoring in the northern Pacific Ocean in April did not show much reason for the Pacific Decadal Oscillation (PDO) to weaken from its strongly negative level attained in March. When the PDO index is updated by the US National Oceanic and Atmospheric Administration showing the April average index value, World Weather, Inc. believes it will show little change with a slightly weaker bias. That will leave the opportunity for PDO to influence the development, position and intensity of a mid-US high-pressure ridge this summer. The persistence of strongly negative PDO in the absence of any other dominating weather pattern suggests that as the jet stream weakens and shifts northward this spring a ridge of high pressure is expected to evolve in the central United States that will dominate the summer. El Niño will evolve, but by the time it has any influence summer will be half over. Strongly negative PDO years typically have a deep trough of low pressure over the Pacific Coast states during the warm season, resulting in cooler-biased conditions in the far western United States and a stronger-than-usual monsoon flow coming northward from Precipitation anomalies associated with PDO phases PDO+ 4 3 2 1 PDO - -1 -2 -3 -4 -.60 -.40 -.20 0.0 .20 .40 .60 44 May 2023 / World Grain / World-Grain.com Pacific Decadal Oscillation through March 2023 Mexico that will bring moisture to the Rocky Mountain region and possibly Canada's Prairies and the northern US Plains. That trough of low pressure also tends to help reinforce a high-pressure ridge that is often present in the middle of the United States. The persistence of negative PDO is likely to shut off some of the precipitation that has been evolving in portions of the central United States and a part of the Midwest later this spring. Totally dry weather is not likely, but precipitation may have a tough time countering evaporation during July and August. That could lead to a notable decline in soil moisture and precipitation for a while this summer, possibly leading to some stress for crops. The change in weather may begin to evolve late this spring but because of cool weather in the higher latitudes, the ridge may not be allowed to expand northward in a blocking manner until July and August. This year's summer weather should have more cool weather over portions of Canada than usual. That cool air occasionally will move over the top of the central US ridge of high pressure resulting in some periodic precipitation in southern Canada and the northern US Dec 1902 Dec 1908 Dec 1914 Dec 1920 Dec 1926 Dec 1932 Dec 1938 Dec 1944 Dec 1950 Dec 1956 Dec 1962 Dec 1968 Dec 1974 Dec 1980 Dec 1986 Dec 1992 Dec 1998 Dec 2001 Dec 2007 Dec 2010 Dec 2016 Dec 2022 Graphics courtesy of National Oceanic and Atmospheric Administrationhttp://www.World-Grain.com