Atlantic hurricane outlook for the last half of July

Atlantic tropical cyclone activity is usually low during the last half of July. Since the current active hurricane period began in 1995, eight of 12 years have had one or two named storms form during the last half of July. Two named storms formed in both 1995 and 2005. In 2005, we were already up to "E" in the alphabet at this point, so 2007 is certainly not going to be a repeat of 2005, thank goodness! As seen in Figure 1, most of the late July activity occurs in the Gulf of Mexico, but there are a few long-track "Cape Verdes" hurricanes beginning to occur. These are spawned by tropical waves that come off the coast of Africa. African 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 16-31. The Gulf of Mexico coast is the preferred strike location.

Sea Surface Temperatures
Sea Surface Temperature (SSTs) remained near average over the tropical Atlantic between Africa and the Lesser Antilles in July, thanks to plenty of African dust keeping sunlight from heating up the ocean. However, SSTs are 0.5-1.0 �C above average over much of the Caribbean and Gulf of Mexico, so there is still good reason to expect an above average number of tropical storms and intense hurricanes this season--but not the 15 or 16 more named storms predicted by the Klotzbach/Gray team and TSR. I think it is more likely we will see 10-12 more named storms in the Atlantic this season, for a total of 12-14 when we include Andrea and Barry.


Figure 2. Sea Surface Temperature (SST) departure from average for July 16, 2007. Image credit: NOAA.

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 3) is a measure of this total heat content. A high TCHP over 80 is very beneficial to rapid intensification. 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. In the unlikely event we get an intense hurricane in late July, it would probably be in the Western Caribbean.


Figure 3. Tropical Cyclone Heat Potential (TCHP) for July 14 2005 (top) and July 14 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 by tearing a storm apart. Wind shear below 8 knots is very conducive for tropical storm formation.

Wind shear during most of June and July has been above 20 knots along the two branches of the jet stream--the polar jet, which has been positioned along the U.S.-Canadian border, and the subtropical jet, which has been over the Caribbean. This pattern is apparent in this morning's wind shear map (Figure 4, top). However, a major shift the Northern Hemisphere weather pattern is expected over the next two weeks. The GFS model is predicting that the persistent trough of low pressure that has been over the Eastern U.S. will move off and be replaced by a ridge of high pressure about ten days from now. The subtropical jet will weaken, bringing pockets of very low wind shear all across the tropical Atlantic by the end of the month (Figure 4, bottom image). The shear will remain high enough to discourage tropical storm formation over the coming week, but chances for a named storm will increase sharply by the beginning of August.



Figure 4. Top: Wind shear analyzed by the GFS model at 00 GMT Monday, July 16 2007. Bottom: Forecasted wind shear for August 1, 2007. 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 light purples in the image) will act to inhibit tropical storm formation. Wind shear below 8 knots (4 m/s, the lightest red color) is very conducive for tropical storm formation. Note the large increase in low wind shear areas expected by August 1 (bottom image, red colors).

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. This activity continued over the tropical Atlantic during the first half of July, and I expect this activity to continue for the remainder of July. This dry air and dust will act as a major deterrent to any storms that tries to form between Africa and the Lesser Antilles Islands the remainder of July. With the coming Northern Hemisphere weather pattern shift, it is possible that the dry air coming off of the Sahara will fade some at the end of the month, though.

Steering currents
The steering current pattern for June and the first half of July 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. As I discussed in my blog on Friday the 13th, this steering current pattern is expected to shift next week, bringing a ridge of high pressure over the Eastern U.S. and an extension of the Bermuda High westwards over the U.S. This pattern will act to block recurvature of any tropical cyclones that might form in the last half of July. Such a pattern puts the Western Caribbean, Gulf of Mexico, and east coast of Florida at highest risk, as we saw in 2004 and 2005. Note that the the northern Lesser Antilles Islands, Puerto Rico, and the U.S. East Coast from the Carolinas northwards can expect reduced risk under this steering pattern. Steering current patterns are not predictable more than about two weeks in advance, and there is no way to tell if this new steering current pattern will remain in place for a few days or a few months.

Summary
Recent history suggests a 75% chance of at least one named storm occurring in the last half of July. This July, I put the odds at 50%, due to the unfavorable conditions for the coming week. Any storms that occur this July will probably be towards the end of the month, due to the lower shear, warmer SSTs, and potential for less African dust and dry air then. The areas of highest risk are the Western Caribbean, Gulf of Mexico, and east coast of Florida.

Jeff Masters