Monday, September 2, 2019

Hurricane Dorian is a VERY Small Storm That is Moving Very Slowly

There has been a  lot of coverage in the media about Hurricane Dorian, but most have not been stressed how SMALL the storm is.   And size matters, limiting the damage to the immediate vicinity of the center of the storm.  A recent image from the Miami radar shows the storm about 120 miles offshore, with a small eye and circling eyewall rain bands.


According to the National Hurricane Center, hurricane-force winds (64 knots or more) extended only 45 miles from the center--the diameter of hurricane winds was only 90 miles.

Compare that to Hurricane Irma, which hit Florida in 2017--it has a diameter of 420 miles ifor the hurricane winds!  Hurricane Sandy in 2012 had a hurricane-force wind diameter of 1000 miles at one point.

The small size of Dorian is shown by the wind forecast for 5 AM PDT Tuesday morning-- the hurricane force winds (purple colors) stay way offshore and don't reach the Florida mainland.


The maximum gusts today (see figure) show gusty, but modest, winds of 30-40 mph along the eastern coast of Florida.  Makes sense considering the small diameter of the storm.


Dorian has essentially not moved during the past 24-hours, keeping the strong winds over the Bahamas, where terrible damage is occurring.  But the small size has another benefit... it limits the growth of large waves, which increase with fetch--the distance over which strong winds work on the water.  This is illustrated by the wave prediction for 3 PM PDT Monday afternoon from the NWS WaveWatchIII system.  Small area of large waves, with none larger than 10 meters.

Staying in one place is bad for a hurricane.   Hurricanes depend on warm water and they churn up the ocean, bringing cooler water from below.  If the hurricane doesn't move it will weaken...and Dorian has already declined from a category 5 to 3 storm.   A NOAA buoy in front of the storm illustrates the cooling.

The latest forecasts are for the storm to slowly move north-northwestward on Tuesday, with the storm gradually increasing in size.  To illustrate,  here is the NOAA/NWS HWRF forecasts (their high-resolution hurricane model), showing some increase in size, with hurricane strength winds just offshore of the coastline.  Really close.  


 Florida will probably escape serious damage from Dorian, but the show is not over, as the storm expands in size, speeds up, and then makes a grazing landfall on North Carolina.

4 comments:

  1. "but the show is not over, as the storm expands in size, speeds up, and then makes landfall Wherever it may." *Ftfy.

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  2. Thank you, Cliff, for helping us keep events like this in perspective!

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  3. Fascinating, Cliff! I don't remember ever being exposed to the "mechanics" explanation of hurricanes and a near real time assessment of the resulting effects. Also, I like the compare and contrast approach of other well-known hurricanes as it provides perspective.

    Thanks for making it so easy to understand for a lay person.

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