August hurricane outlook, part I: SSTs

By: Dr. Jeff Masters , 3:28 PM GMT on August 07, 2008

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The tropical Atlantic is quiet and there are no threat areas to discuss today. The UKMET model foresees that a tropical depression could form Sunday off the Carolina coast, at the tail end of an old cold front. Any such development would likely move northeastward out to sea. The GFS model predicts formation of a tropical depression off the coast of Africa about 6-7 days from now. The other two reliable models, the NOGAPS and ECMWF, do not predict any tropical storm formation in the coming week.

August Sea Surface Temperature Outlook
Eighty-five percent of all major hurricanes form in the Main Development Region (MDR) of the Atlantic, from the coast of Africa to the coast of Central America, between 10° and 20° latitude. This region also spawns 60% of all weaker hurricanes and tropical storms. Sea Surface Temperatures (SSTs) in the MDR are near average over the Caribbean, and 0.5-2.0°C above average between the Lesser Antilles Islands and Africa (Figure 1). SSTs are well below the record levels observed in 2005 and 2006, when they were up to 2°C above average over large portions of the Main Development Region. Still, the very warm waters between the Lesser Antilles and Africa are cause for concern, and have already fueled near-record levels of July activity. The SST anomalies over most of the MDR have been increasing in recent weeks, which is not good news. SSTs are near average in the Gulf of Mexico due to the passage of Hurricane Dolly and Tropical Storm Edouard, which stirred up cool waters from the depths. SST anomalies should return to above average levels in the Gulf of Mexico in coming weeks as the surface waters recover from the passage of these storms.

Figure 1. Departure of SST from average for August 4, 2008. Image credit: NOAA.

Total heat content of the ocean
Hurricanes generally require sea surface temperatures (SSTs) of at least 80° F (26.5° C) to exist, and the hotter the water, the better. Hurricanes also like to have these warm ocean waters extend to a depth of several hundred feet, since the winds of a hurricane generate ocean turbulence that stirs up colder water from the depths to the surface. Hurricane that pass over a region of ocean with very deep warm waters can intensify explosively; this happened with Katrina, Rita, and Wilma in 2005. A good way to monitor this total oceanic heat is with the Tropical Cyclone Heat Potential (TCHP) imagery prepared daily by NOAA using satellite measurements of the height of the ocean surface. Hotter water expands, creating a higher water surface that the satellite can measure.

Let's look at the TCHP data from August 5 this year, and compare it to the record-breaking Hurricane Season of 2005 (Figures 2 and 3). The units of measurement are in kilojoules per square centimeter, and any value greater than 20 kJ/cm**2 (a medium blue color) is high enough to support a Category 1 hurricane. A TCHP greater than 80-90 kJ/cm**2 (orange color) is often associated with rapid intensification of a hurricane. The TCHP images from the two years are quite similar. There was more heat available in the Western Caribbean in 2005, but this year's THCP is higher in the Bahama Islands. An orange bullseye in the Gulf of Mexico in 2005 marks the Loop Current Eddy that broke off in July of that year, and helped fuel Katrina and Rita to record intensities. There is a similar bullseye visible in this year's image, due to a Loop Current Eddy that broke off in July. This eddy is not quite as big or as hot as the 2005 eddy, but is still capable of fueling rapid intensification of any hurricanes that might pass over it. There is also an eddy in the western Gulf of Mexico that broke off in April, but this eddy is not quite as warm, since it broke off before the peak heat of summer. It is unusual to have two eddies break off just three months apart; usually there is a lag of at least six months betweens eddies. The presence of two Loop Current Eddies in the Gulf of Mexico this hurricane season provides an unusual amount of warm water to great depth that can help fuel rapid intensification of any hurricanes that cross the Gulf. Overall, the total heat content of the ocean in the Atlantic Main Development Region and Gulf of Mexico this year is not much less than during the record breaking Hurricane Season of 2005. Thus, we can expect an unusually high number of intense hurricanes this year, assuming that wind shear and dry air are average or below average.

Figure 2. Tropical Cyclone Heat Potential (TCHP) for August 5, 2005.

Figure 3. Tropical Cyclone Heat Potential (TCHP) for August 5, 2008.

What are SSTs likely to do in August?
The latest long range forecast from the NOAA CFS seasonal forecast models shows a continuation of above average SSTs through the remainder of hurricane season. The latest 15-day forecast from the GFS model predicts a weaker than average Bermuda High, which will drive weaker trade winds than normal. Weaker trade winds result in less evaporative cooling of the ocean, and less mixing of cool water from the depths, resulting in higher SSTs. An additional factor affecting SSTs is the amount of African dust coming off the Sahara Desert. Higher dust levels than usual will block more sunlight than usual, resulting in cooler SSTs than average. So far, the August Saharan dust levels appear near average. It is difficult to predict how these dust levels might change later in the month.

I'll summarize wind shear, steering currents, and dust levels in Friday's blog.

Jeff Masters

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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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