Weather Forecasting: It Often Boils Down to Just a Guess

Barry Satchwell, his leg bobbing restlessly, flicked his gaze back and forth from the word processor in front of him to two sophisticated computers that melded hundreds of thousands of bits of data into maps of the world’s weather.

He had a tough call to make.

It was Thursday afternoon, March 5, and Satchwell, a 16-year veteran of National Weather Service forecasting, had spent the morning studying winds, humidity, barometric pressure, satellite pictures, radar observations and the weather service’s sophisticated projections of the next 48 hours.

He had concluded that a moist, slow-moving cloud band that had lumbered northwest from the lower latitudes all week was going to dump an unusually heavy load of rain--about two inches--on Southern California’s urban areas beginning late this afternoon. A flash flood watch had already been issued.

But now, half an hour before Satchwell’s 2:30 p.m. deadline for releasing the weather service’s afternoon forecast, some new contradictory data had to be considered. The storm would definitely hit, but it might-- might-- be significantly weaker.

“We’re going to ride with what we’ve got,” Satchwell decided. The forecast did not change.

Unfortunately, the weather did.

The big rains that had pounded Santa Barbara and Ventura Counties with up to 5 1/2 inches earlier in the day never found their way to Los Angeles and Orange counties. At the Los Angeles Civic Center, only .42 of an inch fell.

It was, in the jargon of the professionals, a busted forecast.

It was also a lesson in the frustrations of trying to make short-term weather forecasts for specific areas. It is a slice of meteorology that, amid a gleaming network of electronic data collection and analysis, is often just plain iffy.

Advances in the use of powerful computers have allowed government and private forecasters to significantly improve their predictions of next week’s weather, specialists say. However, the question of whether it is going to rain tomorrow in Oxnard or Santa Ana or Long Beach--and at what time, and how much--remains elusive.

One reason is that so far, many of the advances in forecasting have yet to be widely tailored to a scale conducive to local predictions.

For example, the weather service’s system of predicting global conditions by “computer modeling,” in which the laws of nature are distilled into mathematical equations, presents the world in sections hundreds of square miles in size, a necessity if computations are to be made quickly enough to have value. However that is not always helpful to a forecaster trying to pare down his view.

Beyond that, for all the orbiting satellites and the high-altitude monitoring balloons and the radar that senses the guts of clouds, the atmosphere itself--12 miles thick and agonizingly fluid--is simply too complicated for anyone to understand completely.

“Not in my lifetime. Not in my children’s lifetime,” said Art Lessard, Satchwell’s boss at the weather service’s Westwood office, where 30 employees are responsible for forecasting land, sea and aviation conditions for the southern half of the state.

Forecasting remains, as weather service prediction branch chief Donald Gilman said a couple years ago, not a science but “a technical art. . . . The tools we have to use are scientifically sound but they’re rather weak.”

All of which was on Barry Satchwell’s mind when he wrapped up his shift on that Thursday afternoon by saying good-naturedly, “I’m glad I’m not working tomorrow. That way I won’t have to wipe pie off my face.”

Satchwell, a well-regarded forecaster who gracefully translates the dialects of meteorology and computers into simple English, was not trying to be cavalier. He had, he believed, made the best call he could with the information he had to work with.

On its face, it was an impressive array of data.

When Satchwell came to work around 6 a.m., it was evident that the mass of tropical air was moving over the coast and producing rain in the Southland’s northern counties as it interacted with a low-pressure “trough” of lighter, relatively cooler air that created the instability needed to pull the moisture out of the band of clouds.

A colleague of Satchwell’s, Brian Finke, who specializes in monitoring a 1 1/2-year-old system that automatically reports the slightest trace of rain from 245 gauges in Southern California to a U.S. Weather Service computer, saw the pattern forming.

“We’ve had about 1 3/4 inches in the mountains of Santa Barbara County and three-quarters of an inch in the mountains of Ventura County,” Finke said as he checked the white numbers darting onto the his computer’s blue display screen.

