KANSAS CITY, MO. – Ocean surface temperature monitoring in the northern Pacific Ocean this month 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 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 Plains. A few bouts of strong thunderstorms will be possible as well and small disturbances will move southeast from the peak of the high-pressure ridge into the eastern Midwest, where rainfall is expected to occur most often. The western corn and soybean production areas in the Midwest and central Plains will have some potential for warm and dry-biased weather this summer once the ridge of high pressure is fully developed.

Rain evolving now over the central and southern Plains will play a significant role in the late spring/early summer ridge of high pressure.

The more moisture that falls in these next few weeks across the Plains and western Corn Belt, the weaker the high-pressure ridge will be — at least initially, this late spring and summer.

The reason for that will be the feedback of moisture from the soil into the atmosphere. A high-pressure ridge will develop, but as it does the temperatures will rise enough to stimulate strong evaporation. If the ground is wet, the moisture flux from the soil to the atmosphere will be great enough to introduce showers and thunderstorms beneath the high-pressure ridge.

If this phenomenon evolves, the showers and thunderstorms will break down the ridge and worry over a drier and warmer summer might be reduced — at least until moisture in the soil is depleted. If, however, the soil is kept dry or moisture levels are below average when the high-pressure ridge evolves, then the potential for a stronger ridge will rise greatly.

The bottom line to this discussion is multi-faceted. First, a high-pressure ridge should evolve in the US Plains and a part of the western Midwest in late spring and early summer. Negative PDO will attempt to enhance the high-pressure ridge and if soil conditions are already drier biased, a quick transition to drier and warmer-biased weather conditions will evolve and moisture stress could become an issue early in the summer.

More likely, May and early June precipitation will be sufficient to put some moisture into the western Corn Belt and Great Plains, possibly delaying the start of warm and dry conditions until later in June. In the meantime, the moisture in the Plains and western Corn Belt should get crops off to a good start.

Delaying the onset of dry and warm weather in the Corn Belt and central Plains will help crops get off to a good start and when the drier and warmer days finally do arrive, root systems may develop downward in the ground enough to support crop development when the ridge of high pressure becomes more dominating. That assumes moisture will be present down deep into the ground.

Recent soil assessments suggest deep moisture is not that prevalent and it might take a while for repetitive rain events to get the ground wet enough for the moisture feedback scenario noted above to kick in and protect crops in the early days of summer.

Regardless of May and early June rainfall, the most challenging months for crop development this year likely will be July and August, and it will be mostly in the western Corn and Soybean Belt while the eastern Midwest does a little better.

The northern Plains and Canada’s Prairies likely will see greater rainfall with the wet bias in the east in May shifting westward in June and July.

Many of the post multi-year La Niña events in this solar cycle have produced lighter-than-usual precipitation in the lower Midwest and that has verified well so far this spring. The same can be said for years moving from a multi-year La Niña into an El Niño. Another parallel with northern California heavy rain in December and January predicted a lighter precipitation bias in the lower Midwest as well. If these parallels continue, including that of PDO, confidence is high about where we are going in this growing season.

Drew Lerner is senior agricultural meteorologist with World Weather, Inc. He may be reached at worldweather@bizkc.rr.com. World Weather, Inc. forecasts and comments pertaining to present, past and future weather conditions included in this report constitute the corporation’s judgment as of the date of this report and are subject to change without notice.