WunderBlog Archive » Category 6™

Category 6 has moved! See the latest from Dr. Jeff Masters and Bob Henson here.

July Atlantic hurricane outlook

By: Dr. Jeff Masters, 5:31 PM GMT on June 30, 2008

The first half of July is usually a quiet period in the Atlantic for tropical cyclone formation. Since 1995, six of 13 years (46%) have had a named storm form during the first half of July. 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 about 0.5°C in the past month 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 (Figure 2). SSTs are now about about 0.5°C below average in this region (compare the May 29 SST anomaly image. One notable exception is the region closest to the African coast, which is about 2°C above average. The reason for the cooling over most of the tropical Atlantic is that the strength of the Bermuda-Azores High has increased since May, driving stronger trade winds. These stronger winds cause more evaporative cooling of the sea surface (just like blowing on your wet skin cools it off). In addition, levels of Saharan dust coming off the coast of Africa in June increased dramatically compared to May, which had dust levels about 30% below average. In particular, a major dust storm that began about June 20 off the coast of Africa crossed all the way to Florida by Friday last week, bringing hazy skies across the entire tropical Atlantic (Figure 3). All that dust blocked sunlight, preventing the water from heating up as much as usual. Another large dust storm was observed leaving the African coast around June 14.

Figure 2. Sea Surface Temperature (SST) departure from average for June 30, 2008. Image credit: NOAA.

Figure 3. Satellite image of the West African coast from June 21, 2008, at 14:50 GMT. A huge dust storm moved off the coast of Africa June 20, and arrived over South Florida on June 27. Image credit: NASA.

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 during almost all of June was above average over the tropical Atlantic, making it a very quiet month. This shear was predominately caused by 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. Indeed, the GFS model is predicting that very low levels of wind shear will develop in the region between Africa and the Lesser Antilles Islands late this week (Figure 4). This is echoed by all the other major global forecast models, which unanimously predict that a tropical depression may form between Africa and the Lesser Antilles Islands by late this week or early next week. The latest 2-week GFS forecast keeps wind shear below average levels through mid-July.

Figure 4. GFS model wind shear forecast for Friday, July 4, 2008. 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 below 12 knots (6 m/s, the lighter red colors) is conducive for tropical storm formation.

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 three straight years of average to above-average rains, which should result in soil stabilization and fewer dust outbreaks, June 2008 had 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
During June 2008, the Bermuda-Azores High (Figure 5) extended farther west than usual. Its strength was about 1 mb below average, which normally would drive slower trade winds than average. This did not occur, as the trade winds were about 2 m/s (4 knots) above average over most of the tropical Atlantic in June. For the first half of July, both the GFS and ECMWF long-range models are predicting that the Bermuda-Azores High will not extend so far west, as the jet stream will bring a persistent trough of low pressure over the East Coast. This pattern will tend to recurve most tropical storms 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 this favorable steering current pattern will persist into the peak of hurricane season.

Figure 5. Sea level pressure for June 2008 (left), and average sea level pressure from climatology (the years 1979-1996). The Bermuda-Azores High extended farther west than usual in June 2008, keeping low pressure entrenched in the U.S. Midwest, leading to major flooding. Image credit: NOAA/ESRL.

Recent history suggests a 46% chance of at least one named storm occurring in the first half of July. All of the major computer models predict the possible formation of a tropical depression in the region between Africa and the Lesser Antilles late this week or early next week. Given the high degree of model unanimity, marginally favorable sea surface temperatures, favorable wind shear, and unfavorable dust levels expected in this region, I put the chances of a tropical storm forming in the region at 30% over the next two weeks. I put the odds of a tropical storm forming elsewhere--in the Caribbean, Gulf of Mexico, or off the U.S. East Coast--at 30% as well, for a combined roughly 50% chance of a first half of July named storm.

I'll have an update Wednesday morning.
Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.