First half of July hurricane outlook

The first half of July is usually a quiet period in the Atlantic for tropical cyclone formation. Since 1995, six of 12 years have had a named storm form during the first half of July, giving a historical 50% chance of a first half of July storm. The busiest first half of July occurred in 2005, when three hurricanes formed. These included Hurricane Dennis and Hurricane Emily--the strongest hurricanes ever observed so early in the season. As seen in Figure 1, most of the early July activity occurs in the Gulf of Mexico, Western Caribbean, and Carolina waters. However, a few long-track "Cape Verdes" hurricanes begin to occur. These are spawned by tropical waves that come off the coast of Africa. Tropical waves serve as the instigators of about 85% of all major hurricanes.


Figure 1. Tracks of all tropical storms and hurricanes since 1851 that formed July 1-15. North Carolina and the Gulf of Mexico coast from the Florida Panhandle to Texas are the preferred strike locations. Oddly, the Florida Peninsula has been struck by only two storms that formed in the first half of July.

Sea Surface Temperatures
Sea Surface Temperatures (SSTs) have cooled considerably in the past two weeks over the region we care about the most--the hurricane Main Development Region that extends from the coast of Africa to the coast of Central America, between 10� and 20� latitude. SSTs were about 0.5-1.0 �C above average over this region in mid-June, and have now cooled to near normal, when one averages over the entire region. A large region of below-average SSTs has formed between Africa and the Lesser Antilles Islands. Why? African dust storms! Levels of Saharan dust coming off the coast of Africa in June were five times those observed in June 2006, and were the highest observed since at least 1999. All that dust blocks sunlight, preventing the water from heating up as much as usual. One dust storm that was particularly noteworthy exited the African coast June 21-22, and made it to the Caribbean and South America June 25 (Figure 3). However, all that dust also interferes with the accurate measurement of ocean heat energy by satellite, so the SSTs shown here may not be quite as cool as indicated.


Figure 2. Sea Surface Temperature (SST) departure from average for July 1, 2007. Image credit: NOAA.


Figure 3. A huge dust storm moved off the coast of Africa June 21-22, and arrived at the Caribbean on June 25. Image credit: NASA.

Tropical Cyclone Heat Potential
It's not just the SSTs that are important for hurricanes, it's also the total amount of heat in the ocean to a depth of about 150 meters. Hurricanes stir up water from down deep due to their high winds, so a shallow layer of warm water isn't as beneficial to a hurricane as a deep one. The Tropical Cyclone Heat Potential (TCHP, Figure 4) is a measure of this total heat content. A high TCHP over 80 is very beneficial to rapid intensification. As we can see, there is less heat energy available this year than in 2005, which recorded the highest SSTs and TCHP ever measured in the tropical Atlantic. However, this is not true in the Western Caribbean, where we have very high TCHP this year. The African dust storms have not penetrated all the way to the Western Caribbean, and SSTs and TCHP have stayed above average. If we do get an intense hurricane in early July, it will likely be here.


Figure 4. Tropical Cyclone Heat Potential (TCHP) for July 1 2005 (top) and July 1 2007 (bottom). TCHP is a measure of the total heat energy available in the ocean. Record high values of TCHP were observed in 2005. Image credit: NOAA/AOML.

Wind shear
Wind shear is usually defined as the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude). In most circumstances, wind shear above 20 knots will act to inhibit tropical storm formation. Wind shear below 12 knots is very conducive for tropical storm formation. High wind shear acts to tear a storm apart.

Wind shear over the past 11 days (Figure 5, top image) has been above 20 knots along the two branches of the jet stream--the polar jet, which runs along the U.S.-Canadian border, and the subtropical jet, which runs through the Caribbean to North Africa. This is very typical for June, when the jet stream is still very active and quite far south. The jet stream will gradually weaken as summer progresses, bringing lower wind shear and greater chances for tropical storm formation. Shear has been average to above average over nearly the entire North Atlantic during the last half of June(Figure 5, bottom image). The latest two-week forecast from the GFS model predicts that the subtropical jet stream will gradually weaken through mid-July, resulting in lower than average wind shear over much of the tropical Atlantic. This should result in a greater than average chance of a named storm occurring.


Figure 5. Top: Average wind shear over the past 11 days. Wind shear is the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude) in meters per second (multiply by two to get the approximate wind shear in knots). In most circumstances, wind shear above 20 knots (10 m/s, the blue colors in the top image) will act to inhibit tropical storm formation. Wind shear below 12 knots (6 m/s, the orange colors) is very conducive for tropical storm formation. Bottom: Departure of wind shear from average for the past 11 days in meters per second. Note that wind shear has been above average over most of the tropical Atlantic and Gulf of Mexico over the past 11 days. Image credit: NOAA/CPC.

Dry air and African dust
June and July are the peak months for dust coming off the coast of Africa. Despite the fact that the Sahel region of Africa has seen two straight years of above-average rains, which should result in soil stabilization and fewer dust outbreaks, 2007 has seen very high levels of dust coming from Africa. Expect dust from Africa to be a major deterrent to any storms that try to form between Africa and the Lesser Antilles Islands in July.

Steering currents
The steering current pattern for June featured a pattern much like we saw in 2006, with an active jet stream bringing many troughs of low pressure off the East Coast of the U.S. The position of the Bermuda-Azores High (Figure 6) was pretty close to average. Its strength was only 1 mb below average, driving slightly slower trade winds than average across the tropical Atlantic. I expect this pattern to continue for the first half of July, and the troughs should be frequent enough and strong enough to recurve any tropical storms or hurricanes that penetrate north of the Caribbean Sea. Steering current patterns are not predictable more than about two weeks in advance, and there is no telling if we are in for a repeat of the favorable 2006 steering current pattern that recurved every storm out to sea. It is encouraging to note that in 2006 the steering current pattern locked into place in late May and stayed that way for almost the entirety of the hurricane season. The atmosphere often stays locked in to a particular steering pattern for an entire summer, and it would not be a surprise if that occurred again this year. If this pattern holds, expect a below-average chance of hurricane landfalls along the U.S. Gulf Coast, and normal to above normal chances along the U.S. East Coast.


Figure 6. Sea level pressure for June 2007 (left), and average sea level pressure from climatology (the years 1979-1995). Note that position and strength of the Bermuda-Azores High during June 2007 was very close to average. Image credit: NOAA/ESRL.

Summary
Recent history suggests a 50% chance of at least one named storm occurring in the first half of July. Wind shear is expected to be below average and SSTs are near average, so I expect a 70% chance of a first half of July named storm this year.

Jeff Masters