• Everything You Need to Know About Hurricanes, Plus What to Expect in 2019
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Everything You Need to Know About Hurricanes, Plus What to Expect in 2019

May 01, 2019

How do hurricanes develop? What conditions cause them to form—specifically, why do they need warm waters?

Hurricanes develop over warm tropical waters, primarily during the summer and fall. To develop, a hurricane needs:

  1. Water temperatures of 27°C (80°F) or greater – Hurricanes are basically heat engines, in that they derive their energy from the release of latent heat in the condensation process. As warm, moist tropical air rises, it condenses into clouds and rain, releasing heat into the atmosphere. This heat results in more rising air, squalls (sudden violent gusts of wind), and a lowering of the surface pressure. As the surface pressure drops, air begins to flow towards the low pressure area. The Coriolis Effect causes this air to rotate (counter-clockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere). Given other favorable conditions, the rotating area of squalls may become a tropical storm or hurricane.

  2. Low wind shear – Hurricanes are very delicate structures in that they do not tolerate winds blowing at different speeds or different directions above the center (wind shear). To develop into a hurricane, the winds aloft need to blow from approximately the same direction and at the same speed. This gives the young disturbance time to build a vertical column of squalls.

  3. Pre-existing disturbance – Hurricanes do not develop out of “thin air,” they require some type of pre-existing weather feature to initially focus the squalls. This feature could be a tropical wave, an area of low pressure that moves off the west coast of Africa and tracks across the tropics, or it could be residual squalls along the trailing end of a cold front that dips into the tropics.

How are hurricanes predicted? How early can we predict with accuracy? How can early prediction help businesses?

Data from around the world are fed into computer models, which use complex equations with many variables to predict weather patterns. Just as these models can predict the development of storm systems that bring rain or snow in the winter months, these same models can predict the development of low pressure areas in the tropics that can develop into hurricanes. However, accurately predicting the precise track of a hurricane days in advance remains a problem. The difficulty lies in obtaining accurate atmospheric data across the tropical oceans. Without direct observations, we must estimate what is happening at all levels of the atmosphere across the oceans. As with any computer program, bad or insufficient data will result in a poor forecast. This is why aircraft reconnaissance is so important: the more accurately we can define the initial conditions in the tropics, the better we can predict the path and intensity of a tropical cyclone.

Modern computer models can predict day-to-day weather as far as two weeks out. Often, they can pick up on potential tropical cyclone development 7-10 days in advance. This gives us the ability to initiate advisories on tropical disturbances days before they are classified as tropical depressions or tropical storms. In 2016, StormGeo initiated advisories on a disturbance in the eastern Atlantic nine days before it developed into Hurricane Hermine. This provided businesses in the eastern Caribbean Sea and the Gulf of Mexico time to prepare for the storm.

tropical cyclone tracks

Why is the Gulf of Mexico prone to hurricanes?

The Gulf of Mexico is a body of very warm water that lies in the tropics, making it an ideal environment for hurricanes.

How is global warming affecting the amount, size or power of hurricanes?

There is no conclusive evidence that global warming is having any effect on hurricanes. Looking back over the past 100 years, there has been a definite increase in the number of hurricanes, however, this is likely due to better detection techniques. Prior to observing the Tropics with earth-orbiting satellites (the first weather satellite was launched in 1959) the knowledge of the existence of hurricanes and other tropical systems relied mainly upon the human eyes of those aboard ocean-going ships.

We now know with certainty that for the past 40 years, worldwide tropical cyclone activity has actually been declining. As for intensity, there are more strong hurricanes now than in the past. Again, this may be contributed to poor observations in the past. Without reconnaissance and satellite data, many strong hurricanes were likely missed prior to the mid 1900s. One thing we do know is that hurricane impacts along the U.S. coast have actually shown a slight decrease over the past 100 years.

subtropical storm Andrea

Subtropical storm Andrea, May 2019

What is a subtropical storm?

A subtropical storm is a low pressure area that forms in the tropics or subtropics. The tropics are a region identified as the area north of the Equator to the Tropic of Cancer and south of the Equator to the Tropic of Capricorn. The subtropical regions are defined as areas extending approximately 12 degrees of latitude north and south of the tropics. Unlike a tropical storm, a subtropical storm typically does not have its strongest winds or heaviest squalls over the center. This makes it more difficult for the storm to strengthen.

If hurricanes slow down, how does that make them more dangerous?

A slower-moving hurricane increases the amount of time that its strong winds impact a location. In addition, the amount of rainfall that a hurricane produces is directly related to its speed of movement. For example, a hurricane moving at 10 mph may produce 10 inches of rain. This is true for any tropical low pressure area, not just hurricanes. Any tropical low can produce extremely heavy rainfall if it moves slowly across an area.

What is the difference between a tropical disturbance, tropical depression and tropical storm?

