Peak of 2016 Atlantic Hurricane Season Is Approaching

It’s hard to believe, but the Atlantic hurricane season began 189 days ago when Hurricane Alex formed on January 13 and went on to become the strongest hurricane ever to form in the month of January in Atlantic Basin. The early season continued with three landfalling named storms that formed this June, making it easy to assume that an early season means an active season. But an early start to the season does not necessarily mean that the heart of the hurricane season will be active. A current lull in the basin since June 21 and long-range forecasts suggest there is limited opportunity for development for the reminder of July. However, climatology suggests we are not out of the woods yet. Instead, we are only just approaching September 10: the peak of the season when storm formation becomes much more frequent.

This is the time of year when eyes are trained on the massive cloud clusters that move off the West African coastline. These clusters have been limited so far this season. The only movement off this region is massive plumes of Saharan dust.

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Current CIMSS Tropical Cyclone Team imagery that is useful for monitoring the position and movement of dry air masses such as the Saharan Air Layer (SAL) and mid-latitude dry air intrusions. Animations of the imagery are useful for tracking these features and can also help identify the source of the dry and/or dusty air that is indicated in the imagery.

In some cases, these dust plumes have traveled all the way to the Texas Gulf Coast. Known as the Saharan Air Layer (SAL), this dry, dusty air has about half the moisture of the typical tropical atmosphere and can discourage the clouds and tropical convection needed for named storm development. Although we still lack a full understanding of how the SAL affects tropical systems, there are likely several reasons why it limits cloud and tropical convection development, and thereby limits the likelihood of named storm development.

  • Dry air can enhance downdrafts (sinking air), suppressing convection around the system.
    As tropical waves move off Africa, any resulting convection quickly reaches the bottom of the SAL (typically at altitudes between 5,000 and 15,000 feet); then entrains dry air which limits further convection.
850mbPlotTempAnoms

Here is the current 850 mb temperature normalized anomalies which shows that areas of heavy dust have warmed the atmosphere at this level 3 – 5 degrees Fahrenheit.

  • Typically atmosphere cools with height, but the SAL absorbs sunlight, which retains warmth and creates an inversion for thousands of miles across the Atlantic basin. This essentially caps the development of the showers and thunderstorms that are needed in tropical cyclone development.
  • Dusty conditions can be enhanced by stronger easterly winds that increase wind shear and tilt or outright displace the convection aloft from low-level circulation, thus limiting convection and tropical cyclone development.
  • The SAL shields sea surfaces from the sun and can keep the sea surface temperatures cooler than normal across the main development region.

Data suggests decadal variability in the SALs that may impact tropical activity (unfortunately, since the insurance industry doesn’t need another multi-year decadal pattern that could influence tropical cyclone development). Records going back to the 1960s and 1970s that were collected by satellites and island stations (using dust as a tracer) show that SAL activity have ebbed and flowed over the years. In the 1980s, some studies point to quiet periods that coincided with a stretch of increased dust outbreaks. In the 1990’s, dust activity decreased and tropical cyclone activity began ramping up.
At this time it is difficult to say if the dry, dusty air will continue into the heart of the Atlantic Hurricane season which effectively peaks around September 10th. We are only one-third of our way through hurricane season, and June and July are not usually good indicators of what is to come: on average, those months account for only 4% of Atlantic major hurricane activity.

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— Philip Klotzbach (@philklotzbach) July 20, 2016

And just as a reminder, the 2004 named storm season didn’t have its first named storm form until August 1, but 2004 ultimately became one of the most active seasons on record for U.S. Florida landfalls.

The insurance industry shouldn’t let its guard down. If the dust persists and Cape Verde storms are hampered by the SAL, storms may instead develop in the western Atlantic which provides a higher likelihood of making U.S. landfall. And given the Western Caribbean and Gulf of Mexico is experiencing record warm surface temperatures and heat content, there is plenty of energy for these storms to become powerful hurricanes should they track over such warm waters.

tchp-2005-2016

Total oceanic heat content (called the Tropical Cyclone Heat Potential, or TCHP) in kilojoules per square centimeter (kJ/cm^2), for July 15 for the years 2005 – 2016. TCHP was at near-record or record values over much of the Caribbean, Gulf of Mexico, and waters surrounding the Bahamas in July 2016. TCHP in excess of 90 kJ/cm^2 (orange colors) is commonly associated with rapid intensification of hurricanes. Image credit: NOAA/AOML.

In summary, El Niño is gone and La Niña conditions are slowly building in the Central Pacific, and the SAL is currently hampering development in the main development region of the Atlantic. Long range forecast models remain quiet with little development chances. However, the warm ocean heat content and sea surface temperatures near the U.S. coastline would provide plenty of fuel for a strong hurricane if one were to track over these waters.