Tracking tropical waves

GOES satellite image from July 19, 2013.
GOES satellite image from July 19, 2013.

MIAMI – Some people have asked me why we bother talking about tropical waves when they are thousands of miles away from South Florida. We don't want to over focus on these disturbances, but it is important to understand that tropical cyclones don't develop out of nowhere. A tropical cyclone develops from a pre-existing disturbance -- such as a frontal trough or an upper-level low -- that eventually works its way down to the surface, or from a tropical wave.

According to NOAA's Hurricane Research Division, about 60 percent of Atlantic tropical storms and Category 1 and 2 hurricanes originate from tropical waves, and nearly 85 percent of major hurricanes (Category 3, 4 and 5) develop from tropical waves.

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Tropical waves are generated by an instability of the African easterly jet. The jet develops as a result of the temperature gradient over western and central North Africa due to extremely warm temperatures over the Saharan Desert and the cooler temperatures along the Gulf of Guinea coast. The waves typically emerge from the coast of Africa every three to four days, are most pronounced in the low to mid levels of the atmosphere, and move generally westward across the Atlantic Ocean. The waves are most easily seen starting in April or May and continue through the hurricane season. On average, about 60 waves are tracked every year. As far as we know, there is no relationship between the number of waves tracked and total Atlantic tropical cyclone activity for the year.

A large-amplitude tropical wave could be seen over the mid-Atlantic on Friday's satellite imagery around 1,500 miles east of the Lesser Antilles. This wave had the characteristic "inverted-V" pattern but had limited thunderstorm activity thanks, in part, to a large area of surrounding dry air.

This tropical wave is not expected to develop by any of the computer models, but we will be watching these disturbances closer as we approach the peak of the hurricane season.