What if a forecaster doesn’t bring up a percentage but instead refers to a “slight” or “likely” chance of rain? Those qualifying terms actually refer to a range of probability-of-precipitation percentages. “We do want to highlight potential risks that could have an adverse impact.” Regardless, it’s information the weather service has to relay to the public. So, if no precipitation shows up, it doesn’t mean the forecaster was wrong. “Because we live in a place where it’s much more common for it not to be raining, people tend to focus on the thing that’s less frequent and could have a higher impact,” Blier said. What’s also important to consider is how most people perceive the term “chance of rain.” If a meteorologist says there’s a 30% chance of rain, that also means there’s a 70% chance - and a higher likelihood - that it won’t rain at all. “The output is provided in a way that there still are places where the human forecaster can make adjustments,” Blier said. The computer-generated models are only a starting point, though. And it’s not just used for rain it also helps meteorologists get a better probabilistic take on other weather parameters like temperature, cloud cover, wind and humidity, Rowe said. The latest version, which was adopted by meteorologists across the country in early 2020, is “a sophisticated, post-process computer weather forecast model output” calibrated by a series of numerical weather prediction models, Blier said. In recent years, the weather service has shifted toward a newer system known as the National Blend of Models. However, both Blier and Rowe described that method as more of a hypothetical and bygone way of determining the forecast. Douglas Zimmerman/SFGATEīut how do forecasters calculate rain percentages? There’s a formula that goes something like this: Forecaster confidence times aerial coverage of expected precipitation equals the chance of rain. Pigeons sit atop a structure at Portsmouth Square during a rainstorm in San Francisco, on March 28, 2023. “It might be a chance of rain across an area in which many locations are included, but I would do the interpretation from a spot-on-the-ground standpoint: ‘Hey, where I am, there is a measurable chance of rain or will be a measurable chance of rain where I will be.’” “When we say ‘chance of rain,’ in a sense, we’re really saying a chance of rain for a particular location,” Blier explained. This is likely due to the design of rain gauges, which are marked with clear, easy-to-read lines that count every hundredth of an inch of collected rainfall. Warren Blier, who has been a science and operations officer for the National Oceanic and Atmospheric Administration office in Monterey for 25 years, said that measurable quantity of rain has been the standard for as long as he can remember. The phrase actually refers to the probability of precipitation, Rowe explained - that at least one-hundredth of an inch of rain will fall somewhere in the forecast area within a given time frame. “It’s rather complicated and complex and depends on who’s talking,” said Scott Rowe, a meteorologist with the National Weather Service.
0 Comments
Leave a Reply. |