Handle weather Forecasting terms can be confusing
No matter how you get your weather information, be it TV, radio, print or the Internet, you are sure to come across plenty of weather jargon when it comes to forecasting precipitation. I’ve discovered that for the most part, people just want to know if it’s going to rain (or snow) or not. It would make sense that this piece of information would be clearly presented in one way or another. Unfortunately, the use of percentages as well as some common “weather phrases” can be confusing. Would you plan an outdoor picnic under the threat of “isolated showers?” What about “scattered showers,” “widespread showers,” or “occasional rain?” Just how much is a slight chance anyway? And finally, how can the forecast call for a 60 percent chance of rain when it is currently raining cats and dogs at my place? Don’t feel bad if you are confused. In my 10-plus years of being involved in weather and television, I’ve heard the myriad of ways that meteorologists try to convey the rain/no rain scenario. I am guilty myself at throwing out terms such as “a few showers,” which may seem vague to the listening public.
The good news is that there are actually some real guidelines when it comes to stating the probability of precipitation (or PoPs as the meteorologists call it), the percentages used and the accompanying terminology. The National Weather Service forecasters follow these guidelines, and more often than not, the forecast you hear from various media outlets, comes from them. The bad news is, that unless a meteorologist is working for the government, he or she can throw out whatever numbers and phrases he or she wants. This leads to many inconsistencies in the way a weather person conveys your chance of getting wet.
The most confusion seems to stem from the use of percentages in a precipitation forecast. Just what does a 70 percent chance of rain mean? The weather service actually has a two part definition for this percentage. Before any precipitation (let’s assume rain for the remainder of this discussion) has begun, the 70 percent indicates the probability that a particular forecasting point will receive at least .01 inches of rain during a designated time frame. This percentage is also matched to the phrase “likely chance” in the forecast. After rain has begun to fall, the 70 percent shifts from indicating a probability, to describing an area coverage, which in this case would be considered “numerous” or “widespread” showers. The complete list of percentages and their matching terms are as follows:
“10-20 percent refers to a “slight chance” or isolated coverage
“30-50 percent refers to a “chance” or scattered coverage
“60-70 percent refers to a “likely chance” or numerous or widespread coverage
“80-100 percent pretty much means it’s going to happen, and everyone’s going to get wet
The waters start to get muddy (no pun intended) because some weather people have a different interpretation of PoP forecasting, and use the percentages based on their own definitions. I have heard that a 70 percent chance of rain means that it will rain over 70 percent of the forecast area. I have also heard that it means it will rain for 70 percent of the designated time period.
In the end, sometimes the weather person can’t win no matter what he says. In a “showery” weather event, it is likely that even within one city wide area, some people will get rained on and some won’t. There will always be folks who think we got the forecast wrong. I’ve been told by my predecessors, however, that it is much better to err on the side of bad weather. You will hear fewer complaints if a forecast rainy day turns out mild and sunny, than the other way around.