The circled area above represents a tropical wave which emerged off the West Coast of Africa this weekend. The National Hurricane Center (NHC) rates the chances of it developing into a tropical depression or storm over the next few days at 20 percent. That seems fairly low, but this system is moving over the warmest part of the Atlantic Ocean and is being pushed steadily eastward by the sprawling Bermuda High. It is also encountering less opposing wind at higher altitudes, so it is in a favorable environment for development. I expect we'll see the chances that this system will become Tropical Storm Ernesto grow by week's end.
Whenever an area is being investigated, it receives a designation by the NHC with a number then a letter. Numbers run from 90 to 99, then return to 90. It is, therefore, possible to have several invests with the same name. All invests in the Atlantic get the letter "L"; this one is called 99L. An array of computer models is then run to determine where the system might go. These so-called spaghetti models (they look like strands of spaghetti) trace the location of 99L's center in time. The model suite is run four times a day, every six hours. Here's what the afternoon run shows:
Each colored line is a different model. Some are based on persistence (moving in the same speed and direction); others on history (where storms forming this time of year end up) and others model the condition of the atmosphere. In tropical meteorology, there is comfort in consensus. The more uncertainty the models display, the less sure we are about a storm's destination. This early in the game, there's already a split, with some models taking the system toward Puerto Rico and other set keeping it in the southern Caribbean.
This is the time of year as we head toward the "peak" of the season that we watch for waves (former desert storms) to move over the Atlantic Ocean waters and intensify. Of the over 100 that emerge, a small fraction (less that 10 percent) become systems that strengthen enough to become tropical storms or hurricanes.