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Environmental Issues

The truth about the weather

Meteorologist Dave Unger offers advice on what to believe when it comes to predictions.

June 24, 2014
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Every year, we are reminded that the biggest concern for carwash operators is the weather. And, as climates, conditions and temperatures can greatly vary from year to year, the question is: What are carwash operators supposed to do? Can local weather forecasters be trusted? How accurate are their predictions? And, how far in advance can you plan for a rain-filled day? Meteorologist Dave Unger of the Climate Prediction Center, offers up the following insight to help you figure out your local weather forecasts and plan accordingly.

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When are predictions accurate?

The weather is predictable to around 14 days, said Unger. However, after about eight days we can be off by one day. “That makes relying on weather forecasts difficult when planning specific events,” he said. “Imagine, for example, that an event is moved from Saturday to Sunday on the basis of a forecast of rain eight days ahead. We meteorologists know that the weather system that is predicted to cause the rain can easily come one day early or one day later than predicted, since the event is more than a week away. If the rain event comes later, Saturday could have been a nice day, and Sunday could have been rainy.” So, in spite of the fact that weather systems are predictable out to a good part of what is called "week 2" (The period that begins eight days from the forecast issue time and extends to 14 days), you can only get a general sense of the weather on a day-to-day basis for about seven days or so.

Climate forecasts vs. weather forecasts

Beyond about 14 days the "weather" becomes unpredictable except in some unusual circumstances, said Unger. “We then have to rely on ‘climate forecasts,’ which are quite different from weather forecasts.”

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Climate forecasts, according to Unger, give a range of possible weather events, but this is not an actual forecast of specific weather events. “We usually express this range as the change in the odds or probabilities of the weather. In most cases the shift is slight. So, for example, if we expect the winter to be warm, we would expect that the range of weather events in, say, Philadelphia, to be more typical of the range of weather in, say, Washington, 100 miles to the Southwest.” Therefore, a Philadelphia resident who moved to Washington over the winter would hardly notice the difference. “Over the long run they would notice the heating bills to be a little lower, but most likely would not notice differences in day-to-day weather.”

Likewise, Unger added, the weather in Washington varies from year to year. A cold year in Washington is still cold by Philadelphia standards, it is just that there are fewer of them.

“Any strategy to take advantage of a climate forecast will have to be a long term strategy, plans that yield results after years, and not a ‘bet the farm’ type strategy,” said Unger.

Are major weather phenomena events predictable?

Once in a while we can make a forecast of a noticeable climate anomaly, (an unusual climate event) even in a single year, said Unger. This mostly happens in an El Niño or La Niña year. “Florida, for example, gets quite a bit drier in the wintertime during La Niña years, and much wetter than average in an El Niño winters. Even a single year of a moderate or strong El Niño or La Niña is usually quite noticeable. This is true in Arizona, New Mexico, Texas and Florida (and surrounding areas to a lesser extent.)” Unger said they express these by making high probabilities (high confidence) in their forecasts and these probabilities are displayed in their seasonal outlook maps.

The advancements of weather predictions

Weather forecasting is a lot better than it was 10 or 15 years ago, said Unger. “Today's seven-day forecasts are about as good as a five-day forecast was in the year 2000, and is better than three-day forecasts were in the 1970s.”

Precipitation forecasting, according to Unger, is a bit more difficult than temperature forecasting. “Our methods of precipitation forecasting have changed over the years, making it difficult to get a long-term series of forecast verification in the same way that we have for temperatures.

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