There are a number of different ways to gauge a hurricane season. One of these is the Accumulated Cyclone Energy index, which is widely referred to as ACE.
It expresses the combined intensity and duration of individual cyclones and provides a measure of activity for an entire hurricane season. For a single storm, it is calculated by summing the squares of the maximum sustained wind speeds measured every six hours while they are at least tropical storm strength. This number is then divided by 10,000 to make it more user-friendly. Overall, the stronger and longer-lived a storm is, the higher its ACE value.
The ACE for a season is the sum of the ACE values from individual storms that occurred that year. NOAA considers a season with an ACE of 111 or higher to be above average, while an ACE of 66 or lower is regarded as below average.
The Atlantic hurricane season runs from June 1 through November 30.
(Note: Year to date the Atlantic Basin has had 13 storms with a combined ACE of 202 and there are still two months left in the 2017 hurricane season.)
Coming ashore with sustained winds measured up to 155mph, Maria was classified as a high-end category-four hurricane. Its powerful winds sheared roofs and balconies off buildings, uprooted trees, and toppled power lines. In addition, the storm’s torrential rain unleashed devastating flooding across the island. More than 30 inches of rain was reported near Caguas, PR.
Local authorities say the entire island, home to 3.5 million people, is without power. They do not expect it to be fully restored for four to six months. This unusually lengthy timeframe is largely due to the fact that Puerto Rico is an island with rugged terrain and an electrical grid that has been crumbling for years from a lack of maintenance. The Puerto Rico Electric Power Authority, the territory’s sole provider of electricity, effectively filed for bankruptcy in July.
Before making landfall in Puerto Rico, Maria roared through the Caribbean and reached category-five strength. It battered several other islands, including the US Virgin Islands, which were still reeling from the devastation caused by Hurricane Irma less than two weeks ago. As of Thursday, the death toll from Maria stands at seventeen. But sadly, this number is expected to increase as search and rescue efforts continue across the region.
Maria was the third category-four hurricane to hit the US, or one of its territories, in less than a month. In 166 years of record keeping, this has never happened before.
Hurricane Maria moves across Puerto Rico. Credit: NOAA/NASA
Hurricane Irma, the 9th named storm of this Atlantic Hurricane season, made two landfalls in Florida on Sunday. First, its eye crossed Cudjoe Key, just east of Key West, as a category-4 hurricane. Then, a few hours later, it came ashore at Marco Island on the state’s Gulf Coast as a category-3 storm.
Measuring about 425 miles in diameter, Irma was wider than the Florida peninsula and its effects were felt across the entire Sunshine state. Powerful winds, heavy rain, and storm surge flooding caused catastrophic property damage in the Keys and along both coasts.
In the west, Naples, FL experienced wind gusts up to 142mph – the strongest recorded during the storm. On the east coast, gusts were clocked up to 109mph in the Miami/Fort Lauderdale area, where three high-rise construction cranes collapsed in the strong winds. The storm surge in both Biscayne Bay and Naples was about four feet above normal tide levels. In the hard hit Keys, according to FEMA, 25% of homes were completely destroyed and about 65% sustained serious damage.
Reports of toppled trees and downed power lines are widespread. The Department of Homeland Security says 15 million people lost power, making it one of the largest power outages in US history.
Moving north and shifting inland, Irma was downgraded to a tropical storm on Monday. Nevertheless, it continued to pack a powerful punch. Jacksonville, FL reported a record six-foot storm surge and the worst flooding it has seen since Hurricane Dora moved through the area in 1964.
Irma’s wrath was also felt in parts of Georgia and the Carolinas. Along the coast, storm surge flooding inundated communities from Savannah, GA to Charleston, SC. Further inland, the NWS issued its first ever tropical storm warning for the metro-Atlanta area.
Coming just sixteen days after Hurricane Harvey, Irma was the second category-4 storm to make landfall in the US this season. In 166 years of record keeping, never before have two Atlantic hurricanes of such intensity hit this country during the same year.
The death toll from this historic storm currently stands at forty-three, including five fatalities reported in Florida, three in Georgia, and one in South Carolina. The rest are from the Caribbean, where Irma hit multiple islands as a Category 5 storm late last week. Sadly, these numbers are expected to rise as search and rescue efforts continue. The entire region faces a long and expensive road to recovery ahead.
Hurricane Irma makes landfall in southern Florida as a Category-4 storm. Credit: NASA
Hurricane Irma’s final chapters have yet to be written, but the storm has already earned its place in the history books.