Tropical Disturbance: An area of squalls which may be associated with westward-moving low pressure areas that move off the west coast of Africa in the summer months, or they may be associated with weak frontal boundaries that extend into the tropics or subtropics. There is no circulation center at the surface.

Tropical Depression: A low pressure area over the tropical ocean that has a well-defined low-level circulation center along with organized squalls around the center. Winds are less than 39 mph.

Tropical Storm: A low pressure area over the tropical ocean that has a well-defined low-level circulation center along with organized squalls around the center. Winds are 39-73 mph.

Hurricane: A low pressure area over the tropical ocean that has a well-defined low-level circulation center along with organized squalls around the center. Winds are 74 mph or greater.

tropical storm types

What is the "eye" and "eyewall"?

The eye is a feature which is unique to strong tropical cyclones like hurricanes (or typhoons). Warm, moist air spiraling into the center of a hurricane reaches a point where it no longer tracks into the center. Instead, it rises up into the atmosphere as high as 10 miles, forming the eyewall. The eyewall is a circular ring of intense squalls surrounding the center of a hurricane and contains a hurricane’s strongest winds. This ring of intense squalls can be as small as one mile or as large as 100 miles in diameter. Inside the eye, close to the center, the winds are relatively calm, often less than 25 mph.

eye

What is a popular hurricane myth?

A particularly dangerous “myth” is the misconception that a hurricane’s storm surge is a direct function of its Saffir-Simpson category (i.e. a Category 3 storm). Many hurricane response plans are based on a hurricane’s Saffir-Simpson ranking. For example, a coastal business may have a plan that only requires evacuation for a Category 3 hurricane or greater. There are a number of variables that contribute to storm surge, and its Saffir-Simpson ranking is only a slight factor. Much more significant is the size of the hurricane’s wind field—a large Category 2 hurricane will produce a greater storm surge than a small Category 4 or 5 hurricane. Every hurricane is different, so never rely solely on a hurricane’s Saffir-Simpson ranking to estimate storm surge.

Tropical Storm Forecast for 2019

For a full look into our predictions for the 2019 Atlantic Hurricane season, download our forecast here. In the meantime, here are a few key takeaways:

StormGeo’s tropical experts monitor and project several regional and global factors when developing the tropical season forecast. These factors include Atlantic and Pacific Ocean water temperatures, global wind and pressure patterns and past tropical seasons which share the weather features of the current tropical season. Based on these factors, StormGeo tropical meteorologists expect slightly fewer storms than the 30-year seasonal average for the 2019 Atlantic Hurricane Season. 

Atlantic Ocean water temperatures play a crucial role in tropical development, as warmer water translates into more energy for developing storms. However, the Atlantic endures decades-long cycles of cooler- or warmer-than-normal temperatures which have a long-lasting effect on seasonal storm counts. The most recent warm phase of the Atlantic Multidecadal Oscillation (AMO), which began in 1995, was noted by a significant uptick in hurricane activity. However, the warm phase has likely ended as cooler-than normal temperatures have been noted since late 2013. This likely indicates the beginning of a new cool cycle that should translate into less hurricane activity, on average, for the next 20+ years.

Next, we look to the Tropical Pacific for signs of significant warming or cooling of the ocean. A warming of the Tropical Pacific is referred to as an El Niño. During an El Niño, hurricane activity typically increases in the East Pacific but decreases in the Atlantic. This decrease is the result of a few different things.

First, rising air in the Atlantic generally sinks in the Gulf of Mexico and the Caribbean Sea, inhibiting tropical storm development. Second, an El Niño tends to produce unfavorable wind shear across the Tropical Atlantic which can inhibit storms. With a strong El Niño in place this year, tropical forecasters anticipate diminished tropical activity in the Atlantic, as compared to normal. 
 

Hurricane FactsForecast 1

StormGeo's 2019 Atlantic Hurricane Season Predictions


Finally, our tropical meteorologists have compared the weather features of previous tropical seasons to the current global weather patterns. Determining these ‘analog years’ requires significant research by tropical and climate experts. For 2019, analog years identified by StormGeo suggest below-normal activity with little activity between the eastern Caribbean and the coast of Africa. These same analog years point to a potential risk to the same parts of the southeastern U.S. hit so hard by storms in 2017 and 2018, as well as southern New England, the western Caribbean and eastern Gulf of Mexico

Where can I learn more about hurricanes and get a full hurricane forecast for this year?

To hear Fred Schmude, StormGeo Sr. Scientist for Long-Range Weather Prediction, and myself discuss our predictions for the 2019 Atlantic hurricane season, you can download our recent webinar. Alternatively, we have created a handy, printable forecast PDF that includes the predicted number of named storms, number of hurricanes and number of intense hurricanes, along with more information on which regions are most likely to be impacted.