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Where does Dorian go from here? The forecast and ties to climate

Dorian is causing storm surge and beach damage to the northern Florida east coast midday on Wednesday, and will begin moving toward the Carolinas Thursday. But it’s safe to say things have gone much better for Florida than what we were looking at a week ago in computer models, including the European. It has stayed far enough offshore to spare Florida its direct eyewall impacts, as more recent models have indicated for days. By comparison, here is the way things looked on August 28th.

Nonetheless, its devastation on Grand Bahama and Abaco islands is beyond anything most of us have seen, due to its ferocity and unprecedented lengthy stalling. More on that in a bit.

Where does Dorian go from here, and how big a threat does it pose? As the SE coast extends farther out to the east, and storm recurves more to the NE, it is inevitable its center will draw nearer to the coast and the chances of an actual landfall increase.

The National Hurricane Center/NHC hurricane warning now extends from part of the Georgia coast up to the North Carolina-Virginia border. Tropical storm to hurricane-force winds will become more likely, particularly by Thursday afternoon into Friday near the South and especially North Carolina coasts. The storm surge warning extends north to the NC-VA border as well. The greatest risk of an actual eyewall landfall appears to be on the North Carolina coast and the Outer Banks. Dorian is expected to remain at Category 2 intensity throughout, with top winds of 100 to 105 mph and its storm surge is very likely to cause significant damage. Its forward motion will be faster than it’s been, but the real acceleration will not begin until Dorian begins moving away from the east coast.

The role of a warming climate in the evolution of Dorian’s monstrous intensity and lengthy stall appears to be significant. No, the warming climate did not cause the genesis of Dorian. As most of you know, late summer is the peak period for Atlantic basin hurricanes, and has been long before warming accelerated. That’s not what is unusual. What has changed are several climate-tied factors. First, there are sea surface temperatures. The top 700 meters of the world’s oceans have absorbed 90% of the excess heating from global warming and are, by far, the greatest collector of this warmth. This is explained in this NOAA link.

This oceanic warming doesn’t mean there are or will be more hurricanes in total. The warming increases the probability of more intense hurricanes, with much more heat energy available to fuel hurricanes in an otherwise favorable environment with low wind shear and abundant atmospheric water vapor. This is not to say all Category 5 hurricanes have strong ties to global climate trends, as there were some ferocious hurricanes prior to this ongoing warming period. But the probabilities for more Category 4 and 5 hurricanes has increased due to the larger oceanic warm temperature anomalies, and the longest string of consecutive years in which at least one Category 5 formed in the Atlantic runs from 2016 up until the present. All the waters colored yellow to orange have near-surface temperatures above the long-term average.

Then there is the issue of the slowing or stalling of tropical cyclones  becoming more common, as has been pointed out to me by distinguished Penn State climate scientist Michael Mann. The bottom line is stated by NOAA scientist James Kossin in a recent study in Nature: “As the Earth’s atmosphere warms, the atmospheric circulation changes. These changes vary by region and time of year, but there is evidence that anthropogenic warming causes a general weakening of summertime tropical circulation.”

In particular, the predicted and verified greater warming in the arctic is now known to have frequently weakening upper air winds, including tropical steering currents by lessening the thermal contrast between the polar region and mid and lower latitudes. This lessened contrast slows those winds, and models are picking up on this new climatology. The virtual stalling of Dorian near the Bahamas was well predicted by global models days in advance of its occurrence. Similar model forecasts were made with the second costliest hurricane to hit the U.S. of all time, Harvey, in 2017 and Florence in the Carolinas last year. There is already confirmed evidence these stallings are occurring with greater frequency as warming has accelerated. As reiterated by Jennifer Francis, formerly with Rutgers and now with the Woods Hole Research Institute, who told the New York Times: “This is yet another example of the kind of slow-moving tropical systems that we expect to see more often as a response to climate change. Upper-level steering winds are slowing over the continents during summer, so stalling weather systems are more likely.”

We have witnessed what occurs with this slowed atmospheric circulation over populated regions. There is prolonged exposure to the horrific elements from a hurricane. There are longer periods of tropical deluges, destructive winds, and storm surge that ran, in the case of Dorian, to as high as 18 to 23 feet above ground level. In the case of Harvey, its stalling led to the worst U.S. flooding yet observed, with $125 billion in damages and 103 deaths in Texas.

Of course, there is also the role of directly climate-related risen and rising sea levels adding to the destructive potential of all storm surges.

Again, the links between hurricane intensities, movement and a warming climate require much additional research for greater detail. Past monster hurricanes occurred before this lengthening era of a mean warming climate began to accelerate, so not all impacts in all Category 4 and 5 storms have strong climate ties in all cases. It must be remembered, though, the physics of a greater supply of heat energy from warmer oceans leading to more water vapor through evaporation and greater available energy for intensification are not overly complex, and are irrefutable when the other environmental conditions for storm development are favorable. Over the coming decades, we may not see more hurricanes in general. But we are likely to see more major hurricanes with more damage.

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