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Florence is now “only” a Category 2 hurricane. That won’t matter much

When it comes to water, wind speed matters less than the size of the wind field. Ten years ago today, Hurricane Ike made landfall as "only" a Category 2 hurricane along Galveston Island near Houston. Though Ike had 110mph winds at landfall, it had grown very large over the Gulf of Mexico, and this large size allowed it to develop an enormous amount of "integrated energy" that manifested itself as a devastating storm surge. With about $30 billion in damages, Hurricane Ike was, at the time, the second-costliest US hurricane on record. As of Thursday morning, Hurricane Florence has weakened to 110 mph as it contends with slightly increased wind shear and drier air. Technically this means Florence is no longer a "major" hurricane, and it may not be when it reaches the North Carolina coast early on Friday morning. Practically, however, that won't matter when it comes to storm surge and inland rainfall. The simplest, most common metric for the measurement of a storm's intensity is maximum wind speed, and certainly this matters in terms of pure destructive potential when it comes to, say, losing a roof or propelling debris through the air. But when it comes to water, the maximum wind speed matters less than the size of the wind field for both storm surge and the destructive power of waves moving onshore. And Florence is a large storm, with hurricane-force winds extending outward up to 80 miles from the center and tropical-storm-force winds outward up to 195 miles. The primary example of this is 2012's Hurricane Sandy, which had modest 80mph winds by the time it reached the New York and New Jersey areas. But because the storm was so large it produced a destructive storm surge in New York City and caused an estimated $65 billion in damage in the United States. (At the time, it became the second costliest US hurricane but has since been supplanted by Harvey and Maria, both in 2017). Thursday, September 13th is the 10 year anniversary of Hurricane Ike. The storm's 15'+ surge devastated the Bolivar peninsula. The outer banks of North Carolina may face a similar threat from Florence. Our thoughts are with you Carolina. Prep for the worst, and hope for the best The area to the north of Florence's landfall along the North Carolina coast—storm surge is always worst to the "right" of a storm's center, where its rotation pushes water onshore—is less populated and developed than the New Jersey and New York shores. But the storm surge will be no less devastating for homes and businesses located there.  Areas from Cape Fear to Cape Lookout face a potential 9 to 13 feet of storm surge, the National Hurricane Center warns, on top of the normal tides. Florence's slow movement over the next two or three days will add another element of uncertainty. Meteorologist Greg Diamond has noted that areas along the coast of North Carolina on the right side of the storm could experience a significant storm surge through multiple high tide cycles, which would have a cumulative battering effect. The National Hurricane Center has, accordingly, warned that "Life-threatening, catastrophic flash flooding and prolonged significant river flooding are likely over portions of the Carolinas and the southern and central Appalachians late this week into early next week, as Florence is expected to slow down as it approaches the coast and moves inland. " It is important to recognize that, while attention will be focused on coastal impacts from Florence, inland flooding will remain a problem for some areas well into next week.

A quick simulation of Hurricane Florence done without climate change

Simple analysis suggests global warming boosts Florences rain by 50 percent. In the last few years, teams of scientists have developed a consistent protocol for rapidly analyzing the influence of climate change on extreme weather events. Within a week of the disaster, reports have been available to inform the conversation about whether we can expect more events like it in the future. But on Wednesday, we saw the first example of something new—an analysis published before the event even happened. A group led by Stony Brook Universitys Kevin Reed ran a very simple computer model experiment on Hurricane Florence—which isnt due to make landfall until Friday—and quickly released the top-line results. The rapid studies weve been seeing are done by examining the historical weather record to estimate how rare and extreme a given storm or heat event would be in that area of the globe. From there, climate model simulations are used to see if climate change is expected to change the frequency of that type of event. In this case, theres obviously no data available for a thing that hasnt happened yet. Instead, the researchers focused on a much more limited question that is faster to answer: how does a warmer world change this storm? In the counterfactual world where global warming never happened, its impossible to say if Hurricane Florence would even have been born. Even small changes can have complex consequences on the atmosphere, such that events would play out completely differently. But thats not the point. Since Hurricane Florence is occurring in this warmer world, we can simply examine the effect of warmer temperatures. To do this, the researchers took the current state of the world on Tuesday, dropped that into their model as a starting point, and pressed play to simulate ahead to Sunday. For a comparison simulation, they took those starting conditions and essentially subtracted out global warming. In this counterfactual world, the storm looks significantly different. Hurricanes are fueled by energy from the evaporation of warm seawater, so its no surprise that warmer sea surface temperatures should give the storm a boost. The size of the boost in this case is pretty remarkable, though. The model analysis showed the real-world Florence dumping 50 percent more rain near the coast than it would in a world without human-caused warming. The modeled hurricane clearly stays stronger when simulated under current-day conditions, but its also larger. The diameter of the storm is about 80 kilometers (50 miles) greater than in the cooler simulation, which would translate into higher storm surge flooding on the coast. Team member Michael Wehner told Ars that the team is working to repeat this analysis with updated observations as the storm barrels down on the Carolinas, so we'll get to see how similar the results are for each iteration. The researchers also plan to repeat their work after the storm and carefully compare with the forecast analyses. That will help show how useful this trial run of pre-storm analysis was. For their part, the group behind the within-one-week studies explained Thursday that they wont be providing an analysis of Hurricane Florence in the near future (for reasons ranging from complex historical data to swamped workloads). But they did comment on these pre-storm results, writing, More analyses are needed to assess the robustness of this quick analysis, although the basic result that global warming increases the precipitation is a very robust one supported by observations and modelling studies.