Daily forecasts limited to two weeks out

Published 7:00 am Saturday, April 20, 2019

By Skip Rigney

How far in advance would you like an accurate weather forecast? A week? A month? A year?

Before sarcastically answering that you would be satisfied if meteorologists could just get tomorrow’s forecast correct, take a moment to consider the forecasts leading up to this past Thursday’s thunderstorms.

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As far back as last Saturday’s column, which I wrote on Friday morning, April 12, I said that, “By Thursday (April 18) or Friday (April 19) a Great Plains storm system and its associated cool front could, once again, bring together the ingredients necessary for some severe thunderstorms in the Gulf South.”

Sure enough, six days after I wrote those words, severe thunderstorms and tornadoes hit Mississippi on Thursday, April 18.

Fortunately, here in Pearl River County, we dodged the severe storms, but some areas in the central part of the state were hard hit, with reports of damaged buildings and flipped automobiles.

Being able to anticipate six days in advance that we might have a heightened risk of severe weather took no special insight on my part. I was merely passing along what forecasters at the National Weather Service in Slidell were already talking about in their publicly available forecast discussions and what both they and I saw in multiple computer model predictions.

The skill of computer weather models has increased about one day every decade since its inception 40 years ago according to a report published in the scientific journal “Nature,” in 2015 entitled “The quiet revolution of numerical weather prediction.”

In addition to huge supercomputers needed to simulate the complex physics of the atmosphere and its interactions with the land and ocean, successful computer weather modeling relies on numerous, high quality weather observations from a wide variety of sources, especially satellites and weather balloons.

Of course, even with the best observations and computers, the accuracy of forecasts is not constant, but varies considerably depending on the complexity of the evolving weather pattern. Still, for most locations in the United States, including here in the Gulf South, weather forecasts three to five days in advance tend to be very good, and even ten days ahead of time can identify trends and weather pattern changes.

Can we expect to continue to see one additional day of accurate forecasts every ten years, so that 40 years from now my grandchildren will be able to confidently plan their children’s outdoor wedding two weeks in advance?

It could happen. But, it appears that forecasts of weather on a particular day will never be made accurately more than about two weeks in advance. Research results published this month in the Journal of Atmospheric Science show that two weeks is the limit of predictability, even with a perfect model of the atmosphere’s physics, perfect observations, and an infinitely large computer.

The team of researchers led by Fuqing Zhang of Penn State University confirmed the theory put forth in the 1960s by scientist Edward Lorenz that the chaotic nature of the atmosphere limits accurate predictions to about two weeks.

So, if you’re still consulting the yearly Farmer’s Almanac for the weather on a particular day, the latest science confirms what you should have already found out. Don’t.