Finke, a forecaster for 27 years, saw something else: the features of this storm system--big, slow, warm--were reminiscent of the violent storms that hit Southern California in January, 1969, and in 1982-83. “We’re in an El Nino regime,” he said, referring to a pattern of abnormally warm Pacific Ocean temperatures that affect weather patterns over the adjacent land masses. Additional flooding problems could be caused when the warm rain melted the mountain snow pack.

Satchwell scanned a drawing copied from a radar screen, showing where the rain had fallen so far. The storm was indeed shifting inland. He looked at the satellite pictures that had come in each half hour. They are printed on an old machine that was out of whack today (“it took me an hour to figure out where we were,” Finke groused), but the cloud mass was clearly intact.

Satchwell checked each of the three computer models sent electronically from the U.S. National Meteorological Center outside Washington, D.C., which receives raw weather data from throughout the world and then “crunches” the numbers into global forecast maps. The models agreed with each other, indicating that while the storm would weaken, it should still have considerable energy left as it headed east. The satellite pictures, which are useful in predicting the next few hours of a weather system’s behavior, confirmed the computer’s longer-range view.

The trough of low pressure that was buffeting the band of clouds was coming from the southwest. Like the clouds, it appeared to be heading east, making rain a 100% probability.

(One hundred percent probability means that it will definitely rain everywhere in a given area. If a forecaster thinks there is an 80% chance of rain and that about half the county will get wet, he expresses it in the daily forecast as a 40% chance. Forecasters know that the public generally does not understand this but, knowing that their words will be compressed by the news media, feel that a more complete explanation would never see the light of day.)

What insured very heavy rain, Satchwell said, was the fact that the back end of the low-pressure trough was moving faster than the front end. “It’s like a scoop,” he said, literally scooping the moisture out of the soggy clouds.

“We were discussing this morning whether we (will get) an ‘abundant’ or ‘copious’ amount of rain during the next 24 hours,” he said. “For the public, it’s going to be wet .”

There was, in the back of the mind of each forecaster involved with charting the weather on this day, the recognition of an odd pattern. During this rain season, most of the Pacific storm systems that headed toward Los Angeles had surprised forecasters by turning north at the last moment. Ventura had received twice as much rain as West Los Angeles, for instance.

Why this was happening was not clear. What was clear was that it could not be used to predict what a new storm would do.

“The first time you try that,” said Finke with the air of a man who has seen it all, “you’ll get your head handed to you.”

There was something else in the back of each forecaster’s mind, too, something that comes into play with every decision made in the weather service’s 11th floor office in the Federal Building: The West Coast is a lousy place to try to predict weather.

The reason is that, due to the way the Earth spins, most of our weather comes from the West. In a place such as Omaha, Neb., forecasters can predict a storm’s impact by what ground stations to the west report. By contrast, West Coast forecasters must stare out at an ocean with little such aid. In past years, the federal government has cut back money spent for buoys and balloons that provide data about ocean and atmospheric conditions in the Pacific, forcing forecasters here to rely more on satellite photos.

“With satellite pictures,” said Los Angeles weather chief Lessard, “you have to infer a lot. Once you do that, you may be inferring the wrong things. We just don’t have the data to play with.”

In addition, forecasters are far less skilled at predicting amounts of rain than they are at figuring whether tomorrow will be wet or dry, according to a study by John Gyakum, a University of Illinois professor of atmospheric sciences who specializes in studying short-term forecasting.

Given these adversities, Lessard was asked, is it realistic for a forecaster to try to predict a specific amount of precipitation?

Lessard, who has done nearly four decades of forecasting between his time in the Air Force and the weather service and has spent his last 10 years in the Los Angeles office, acknowledged the quandary.

“It’s kind of a subjective type thing,” he said. “There’s a lot of instinct. . . . Very seldom does 1 and 1 make 2 to every forecaster. It might be only 1 1/2 to someone else. Are you asking, is it beyond our capabilities (to try to be so specific)? You’re probably right.”

