Ag Weather Forum

A hotter, more humid and largely drier forecast for August is likely to make summer end on a sour note for a lot of agricultural producers across the country, according to long-range weather models.

It has been elusive most of the summer, or rather it has been shifting around, but a consistent upper-level ridge is being forecast to build across the Rockies and Plains next week, leading to a long-standing heatwave that should stick around for most, if not all, of August.

Given that conditions in the Pacific Ocean, the largest driver of worldwide weather, have been complex and shifting, it should come as no surprise that heatwaves in the United States this summer have been mostly fleeting. Especially for those of us east of the Rockies.

Sure, we have had stretches of heat, and much of these areas have had above-normal temperatures for most of the season so far. But stretches of sustained heat for long periods have not been very consistent across the U.S. and usually have come to certain regions rather than being widespread. Those in the south have had days above 100 degrees Fahrenheit and some in the Plains have had it on a couple of occasions as well.

An early June heatwave brought upper 90s F temperatures into the Eastern Corn Belt as well. But these stretches were relatively short-lived by summer standards, lasting less than a week in most cases, and followed by longer stretches of milder or even below-normal temperatures. The true hot-and-humid conditions we expect during the summer have been the exception, not the rule.

That is about to change.

The El Nino Southern Oscillation (ENSO) situation has been neutral all summer, reluctant to switch over to the La Nina (cooler-than-normal sea-surface temperatures in the tropical Pacific Ocean) that was forecast to occur earlier this year. Cold water just below the surface has not made it to the top in any meaningful way and has not affected atmospheric circulation much.

What's more, the Madden-Julien Oscillation (MJO) -- a description of a zone of thunderstorms that circumnavigates the equator -- has also been moving this summer. The MJO has a large, but usually short-term effect on global weather. As it moves around, it sends energy into the upper troposphere, shifting the jet stream. With it being weaker and constantly moving around, upper-level troughs and ridges have moved around in tandem, leading to few blocked up patterns that would make for expansive heat. Without ENSO changing, the MJO will be the primary driver of the global circulation.

During the last couple of weeks though, the MJO has started to slow down and ocean temperatures in the western Indian Ocean have been rising. That has made a more stout ridge across the western U.S. and into the Canadian Prairies, where heat has been noticeable and brutal on some days.

Thunderstorm activity will be more consistent over eastern Africa and the western Indian Ocean. In so doing, the MJO will have a tendency to stall out in the same location for an extended period. When the MJO is in this phase in the western Indian Ocean, upper-level ridging is favored over the U.S. and Canada during August. Hence why models are building a consistent ridge in the Rockies and Plains next week and extending that up into Canada at times as well.

Longer-range models continue to see that ridge throughout the vast majority of August, about a month's-worth of hot and humid conditions after a fairly good weather situation for much of the country so far this summer.

Underneath the ridge, hot weather builds up and with it not likely to move around much, will be a consistent feature during the next several weeks. Temperatures will consistently be above normal across most areas except the Gulf Coast, which should be closer to normal. Daytime highs in the 90s F and approaching or exceeding 100 F look to be a familiar sign for much of the month.

The highest temperatures are likely to be in the U.S. Plains and triple-digit heat is unlikely east of the Mississippi River in the current forecasts. Depending on the precipitation situation, we could or should see drought building as plants rapidly use up the available soil moisture to combat the heat.

Some "good" news is that although the ridge will be in place, disturbances will continue as well. They are unlikely to go through the ridge, instead going over the top of it. From the Canadian Prairies through the U.S. Corn Belt, we should see these disturbances flowing over the top of the heat, being able to tap into some of the heat and humidity to produce clusters of showers and thunderstorms.

This is a good setup for creating severe weather, and forecasters should have a heightened sense of those occurring in the coming weeks. But it could lead to some decent or even good precipitation across portions of the Corn Belt.

That is most likely to occur in the eastern Midwest. States like Indiana, Ohio, and Michigan could be in line for relatively consistent and widespread rain, though not guaranteed. But farther west, showers and thunderstorms will have less access to moisture moving up from the Gulf of Mexico and should be more sporadic. With heat coming in and precipitation being more isolated, we could see building drought and flash drought throughout the region.

Even portions of the Eastern Corn Belt, where showers frequently miss or are too light, could see drought develop in the heat. Southern areas, though they may find showers at times due to proximity to the Gulf of Mexico, are less likely to have consistent rainfall with time, leading to drying concerns there as well.

But the ridge will not be relegated to just the Rockies and Plains and the MJO is not the only factor in determining the weather. The ridge will likely make some movements and will be stronger or weaker throughout the month. It will not be a forecast that you can hang your hat on every day. There will still be challenges and those details will matter.

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John Baranick can be reached at john.baranick@dtn.com

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