90-day predictions could go either way


December 31, 1991|By Douglas Birch

The National Weather Service's climate specialists issued their official prediction yesterday for the Baltimore region over the next 90 days. The result?

Your guess seems as good as their forecast.

There is a 50 percent chance that January, February and March will be colder than normal, said Tony Barnstone, a meteorologist with the prediction branch of the service's Climate Analysis Center in Camp Springs. There is also a 50 percent chance that warmer-than-usual temperatures will reign.

The prediction branch's forecast for rain, snow and sleet over the next three months is only slightly bolder. There's a 54 percent chance that precipitation will be above the 3 to 4 inches a month that typically falls in this area, Mr. Barnstone said.

Better than a coin toss, but not much.

Part of the problem is that Baltimore is sandwiched between Southern states, stretching from Virginia to New Mexico, where it's supposed to be colder and drier than in a normal winter, and Northern states, stretching from New England to the Pacific Northwest, where it is expected to be warmer and wetter than normal.

A narrow slice of the country from Maryland through Arkansas and California could "go either way," Mr. Barnstone said.

Partly, the problem is that long-range weather forecasting is a tricky business. A couple of small thunderstorms can -- you'll excuse the expression -- snowball into hundreds of storms lasting most of a day. Regional weather patterns are so complex that supercomputers can't number-crunch all the variables.

"It's very unstable, it's very non-linear," said Mr. Barnstone, who added that long-range temperature forecasts are never more than 70 percent certain. There is never more than about a 60 percent chance that long-range precipitation predictions will come true.

While uncertain about the future, meteorologists have 20-20 hindsight.

Fred Davis of the National Weather Service at Baltimore-Washington International Airport summed up 1991 this way yesterday: "warm and dry."

Mr. Davis said the past year was the second warmest at the airport since the weather service started keeping records there 41 years ago. Only 1990 was warmer.

If forecasts for the last 36 hours of the year hold, he said, the average temperature would be 57.7 degrees -- compared to the average of 55.1 degrees.

The year is also expected to wind up as the third driest at the airport, he said, with a total accumulation of 30.16 inches as of about 5 p.m. yesterday. The average annual precipitation is 41.84 inches.

The two driest years on record were 1954, with 27.89 inches, and 1965, with 28.22.

There was a lot less snow than usual in November and December. In fact, there wasn't enough to measure.

"I think we had a six-minute snow squall in December, and that was it," Mr. Davis said. In a typical year, 1.1 inches will fall at BWI in November and 3.6 inches in December.

Last winter, the Climate Analysis Center wrongly predicted the development of El Nino, a periodic rise in surface temperatures in the tropical Pacific that affects global climate.

Based on El Nino, meteorologists first predicted cooler than normal winter temperatures in the Eastern United States. But the weather pattern fizzled. The winter of 1990-1991 was much warmer than normal.

"It was a bad bust," Mr. Barnstone said.

El Nino, which occurs every three to six years, finally showed up last summer and is expected to affect this winter's weather.

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