It was 1:10 p.m. Lessard, Satchwell, Finke and another forecaster responsible for aviation predictions had had their daily meeting and had heard Satchwell, that day’s designated “lead forecaster,” shape his proposed forecast. Now they returned to their computers to refine the details. Their small room was filled with the dull hum of the machines. There was little conversation. Satchwell, casually dressed in a short-sleeved, green, knit shirt, corduroys and loafers, gulped a diet soda as he typed out his forecasts.

Brian Finke, thin and bespectacled, stood up. “I have a sneaking suspicion this thing is ridging over us,” he said.

What Finke meant was that intense rains might be delayed in reaching Los Angeles, or might never come, because the barometric pressure was not falling the way it should.

Finke walked over to a computer screen. “I’d almost be willing to stake my merit badge that the pressure’s rising over us,” he said.

He checked the console for the weight of the atmosphere. Normally at this time of day the barometric pressure drops in response to the rotation of the earth. Today it had dropped only half as much as usual.

“The pressure is rising over us,” Finke said. “That’s not good.”

It was not good if you were predicting heavy rain in Los Angeles. The warm, heavy cloud band was stable. It needed a direct hit from the low-pressure trough to create the maximum instability. If the trough veered away, Satchwell thought, it would be no more possible to have heavy rain than to have a blazing fireplace if somebody suddenly closed your chimney.

“The problem now,” he said, “will be determining whether the system continues to move onshore before this smothering effect takes effect and we lose the support for the rain. There’s gotta be upward vertical motion in the atmosphere. The air is so moist that just coming into the coastal mountains will produce that upward vertical motion, so we’ll get rain. But you need that upper-level funnel, that movement upward in the atmosphere, to generate significant rain, and that is on the diminishing side right now.”

It came down to a question of degrees--precisely how would weakening of the low-pressure trough affect its interplay with the clouds?

It was still certain that there would be rain. Outside, it was already coming down. Satchwell had time to downgrade the heavy rain in his daily forecast, but he did not think he had enough evidence.

Part of his reasoning was a forecaster’s reluctance to issue what are called “yo-yo” predictions. It’s part of what Satchwell refers to as the politics of weather forecasting--trying to maintain credibility while being accurate.

“Once you put out a forecast that a significant weather event will happen, Mother Nature has a way of sort of giving you all sorts of input that would make you think maybe that’s not the right thing,” he said. “So you back off, and then it does come, and that actually makes you look worse in the eyes of the public.

“The weather is a very, very fluid situation. The only way a forecast will ever be perceived as accurate is if the public receives it every hour. One of our biggest problems is the time lag. It’s a real Catch 22.”

Besides, he thought, echoing a sentiment voiced by many forecasters, the public stands to lose less if the forecast errs on the side of pessimism. City street maintenance supervisors, county flood control officials, law enforcement--it is better for them to be prepared than to suffer from a false sense of security.

It was 2:30 p.m.. The next shift came on. “I may have left you with an overly wet forecast,” Satchwell said to the man who took his chair, and as the day passed and the light rain fell, that surmise proved all too accurate.

The high-pressure trough veered more to the north than expected, so far less of the clouds’ moisture was shaken out over Los Angeles and parts south.

Simply put, said Lessard, a New Englander given to pleasant informalities, “the system pooped out.”

As far as the National Weather Service’s record-keeping system was concerned, Satchwell’s forecast went into the books as an accurate one. He had predicted rain for Southern California, and it did rain. Anytime a forecaster predicts a better than 50% chance of rain in the coming 24 hours and precipitation is measured, regardless of the amount, it’s a hit. By this system, short-term forecasts are correct about 85% of the time nationwide, the government says.

To Satchwell, though, it was a bust. Of course, he noted philosophically, it could have been worse.

“I remember in ’79 I was forecasting in Louisville, sitting in front of a terminal and I saw a huge thunderstorm developing to the north,” he said. “This thing looked very ominous, and in that area tornadoes are a very big thing, people are very scared of them. So I issued a wind damage warning.

“It was a 4th of July, lots of people out and around, and here all the restaurants are pushing people into the basements, leaving all this food on the tables--and it didn’t even rain.

“Like I said, this wasn’t the first time I’ve gone home with pie on my face.”