Atlantic hurricanes like Irma: How they form
WEST MICHIGAN — We are in peak hurricane season right now as the tropical Atlantic Ocean, Caribbean Sea, and Gulf of Mexico are primed with warm water ready to fuel these monster storms we call hurricanes. Irma now holds the dubious distinction of being the strongest tropical system, the longest anywhere on the planet! That includes it global counterparts known as typhoons and cyclones. It remained a category five with 185 mile per hour winds for some 36 hours. But how and where do these storms form?
Only concerning ourselves with the formation of ATLANTIC basin hurricanes (which can impact the Caribbean, Gulf Coast, and Eastern United States) it’s easy to understand the process of formation or genesis. Take a look at the image below. What you see is the African continent. It’s located well south and east of the United States. Atmospheric disturbances or tropical waves of energy develop over the dry, Saharian desert and move westward off the continent and in to the tropical Atlantic Ocean.
Once this energy gets to the ocean it can begin the strengthening stage by evaporating warm water and becoming a bit more organized in to a cluster of thunderstorms. It becomes an area of disturbed weather. See image below.
As the disturbance and cluster of storms continues to grow, it evaporates more and more ocean water and begins to establish a center of circulation (the eye) around a low pressure area. In order for this to occur, the water needs to be at least 79 degrees (warmer is better). It is this evaporation that literally fuels the strength of these systems. Recall that last week Hurricane Harvey over Texas evaporated and dropped more than 20 trillion (that’s 12 zeros) gallons of water! The other necessary component these systems need are weak upper level winds. Strong winds aloft can deflect or tear apart a tropical system from wind shear. Ideally, these systems require weak upper level winds in order to form, rotate, and continue developing symmetrically. Once the system organizes and has sustained winds less than 39 mph, it is considered a tropical depression. Winds greater than 39 mph make it a named tropical storm. You can see from the image below that the Caribbean Sea and its islands, the Gulf Coast, and the Eastern seaboard of the United States are now visible and possibly in the path.
Notice in my graphics that the entire time of this process, development continues to move contrary to what weather systems across the United States move. U.S. weather systems generally move west to east, but these tropical systems move east to west with little/no coriolis force/effect close to the equator. Click here to read more on coriolis. These tropical systems are carried and moved by the trade winds.
Once sustained winds reach 74 mph or greater the tropical storm is upgraded to hurricane status. Anything over 110 mph is considered a major hurricane. Keep in mind Irma had 185 mph sustained winds (equal to an EF-4 tornado) and gusts of 225 mph. The image below shows the general path these hurricanes can follow once they form. Upper level steering winds can send these storms in to the north Atlantic where they may miss landfall completely. The south central Atlantic path (Irma is following this) tends to track it toward Florida and the East Coast of the United States. The other path with weaker steering winds tends to be further south in the Caribbean Sea…and perhaps eventually in to the Gulf of Mexico.
Sometimes these storms can track through multiple basins…the Atlantic, the Caribbean, or the Gulf of Mexico, but many of them form off the west coast of Africa. That said, some can form from a cluster of storms already directly over the ocean and not as an African wave or disturbance.
Since Irma is such a powerful system and heading directly toward southern Florida, Hurricane Jose is behind it and moving in a similar manner. It’s likely Jose will eventually turn more northerly but still hit some of the same islands Irma hit. Usually when a large, powerful hurricane like Irma moves through an area, a recent second hurricane tends to lack the same strength since the surface water is stirred up and sometimes a bit cooler limiting evaporation and storm strength.