Weather Forecasts

Weather Forecasts

Rain Today, Sun Tomorrow

Short-term predictions are much improved, say the weather pros

The days when you could claim that a flare-up of an old sports injury was a better barometer of weather changes than the forecasts provided by your local television station are fast-disappearing. Experts report that in recent years there have been significant improvements in 6- to 48-hour forecasts. “A two-day forecast is as accurate as a one-day forecast was in the beginning of the 21th century.

Meteorologists developing one- to two-day forecasts have become quite skilled at predicting whether or not storms are on the way, thanks to better short-term modeling techniques. But major errors still occur in predicting weather events that sit on the cusp between rain and snow. David Olson, Chief of the National Weather Service’s Weather Forecast Branch in Camp Spring, Md in the late 1990s, said in one occasion: “We can generally narrow the rain-snow line to some tens of miles, but to say that the southern suburbs of Boston will be rain and the northern suburbs will be snow is slicing it pretty thin.”

An old tree in a park. Photo by Elena

The National Weather Service’s three-to-five forecasts have also gotten better in the last decade due to improvements in t meteorologists’ ability to predict flow and pressure patterns in the middle part of the atmosphere. But measuring the accuracy of many types of forecasts can be very tricky. If a meteorologist says there’s going to be increasing cloudiness with rain before sunset, how can you measure this forecast’s accuracy? It’s almost impossible to put a number on publicly worded forecasts that have so much going on.

Even when forecasters predict the course of an upcoming weather event accurately they may still have difficulty pinpointing the time of arrival. Meteorologists say that they always hesitate to recommend that people call off their plans for a Saturday afternoon picnic on the basis of a prediction made on the Tuesday before. It may end up that the rain comes 4 hours earlier or 4 hours later.

Even more challenging is predicting the weather months in advance. Each month forecasters at the National Weather Service Climate Analysis Center in Camp Spring, MD, issue a forecast of average climatic conditions for the upcoming year. The Weather Service doesn’t try to predict individual storms, but does try to forecast whether the months ahead will bring above or below average temperature and rainfall. The success rates of these forecasts are just slightly better than flipping a coin, according to some meteorologists. They estimate than the National Weather Service’s temperature forecasts are correct 80 to 90 percent of the time, while its precipitation forecasts are correct 70 to 80 percent of the time.

Autumn. Photo by Elena

To be more accurate would be like trying to predict the exact landing spot of a piece of paper held horizontally and the released. Not even the best aerodynamicists could accurately predict where the paper would land. Not because they don’t understand the physics but because the results are inevitably and unbelievable sensitive to the exact position and orientation of when you let it go.

The ultimate goal of long-range weather forecasting is to provide people and governments with time to prepare for severe climatic changes. For instance, knowing that the warming of the tropical Pacific known as El Nino is on the way can prepare people on the West Coast for a tough winter of flooding and mud slides. Forecasters may also be able to prepare farmers for future droughts.

The National Weather Service would be content to be able to predict general climatic conditions up to a year in advance with 80 percent accuracy. It is counting on powerful computer models incorporating even-more data from ocean, land, and atmosphere to bring about such progress. That would be a sea change. Years ago, meteorologists point out, most people thought the weather was driven by something in the atmosphere. Only recently have people begun to realize the oceans’ influence on the wearther around us.