What’s behind the big change in expectations?

The strong upper-level ridge responsible for the heat was supposed to remain throughout the month. Instead, it looks like it will retreat into Mexico and occasionally collide with Texas and the Southwest. This is what will continue the heat through early next week in the Southern Plains.

Meanwhile, another ridge will develop over Canada this weekend and next week and there will be a channel of weakness that will allow disturbances to move across the west and gather in the east. The general low in the east will keep temperatures moderate most of the time.

It will also have a tendency to deflect any threats from the tropics, which includes the rapidly developing Tropical Storm Lee in the mid-Atlantic. That storm is expected to become a hurricane later Wednesday and a major hurricane by Friday as it moves northwest toward the United States but should be pushed away by this protective trough. However, it may come close. So, instead of a ridge over the central United States, we’ll more or less see a bottom until mid-September.

What is the reason for such a major shift in paradigms? We can blame some of that on their inability to accurately predict the impact of the Madden-Julian Oscillation (MJO), a band of thunderstorms that orbit the tropics. This feature has been studied as a link between how the tropics affect weather patterns around the world and especially here in the United States. The MJO traces its beginnings in the Indian Ocean or western Pacific Ocean and travels east, sometimes all the way around the world via the Americas, the Atlantic Ocean and Africa before reaching the Indian Ocean again. This occurs on a time scale of approximately 30 to 60 days, but may also be fairly constant.

Previous forecasts were for the MJO to move across the Pacific Ocean throughout the month of September. History suggests that during August and September, ridges are favored in the central United States when the MJO is in the Pacific Ocean west of the date line, and then across the eastern United States as it crosses the date line. Hence, the forecast is hotter and drier.

However, maritime surveillance has been weaker and more halted in the Indian Ocean and the so-called maritime continent area that includes the island group of Indonesia and the Philippines. Currently, the MJO is not having the impact that the models thought it would have at this point about last week.

The other influence is due to the El Niño phenomenon, which has taken hold over the eastern Pacific Ocean over the past few weeks, and may be partly responsible for delaying the MJO process. The latest weekly sea surface temperature in the El Nino 3.4 region – the region used by forecasters to describe the El Nino Southern Oscillation (ENSO) – rose to 1.6 degrees Celsius above normal. A positive reading of 0.5°C is required to be in an El Nino condition. The current reading indicates the occurrence of a very strong El Nino phenomenon in the eastern Pacific Ocean, which is the main driver of atmospheric movement.

As the MJO and El Niño battle it out, temperature fluctuations have been the norm in the United States. It was a week of extreme heat in mid-late August, followed by other moderate temperatures in late August, and then another heat wave in early September. Followed by another mild period now in early-mid September. Most models predict a return of hills and heat in late September through early October. It’s the changing conditions we normally associate with periods of good rain, but that hasn’t paid off in more than a few lucky areas so far. Drought has been increasing in recent weeks and should grow according to the latest U.S. Drought Monitor update due out tomorrow, September 7.

There is hope that residents of the central and southern Plains will see good rains from September 9 to 13, but this is not guaranteed. Otherwise, look for light amounts of rain across most of the country outside of Michigan and the Northeast to occur over the next couple of days.

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

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