How Forecasters Nailed Harvey’s Massive Rain Dump

Houston generally gets about 50 inches of rain a year, at least before Hurricane came to town. Normally it takes 104 days of rain to get that much moisture, so no one could have imagined that much falling in just a few days.

No one, that is, except America’s principal computer weather-forecasting system. And a bunch of meteorologists, who issued sharp warnings of the disaster to come.

The alert from the Global Forecast System (GFS) of the potential for a seemingly unthinkable 50-inch downpour arrived in meteorologists’ computers on Aug. 23 — two full days before Harvey roared ashore. The prediction went out almost immediately, starting with climate scientist Ryan Maue of WeatherBell Analytics, who tweeted: “Exact amount unknown, but this ballpark extreme value rings alarm bells Harvey.”

Though forecasters hedged a bit from the largest possible totals, they left little doubt about the impending threat. That same morning, a Wednesday, AccuWeather, a consultant to businesses and media, warned of possible “life-threatening” floods. The Washington Post’s weather blog foreshadowed how the deluge would linger: “Harvey will stall over the Texas-Louisiana area through most of the weekend.” Al Roker of NBC News warned of “a major flooding event.” By the next day, the network predicted 30 inches of rain.

The ability of weather forecasters to nail the magnitude of the threat posed by Harvey days in advance was seen as a marked improvement in the forecasting business, one that is being celebrated in their close-knit community. They did a particularly good job in plotting the track of the storm and its rainfall potential, which have traditionally been among the most difficult tasks for forecasters.

But as the devastation from the storm became a reality, meteorologists raised other important questions: Could their warnings have been even stronger? Is more imagination (or virtual reality?) needed to penetrate the public consciousness? And why didn’t public officials do more?



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