Hurricane (and Tropical Cyclone) Update, Wednesday Morning

Although people in the US are focused on Dorian, there are currently seven active tropical systems and a couple more “invest” (potential) areas.  In the Atlantic, we have Dorian, Fernand (no “oh” at the end) is a weak tropical storm making landfall near the Mexico/Texas border.  Gabrielle, also a tropical storm is in the mid-Atlantic, and is not forecast to be a threat to land.  Here the big picture .. can you identify which are the storms and which are just blobs of clouds ?

Then there’s Dorian … here is the 542am scan from Melbourne, Florida …

Dorian is now a Category 2 hurricane, with 105mph winds.  A lot is being made of the fact the storm is “bigger”, and I’m hearing on the radio that “tropical storm winds are extending 175 miles!”  Let’s take a look at that statement, which while true, is also misleading.

The left side of the storm is the weaker side for a storm moving south to north.  The definitive NHC product is the “Forecast Advisory.” For Dorian, the current NHC advisory is  that on the west side of the storm, tropical storm winds are extending between 100 and 140 nautical miles, or 115 to 160 miles.  Only in the stronger NorthEast side of the storm do the tropical storm force winds extend as far as 175 miles. But here’s a key missing piece of the puzzle: that is over water.  Over land, winds decrease significantly due to friction between the air and land.  A rule of thumb is that over land winds will be about 20% lower than over water, so while the winds over Tybee might be 50mph, by the time you get to downtown Savannah they are only 40mph.  And remember that wind energy is proportional to the square of the wind speed, so a 40mph sustained wind has far less damage potential than a 50mph wind (a 40mph wind has 64% of the energy of a 50mph wind, to be exact). While watch and warning areas are based on tropical storm force (40mph), significant damage to a normal home such as roof damage really starts to pick up at 50 to 60 mph sustained winds. So from a risk standpoint, and given that forecast track errors for a storm like Dorian are far more likely to be to the right than the left, the situation of the Georgia Coast, while the situation is hazardous, is not as dangerous as one might think unless you are in an area that floods.

To wrap up, here’s the obligatory impact map based on the official forecast track:

In the Coastal Georgia and Lowcountry of SC area, nothing much has changed.  The storm will be traveling parallel the the shoreline.  The barrier islands will see tropical storm force winds, and high tides will be about 3 feet above normal with perhaps some spots a bit higher; in other words,  more than likely a foot or so below the worst of what was seen in the storms we have seen in the last two years.  Inland winds will decay rapidly.  There will be some rain bands, there is one parallel to the shore right now (6am) that will drift through the area.  But all in all, aside from the usual impacts (power outages, limbs down, a few random trees might blow over) unless something breaks that shouldn’t that should be it.  Conditions will deteriorate this morning, be crummy this afternoon and overnight, but by noon Thursday the sun will probably be out and it will all be over.  If you are in a sturdy structure that doesn’t flood, you are likely to ride out this storm fine.  Those in areas that flood, or in manufactured homes within 10 miles of the coast should seek better shelter.  If you are not comfortable where you are, the time to leave is now – after noon, travel will likely become hazardous and you will be better off staying put.

Further north, from Charleston to the Outer Banks of NC, things will be progressively worse.  Myrtle Beach, Wilmington NC, and the outer banks to Cape Hatteras will likely see minimal hurricane conditions and should prepare for that and, if in areas in danger of flooding, evacuate now.

As we watch the impacts on the Mainland US, please don’t forget The Bahamas, which has been devastated.  It will take a decade for them to recover from this storm, which in the course of 72 hours wiped out half a year of their entire GDP.




6 thoughts on “Hurricane (and Tropical Cyclone) Update, Wednesday Morning

  1. Pingback: Dorian Update Wednesday 4 Sep – rain | Enki Research

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