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Following a record for the earliest formation of four named tropical systems in the Atlantic this hurricane season, there was a lull in tropical activity during July.
So far this hurricane season, with the formation of Ernesto Thursday, there have been four named tropical storms and one hurricane.
Both Tropical Storm Alberto and Tropical Storm Beryl formed before the official start of the hurricane season, June 1. Hurricane Chris and Tropical Storm Debby formed during June. When Debby formed on June 23, it marked the earliest development of four named storms in the Atlantic.
A waning La Niña pattern, which is more favorable for tropical storm development due to less frequent occurrences of high wind shear in development regions of the Atlantic, was present early in the season. La Niña is a phenomenon characterized by below-normal water temperatures in the central and eastern equatorial Pacific.
After an unusually fast start to the season, there were no named storms during July. However, it is not that unusual for no storms to be named in July. The average number of storms in the Atlantic in July over all the years of record keeping is 0.7.
"There are small factors in July that create a more hostile environment," AccuWeather Expert Senior Meteorologist Dan Kottlowski said. "July water temperatures are not consistently warm across the Atlantic basin. You still have lagging wind shear problems as well."
Compared to the past two years, water temperatures have been cooler and surface pressure is higher in the main development regions of the Atlantic. Both of these factors have been less favorable for tropical development in July.
Tropical systems thrive with warm water, so near- to below-normal temperatures in the Atlantic have not been presenting the best conditions for tropical formation.
With surface pressure above normal, it has been harder for tropical systems to spin up.
"It takes more time for pressure to lower [when the ambient surface pressure is higher], so it slows the development process down," Kottlowski explained. "The higher pressure has also been creating areas of stronger trade winds across the Atlantic and Caribbean, which in turn causes upwelling water."
Upwelling water occurs when the wind drives cooler water to the ocean surface. Both strong winds and cool water are unfavorable for tropical development, so this cycle has been reducing the opportunity for tropical development over the past month in the Atlantic.
Abundant dust in the atmosphere is another factor in the quiet July. Strong trade winds off of Africa stirred an abundance of Saharan dust into the atmosphere during June and July. Dust in the atmosphere is associated with large, dry air masses, and the tiny dust particles can remain suspended 25,000-35,000 feet above the surface.
The dust prevents intense thunderstorms from forming, which in turn does not allow the surface pressure to lower enough for tropical storms to wrap up.
It is important not to gain a false sense of security because of the slow July in the Atlantic. The worst is yet to come with a near-normal number of storms forecast for the 2012 Atlantic Hurricane Season.
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