As a meteorologist, I’d like to respond to criticism about weather forecasting.

Readers may have noticed one example in a recent letter to the editor by Wayne Tangway (“Most weather forecasts are all wet” Sept. 20). The idea he believes the forecast was wrong because tornadoes and thunderstorms were forecast but there was only a quarter of an inch of rain at his Scarborough home speaks to the complete misunderstanding of how weather forecasts and probability work.

In the first place, meteorologists do not forecast to the  square-foot-area that a person’s home or condo or apartment building sits on. We forecast for large areas such as the 2,487 square miles of Cumberland and York counties.

When, for example, we forecast thunderstorms and tornadoes in a general area, we do so because there’s a non-zero probability of this type of storm. After decades of these predictions most people realize the “chance of thunderstorms” means you may or may not see one. Imagine if the measure of accuracy of a tornado forecast was a that a tornado would have to strike everyone’s home.

In general the higher the probability, the greater the likelihood your area will experience a particular type of weather. There are two components of probability; I can be 100 percent confident that showers will form, but not sure exactly what areas will receive them. Thus, a 40 percent chance of showers could represent the idea there will be a line of afternoon showers forming, but they could miss your town.  We’ve all seen Sebago lake get a summer shower that dissipates before ever reaching Portland.

This idea of probability in weather forecasting is no different than the probability ratings of the vaccines that most of us have now had.

If a vaccinated individual gets COVID, it doesn’t mean the vaccine itself doesn’t work. The vaccine dramatically lowers the probability you’ll get seriously ill, it doesn’t absolutely prevent it. In the same way epidemiologists get new information changing their recommendations, so too as meteorologists get new information throughout the week we also change our predictions and recommendations.

Consider the forecast of a snow event days in advance. As the forecast gets closer to the storm arriving the forecast is modified. The predicted snow could increase or decrease or the threat could even go away completely.  The uncertainty of a forecast decreases closer to the arrival of a storm.

This is why the cone of uncertainty for hurricanes is so much larger in a 10-day forecast than a two-day one. A week and a half in advance a forecast should not be relied upon to make definitive plans, those are still not much better than flipping a coin. Real statistical studies, which actually measure the overall forecast accuracy, continue to support the dramatic increases in the accuracy of a forecasting.  A seven-day forecast today is about as accurate as a three-day forecast back in the late 1970s.

You should expect a 3-day prediction forecast to provide about 90 percent accuracy. This doesn’t mean we don’t get it wrong some of the time, but those big misses are less and less likely.

As end users of the information we produce it’s always a good idea to check the latest forecast because as we receive new information things can change. Maine and all of New England is fortunate to have some of the talented forecasters in both the public and private sectors.  It may seem to some that forecasts are not accurate but feelings aren’t facts and the facts are meteorologists do an amazing job predicting the chaotic state of the atmosphere, something I’m proud to have been a part of for over 40 years.

— Special to the Telegram

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