By Paul Homewood
Tropical Storm Sally has left more than half a million Americans without power as its torrential rains and storm surges lashed the US Gulf coast.
Sally weakened after it made landfall as a Category 2 hurricane on Wednesday, but the slow-moving storm continues to batter Florida and Alabama.
One person was killed and hundreds were rescued from flooded areas.
Pensacola, in Florida, was badly hit, with a loose barge bringing down part of the Bay Bridge.
"Catastrophic and life-threatening flooding continues over portions of the Florida Panhandle and southern Alabama," the National Hurricane Center (NHC) said.
The storm has brought "four months of rain in four hours" to the city, Pensacola fire chief Ginny Cranor told CNN.
One person died and another was missing in the town of Orange Beach, Alabama, the mayor said without giving further details.
Sally made landfall at Gulf Shores, Alabama, at 04:45 local time on Wednesday, with maximum wind speeds of 105mph .
According to the NHC, Category 2 hurricanes have sustained winds of 96 to 110 mph. The NHC says a Category 2 storm’s "extremely dangerous winds" usually cause damage to homes and shallowly rooted trees.
The storm later become a tropical depression with winds decreasing to 35mph, but it has been the torrents of rainfall and high storm surges that have caused most damage.
Rainfall is being measured in feet rather than inches in some places, but 18in (45cm) has been recorded across many areas.
Is climate change causing the storm’s slow pace?
John De Block, at the National Weather Service in Birmingham, Alabama, told the New York Times that Sally was drifting "at the speed of a child in a candy shop".
Sally’s pace may be linked to climate change, according to experts. A 2018 study in Nature magazine found that the speed at which hurricanes and tropical storms move over an area had decreased by 10% between 1949 and 2016, a drop that was linked to an increase in total rainfall.
"Sally has a characteristic that isn’t often seen, and that’s a slow forward speed and that’s going to exacerbate the flooding," NHC deputy director Ed Rappaport told the Associated Press.
It’s funny how global warming always makes the weather worse. As there is no evidence hurricanes are getting more powerful, the BBC dredge out obscure claims that they are slower moving, and therefore dump more rainfall.
The highest rainfall from Sally is given as 18 inches, but this is par for the course for hurricanes. The daily record for Alabama is 32.52 inches in 1997 during Hurricane Danny. NOAA point out that many higher totals have been recorded in the US, but not at official stations.
The storm surge was also pretty low at around five feet. Really powerful storms can often bring 20 feet and more. In reality, Sally was no more than a run of the mill hurricane, with a minimum central pressure of 965mb.
As for claims of higher rainfall from hurricanes, there is no data to prove this, given that by their nature hurricanes are rare, small scale events. But we can get a clue from NOAA’s Climate Extremes Index.
The chart below measures :
Twice the value of the percentage of the United States (or region) with a much greater than normal proportion of precipitation derived from extreme (equivalent to the highest tenth percentile) 1-day precipitation events.
[Twice the value is used, as other components of the index have two extremes, eg hot/cold or dry/wet. ]
The above chart is for the Southeast during the hurricane season, and indicates that there has been no real trend in extreme daily rainfall, from whatever cause.
And a look at daily rainfall plots for long running sites in that part of the Gulf Coast does not seem to offer any obvious trends either.
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September 17, 2020 at 01:03PM