Fox News Weather Center

Large Atlantic Tropical Disturbance Fights Dry Air

An Atlantic tropical disturbance that moved off Africa Sunday is being monitored for development over the next couple of days and will reach some of the islands of the Caribbean this weekend.

The disturbance, a tropical wave of low pressure, was located near the Cape Verde Islands Tuesday and was producing spotty showers and thunderstorms in the region.

According to Hurricane Expert Dan Kottlowski, "This is the eighteenth and one of the largest tropical waves we have seen so far this season."

Slight circular motion to the cloud cover in the region has been observed Tuesday. Circular motion of clouds is an early sign of development for tropical systems.

"The system appears to be experiencing some problems during the midday Tuesday," Kottlowski said.

Much of the tropical Atlantic was being influenced by dry air and dust this week. Dry air tends to work against tropical development.

As we have seen in the past, sometimes these systems, especially the larger ones, can overcome the surrounding dry air. However, the system would have a better chance of development through the middle of the week, rather than late in the week.

"As the feature continues to drift on a slightly north of west course, it is likely to encounter a zone of increasingly disruptive winds at mid-levels of the atmosphere this week," Kottlowski said.

At present course and speed, the system would be near the Leeward Islands this weekend and could brush some of the northern islands of the Caribbean next week as a tropical wave of low pressure or perhaps something stronger and more organized. Even if it does not develop fully, the system could bring a period of showers and thunderstorms to these islands.

We have already had three named tropical systems so far this season in the Atlantic. The next name on the list is Dorian.

Before a system becomes a tropical storm or hurricane, it must advance past a tropical disturbance (tropical wave) by developing a distinct circulation just above the sea surface.

Once this happens, it is upgraded to a tropical depression and assigned a number.

If sustained winds of 39 mph or higher are found, then the system is upgraded to a tropical storm and given a name.

Next, if sustained winds reach 74 mph or higher, then system upgraded to a hurricane.

Occasionally, a system can go right from a tropical disturbance to a tropical storm. This happens during rapid development and can occur when aircraft cannot investigate the system often enough or sufficient high-resolution satellite data are not available.