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    David Epstein

    Don’t be fooled: Florence’s lower category doesn’t take rainfall into account

    There are generally three components to a hurricane: strong winds, storm surge, and heavy rain. The problem with the Saffir-Simpson category scale is it doesn’t take into account rainfall.

    Even if Hurricane Florence’s category weakens, or if the storm no longer maintains its hurricane status, the amount of rain can still be biblically large. We only need to remember that tropical storm Irene was not a hurricane when it created tremendous damage to homes and covered bridges in Vermont in 2011.

    Next week, when Florence has finally passed the Carolinas and moved into the Ohio Valley, the rain will have been the big story.

    It’s still a big storm

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    Florence continued to weaken overnight, but it’s still a hurricane and will probably maintain that status when it reaches the shoreline, though not as Category 3 or 4 — perhaps not even as a Category 2.

    Hurricane Florence weakened overnight but is still a powerful storm. (COD Weather)

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    Nevertheless, torrential rain will be moving ashore Thursday, continuing into Friday, and dumping 1 or perhaps even 2 feet of rain in some areas. Of course, in these situations there can always be isolated spots that receive significantly more rainfall.

    This morning, the radar loop out of Morehead, N.C., is beginning to show those bands of heavy rain closing in on the coast.

    Rain was headed for North Carolina early Thursday (NOAA)

    It’s the water that’s the most worrisome

    Once the rain hits the coast, it will accumulate very quickly, causing some flash flooding and eventually probably overwhelming streams and rivers in the areas that get the most. The GFS model has 20 or so inches of rain falling over the next couple of days, and you can see how that rain accumulates over time. The axis of heaviest rain may shift further south or west, but the extreme rainfall is going to happen.

    Rainfall of nearly 2 feet will accumulate in North Carolina. (COD Weather)

    Florence’s present track takes it inland Friday. Because the storm was so strong earlier in the week, it built up a wall of water that will push inland as storm surge. The surge won’t be as bad as it potentially could have been, however, and the winds won’t be as strong. All that said, a saturated ground, even with tropical-storm-force winds, will uproot trees. Older homes and less-reinforced homes will sustain some wind damage as well.

    The latest storm surge has some areas under more than 8 feet of water.
    Coastal Risk Assessment
    The latest storm surge has some areas under more than 8 feet of water.

    Are slower storms a thing of the future?

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    The problem from this hurricane is going to be its slow movement.

    Hurricanes have a lot of water, and if the storm is moving at 10 or 15 miles per hour then it would rain itself out over a much larger area, spreading its rainfall over multiple states instead of just a couple. Slower storms mean more rainfall over a smaller area.

    Earlier this year, in the Journal Nature, atmospheric researcher James Kossin said, “The tropical-cyclone translation speed has decreased globally by 10 per cent over the period 1949–2016, which is very likely to have compounded, and possibly dominated, any increases in local rainfall totals that may have occurred as a result of increased tropical-cyclone rain rates.”

    Scientists hypothesize that a warmer world will bring slower storms, so what we saw last year with Harvey — and now this year with Florence — could be a sign of those changes.

    The bottom line is that damage from winds and the storm surge will probably be more typical of past hurricanes, but the freshwater flooding still looks to create major property damage, which could lead to loss of life if residents haven’t heeded the warnings of their local officials.