Computers Improving Forecasts, but Meteorologists Still Peek Outside

Every few minutes, rain or shine, day and night, a giant computer here runs millions of numbers through equations that meteorologists use in forecasting weather conditions around the world.

But high technology isn’t always enough.

“It’s always a good idea to look out the window,” acknowledges Richard W. Schwerdt, glancing into a bright, sunny afternoon outside the National Weather Service’s World Weather Building.

By Looking at Sky

That caution by Schwerdt, who produces forecasts for Washington, reflects the way people have forecast the weather for thousands of years--by looking at the sky.

But a good weather eye is getting a lot of help from sophisticated computers these days. The Weather Service forecasts are becoming increasingly accurate and detailed, looking further into the future.

“Forecasts of three days are now as accurate as forecasts of 1 1/2 days were a dozen years ago,” says Ronald L. Lavoie of the weather service’s Office of Meteorology in Silver Spring, several miles north of Schwerdt’s forecast center and the computer operation. Correspondingly, the five-day forecasts are as accurate now as the two-day predictions once were.

“Our data shows a steady improvement over a 20-year period in local forecasts for 24-, 36- and 48-hour periods,” adds Paul D. Polger, who is in charge of tracking the accuracy of the weather service forecasts. “The improvement is statistically significant.”

‘They Have Improved’

Joe D’Aleo, director of meteorology for The Weather Channel, an Atlanta-based national cable TV service, agrees. “I think they have improved, and the prospect is good for further improvement.

“Their ability to detect and communicate severe weather has improved with satellites and radar. Also, their communications system improvements allow them to more quickly process messages and then transmit forecasts and warnings.”

A big gain has been in the three-to-five-day forecasts, which simply didn’t exist a decade or two ago. That was just too far in the future to predict the weather with accuracy.

Now, Weather Service meteorologists routinely forecast the weather five days ahead with confidence--”with some skill,” in Lavoie’s words.

And, although they won’t call it a forecast yet, a group headed by Donald Gilman is even using computers to generate a weather “outlook” for 30 days and 90 days in the future, although it’s a pretty general picture.

Series of Plateaus

A chart showing how National Weather Service forecasts over the last few decades compare to what actually happened outdoors looks like a series of plateaus--a step up each time a new computer allows more detail to be used in calculating the current and future weather.

“If you’re looking at the progression over a period of time, 30 years ago we used fairly subjective means, drawing lines on charts and moving the lines forward,” Lavoie says.

The big computers came into their own in the mid-1950s and since then the improvement has been in steps. “Each more powerful computer allows a more realistic model to be run, including better physics and so forth,” Lavoie says.

The starting point for any forecast is, of course: What is the weather doing now?

To find out, meteorologists in more than a hundred countries around the world--and at hundreds of places in the United States--take detailed measurements twice a day.

Greenwich Mean Time

This is done everywhere at the same time, at noon and midnight Greenwich mean time--7 a.m. and 7 p.m. EST--so that the result is a detailed picture of the weather of the whole world.

Those calculations are shared by all countries and fed into computers, which calculate what is happening.

Then, using about six basic equations for winds, temperature, humidity, precipitation and other factors, the computer calculates what will be happening in the world’s weather in a few minutes and draws that worldwide picture.

From that, it calculates what the situation will be in another few minutes, and so on into the future, 10 or 15 minutes--or less--at a time.

The length of those forward steps varies, depending on the detail of the information and which weather data is being used, explains John Brown, chief of the development division at the computer operation.

Some steps move forward in 75-second increments while others don’t have to be calculated as often.

‘Faster Than Weather’

“It really takes a supercomputer to do this,” Brown chuckles. “You have to do it faster than the weather evolves, or you aren’t making a forecast.”

The computer calculates these steps for six hours, prints out a weather map of the expected conditions, then goes at it again to extend the conditions to 12 hours.

“The trouble is, of course, that there are simplifications in the physics and there are features that you haven’t been able to fully identify and observe, which grow and begin to make the projection incorrect,” Lavoie explains.

So, he says, “tat essentially produces a limit on the usefulness of the prediction, which is currently at about six or seven days.”

Beyond that, the larger patterns of temperature or moisture trends can be foreseen, but smaller things like a low pressure area or a cold front development may be missed.

Rerun Twice Daily

The basic models are rerun twice a day to use the new worldwide information being sent in, except for the longest term forecasts, which are done only once a day.

American forecasters have three weather models: more general ones cover the world, and a detailed model of the weather over North America looks at smaller areas with more accuracy.

But Lavoie stresses that “the responsibility for the forecast is at the local level, the local forecast office.”

While the big computer compiles worldwide conditions and reports what it thinks will happen across the nation, that information is merely guidance for meteorologists who must use their own local knowledge and training to produce specific forecasts.

Until three years ago, national forecasters added their interpretation to the computer models before sending them out to the local offices. Now the computer models have become so good they can be sent directly to local forecasters, who modify them with local information.

‘Depend on Data’

“Of course, the computers depend upon data, so one has to also give some credit to the improved data, satellite data for example, that is fed into the models,” Lavoie says.

Much of this improvement stems from the World Weather Experiment conducted in the late 1970s, a concentrated effort to see how much detailed weather data could be collected all over the planet.

“The further you go out in time, the more distant the influences that can affect the forecast are, so at five days and beyond we would definitely be affected by features on the other side of the globe,” Lavoie says.

“Most countries recognized the improvements that were available from this enhanced observing system, so they made an effort to maintain this level.”

When most folks think about the weather, they wonder if it will rain on the barbecue, if they’ll need an overcoat or whether snow will mess up traffic the next day.

How Much Snow?

“Of course, what people would like to know is the exact eventual snow depth to the nearest inch, and that’s still a very difficult proposition,” Lavoie says.

“One of the difficulties in this whole business is the variability, which is particularly extreme in the summer with showers and thunderstorms, but even in the winter, when people think that a storm produces fairly uniform results, we find that there are swaths just a few miles across sometimes in which conditions may be quite significantly different than elsewhere.”

Better computers have also improved the performance of the Severe Storms Forecast Center in Kansas City, Mo., which issues tornado and severe storm watches for the whole nation.

And the future looks even more promising with the impending arrival of Nexrad, the next generation weather radar that will give an even better picture of sky and wind conditions. Nexrad is expected to go into service in 1989, jointly paid for by the National Oceanic and Atmospheric Administration, the Federal Aviation Administration and the Department of Defense.

Advanced radars used experimentally in Oklahoma have provided vastly improved tornado warnings while reducing the number of false alarms, Polger says.

Forecasters Chided

Many people chide their local forecasters when they warn of an 80% chance of rain and the sun shines all day.

“When we put out a probability of 80%, I think in the public mind they certainly expect it to rain,” Polger says.

“In order to achieve 100% accuracy, all our forecasts would have to be expressed as either a zero chance of rain or 100% chance of rain and every one of them be correct.

“That’s completely unrealistic, because by nature these precipitation systems are spotty. And if you’re forecasting for an area, and you have showers moving around through the area, there’s no way that those showers are going to wet every part of that area.”