Hurricane Florence’s 3-day rainfall was a less than .1% probability, "1000-year" event, analysis from The National Oceanic and Atmospheric Administration’s National Weather Service found.
"The fact that this event was greater than 1-in-1000 in such a large area is unusual," said Mark Glaudemans, Geo-Intelligence Division Director at NOAA’s National Weather Service Office of Water Prediction. "It’s one thing to have a heavy storm over your backyard or a parking lot in town. Heavy storms happen all the time in very small areas. But to have a heavy storm that’s this heavy over such a large area is an extreme event."
This event is not unprecedented, Kevin Trenberth, Distinguished Senior Scientist in the Climate Analysis Section at the National Center for Atmospheric Research, told NBC. "A thousand year event is based upon the idea that the climate isn’t changing," Trenberth said. "A lot of what used to be 1000 year events are now 100-year events or seventy-year events or maybe even fifty-year events, in the case of places like Houston," he added.
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Texas’ Hurricane Harvey from last year had some areas with "1-in-500- year" or "1-in-1000-year event" rainfall, NOAA reported. Hurricane Matthew, which made landfall in South Carolina as a Category 1 hurricane, also had areas with greater than 0.1% probability events, though they cover less of the map. The NOAA data in the Atlas 14 map is compared to data from up to 2004.
Glaudemans clarified the misleading "1000 year event" terminology. The probability is based on the three-day worst case period, and compares the observed precipitation with the expected probability of future precipitation. Simply put, this event has "a one in one thousand chance of occurring in a given year at a given location," he said, noting it does not mean this event will only occur once in a thousand years. "Next year, it can happen all over again," he explained.
"An event like Florence is extremely rare because you do not normally see tropical cycles with such an intensity, with so much precipitation, make landfall somewhere along the coast," said Dr. Jill Trepanier, associate professor in the Geography and Anthropology Department at Louisiana State University, expertise is hurricane climatology. "However, that rare event can still happen every year and then that rare event no longer becomes rare anymore. With a changing climate, what is rare now will shift."
North Carolina saw 8.04 trillion gallons of rainfall, NWS Raleigh tweeted on Tuesday. NWS Greensville-Spartanburg responded by noting that for three counties, the estimates for rain are too low. Emergency managers issued a new evacuation order in South Carolina Friday morning, AP reported. Hurricane Florence has caused 42 deaths since it made landfall last Friday.
"[These 1-in-1000 year events] are becoming increasingly common, unfortunately," Trenberth explained. "They occur in different places, in different times, and the phenomenon is always a bit different. But the fact is, the environment is warmer, it holds more moisture, and so the risk of these heavy rainfall events is going up."
"When it rains, it rains harder than it used to," he said.