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"In a globally warmed climate, the high latitudes are warming faster than the tropical regions" as a result of the ice loss, he said. The resulting reduction in the difference in temperatures between the Arctic and the tropics reduces the wind speeds of the jet stream, he said. "That diminishment is realized not so much in an overall average [that is] a little bit slower each year but in a less stable set of wind patterns, a more meandering jet stream," he said. That meandering, he added, affects the forward movement of tropical storms and hurricanes. Sally certainly fits that pattern. But Ed Rappaport, deputy director of the National Hurricane Center, said forecasters have already folded any potential effects into their forecasts. "Any such changes on a day-to-day basis are already incorporated by the computer models" used by forecasters to determine a hurricane's track, intensity, rainfall and surge, Rappaport said. "If the atmosphere is warmer, then the computer models start with a warmer atmosphere." The forecast models also captured the blocking high pressure that caused Sally to the stall, with updated models predicting it would move imperceptibly east as it hugged the shore for several days, he said. "They’re working with whatever the weather is at that particular time. And so any impact of climate is already implicitly included in what the models are doing right now," he said. Rappaport added that some of history's more intense storms, including Harvey in 2017, Michael in 2018 and even Camille in 1969, saw effects greater than Sally from traveling over pools of warmer-than-normal water associated with the "loop current," a column of much warmer water that often breaks off the Gulf Stream and floats around the Gulf during hurricane season. Hurricane Sally does not seem to have floated over such a deep, warm water area in the Gulf. Even so, the water temperatures have been in the upper 80s in areas close to the northern shoreline this year. Sally fit a more common pattern of storms approaching the northern Gulf Coast, researchers said. "We're used to storms where they're moving from the east in the tropics, approach from the east for many days, and then they start their dance towards the north as they start to be influenced by the mid-latitude weather systems," said Frank Marks, director of National Oceanic and Atmospheric Administration's Hurricane Research Division in Miami. As a storm approaches the Gulf shoreline, a stronger low-pressure system that's traveling west to east across the U.S. continent will either grab the storm and move it north and east, or pass by too quickly to affect its course, he said. In Sally's case, a low-pressure system moved out of the central U.S. towards the East Coast, allowing high pressure to build in behind it, and that blocked Sally from moving north, he said. "When that happens, all bets are off," Marks said. "If they're not being pushed and shoved around by the atmosphere, and the atmosphere is favorable, they're going to intensify. That's just the nature of the beast." While Sally didn't hit the loop current, the high temperature of the water over which it was traveling, still well in the mid- to upper 80s, likely helped the storm to strengthen -- even though Sally itself was trying to pull cooler water up to the surface with its circular core of thunderstorms. Then it just became a question of how long it would be before a hole in that high pressure to the north opened to allow the storm to begin moving north again. At that point, the effects again were the result of local weather conditions, but with a bit of climate change mixed in. Marks' rule of thumb for rainfall produced by a storm is about 7 to 10 inches falling in its core over 6 to 12 hours. When a storm is moving at more normal speeds, 5 mph to 15 mph, that results in the rain being distributed over a broad area. Sally's 3 mph stall speed resulted in much greater rain production in some areas. And the production was exacerbated by the climate factor: Warmer temperatures in the atmosphere in the Gulf allow a hurricane's thunderstorms to hold more moisture and produce more rain. The result was as much as 30 inches of rain in Pensacola, Florida, where the core's rainbands repeatedly crossed the same areas. Marks is not convinced that the recent spate of hurricanes delivering intense amounts of rainfall means a rapidly increasing trend towards more such events, driven by global warming. He points out that there are similar examples, dating back more than 40 years, of tropical storms and hurricanes delivering huge amounts of rainfall. Louisiana state climatologist Barry Keim points to one much older example, Hurricane Easy of 1950, back when the Weather Service designated storms using Army letter names. Easy did a loop-de-loop over southwest Florida, dumping 45 inches of rain on Yankeetown. But it's clear that global warming played a role in other aspects of Sally, such as storm surge levels that were slightly higher because of sea-level rise. Even there, it was Sally's stall that had the biggest effect, though, with the storm's winds pushing surge water toward the coast through at least two high-tide cycles.
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