The 2013 Hurricane season continues to be slow & weak

1:50 PM, Oct 14, 2013   |    comments
Flooding rain from Hurricane Ingrid washed out a bridge in Acapulco, Mexico.
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The United States has been fortunate so far this year to avoid any significant effects from a tropical storm or hurricane. And, without any major damage in our country, it kind of feels like there hasn't been any activity in the tropics this year! In the DC Metro area, the only storm that really impacted us was Tropical Storm Andrea, all the way back in early June. The storm that brought record rainfall to our area last Friday was somewhat influenced by the remnants of Tropical Storm Karen, but it was in fact a separate system, and not tropical in origin. 

At the beginning of the season, NOAA predicted a 2013 tropical season with 13-19 named storms, with 6-9 of those storms becoming hurricanes. NOAA also predicted that 3-5 of those hurricanes would be major, reaching a status of Category 3 or higher (Cat 3 storms have wind speeds of at least 115mph). With about six weeks left in the tropical season, we have had eleven systems in the Atlantic that reached Tropical Storm status, and only two hurricanes. We haven't had a major hurricane at all in the Atlantic Ocean this year. So, it appears that NOAA's forecast for the number of storms could end up being on-target, since there's still plenty of time for two or more tropical storms to develop. But the forecast for the number of intense storms looks to be way too high. The quiet season can largely be explained by one factor-- dry air! 

Hurricanes develop differently than any other type of storm. A classic hurricane is very symmetrical and almost circular in shape. Compare that to a classic winter storm, which tends to have a comma-like shape. 

Winter Storm

 Symmetry and consistency are very important for hurricanes. If you look at a hurricane's vertical structure, you would see a cylindrical column of warm, moisture-laden air, with the temperature and wind speeds very similar throughout. By contrast, winter storms, Nor'Easters, and any other land-based cyclone have changing temperatures, changing wind speeds, and changing levels of humidity throughout horizontal and vertical cross-sections of the system. The reason for these big differences is because hurricanes get their energy from very warm water, whereas the other types of storms get their energy from differences in temperature, wind, and humidity. So, when a developing hurricane encounters a changing environment, such as strong winds at the jet stream level or an intrusion of dry air, these disruptions tend to kill the system.

In 2013, that's exactly what happened. Both wind shear (stronger winds aloft than at the surface) and dry air (winds blowing into the Atlantic from the Sahara Desert) are making for a barren environment for tropical development. And, the storms that are developing aren't getting any traction because of the aforementioned elements fighting against them. I think this is the main reason why we have had a fair number of storms this year, but only a couple hurricanes and no major storms at all. By the way, I am not ignoring the impact of Hurricane Ingrid on eastern Mexico. The storm, with its flooding rains, was devastating and deadly. But with peak winds of 85mph, the storm was a Category 1 hurricane and never reached major (Cat 3 or higher) status. 

There is little reason to believe that the tropical season will give us a late-season punch. The peak of hurricane season, early September, has long passed. (Not coincidentally, the season's peak is the only time we had any hurricanes at all. Both Humberto and Ingrid formed and fizzled during the month of September.) The current picture is looking pretty bleak for hurricane lovers, as well: 

The NHC has an area outlined in yellow with a 0% chance of tropical cyclone formation within the next 48 hours. Certainly nothing to get excited about there! Though it's certainly not unprecedented to get a late-season hurricane-- for instance, we just had one last year with Sandy's late-October formation-- hurricanes do become much more unlikely once the season is past its peak.

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