According to NOAA, Irma’s winds peaked at 185 mph. That means it is tied as the second strongest hurricane ever recorded in the Atlantic Basin. It shares this dubious honor with the Labor Day /Florida Keys Hurricane of 1935, Hurricane Gilbert of 1988, and Hurricane Wilma of 2005. Only Hurricane Allen in 1980 was stronger with winds clocked at 190 mph. That said, Irma now ranks as the strongest storm on record in the Atlantic outside of the Carribean Sea and the Gulf of Mexico.
Irma’s winds remained at 185 mph for 37 hours. That is the longest any storm on the planet has maintained that level of intensity. The previous record of 24 hours was set by Super Typhoon Haiyan in the northwest Pacific in 2013. Furthermore, Irma remained a Category 5 hurricane for three consecutive days, the longest such measurement since the satellite era began in 1966.
According to the NWS, 51.88 inches of rain was reported at a gauge near Mont Belvieu at Cedar Bayou, about 40 miles east of Houston. This crushes the previous record of 48 inches that was set by Tropical Storm Amelia which hit Medina, TX in 1978.
The record for the entire US, however, still stands. It was set in 1950 when Hurricane Hiki dropped 52 inches of rain on Hawaii.
Days after making landfall as a category-4 hurricane, Harvey, now a tropical storm, is still battering southeast Texas. Relentless rain has unleashed catastrophic flooding across the region, including Houston – this country’s fourth largest city.
While this devastating event is still unfolding, rainfall totals in some areas have already exceeded two feet. Dayton, TX, northeast of Houston, has reported a staggering 39.72 inches of rain since Thursday. Houston Intercontinental Airport (IAH) has posted 25.66 inches of rain so far and reported its wettest calendar day on record on Sunday with 16.07 inches.
This intense rainfall is causing the many rivers, creeks, and bayous in the metro Houston area to overflow their banks and flood homes, businesses, and major roadways – effectively paralyzing the area. Rainfall rates reached as high as 3 inches per hour on Sunday in some areas, prompting the NWS to issue a flash flood emergency – the highest level of a flood alert.
No evacuation order was issued ahead of the storm in Houston and residents were told to shelter in place. Now, thousands of high water rescues are taking place via boat and helicopter to save people trapped in their homes. The situation has many people comparing it to scenes that played out in the aftermath of Hurricane Katrina in New Orleans. The mayor of Houston, Sylvester Turner, has opened the massive George R. Brown Convention Center to be used as a shelter. As of Monday, eight storm-related deaths have been reported. But sadly, local authorities expect this number to increase as the water continues to rise.
Southeast Texas is no stranger to flooding. However, officials in Houston say this event is the worst they have seen since Tropical Storm Allison in 2001. That storm dumped more than 35 inches of rain on the metro area over the course of five days and caused $5 billion worth of damage.
Harvey, wedged between two areas of high pressure, is expected to linger over the region for several more days and dump an additional 15 to 25 inches of rain. Forecasters at the NWS say the rainfall total for this storm could reach an unprecedented 50 inches in some spots.
Eighteen counties in Texas, according to Governor Greg Abbot, have been declared federal disaster areas because of this storm.
Coming ashore with winds measured up to 130mph, Harvey was classified as a Category-4 hurricane on the Saffir-Simpson Scale. These powerful winds along with storm surge flooding and torrential rain caused catastrophic property damage across the Texas Coastal Bend region. It also downed trees and left hundreds of thousands of people without power. To date, according to local officials, only two storm-related deaths have been reported.
Moving inland, toward Houston, Harvey weakened to a Tropical Storm on Saturday afternoon. However, despite this downgrade in wind speed, its rain bands are still drawing moisture from the Gulf of Mexico and unleashing massive amounts of precipitation. Wedged between two areas of high pressure, the storm is essentially stalled over the region. Therefore, even more flooding rain is expected over the next several days.
Harvey was the first major hurricane – category 3 or higher – to make landfall in the Lone Star State since Hurricane Carla in September 1961.
Satellite imagery from NOAA shows Hurricane Harvey along the Texas Coast. Credit: NOAA
Tropical cyclones are fueled by warm ocean water and typically peter out over land. Sometimes, however, their lives are extended by something called the “brown ocean effect”.
This is a phenomenon where a storm derives energy from the evaporation of abundant soil moisture deposited by previous rainfall. Essentially, the saturated soil mimics the role of the ocean allowing a tropical cyclone to maintain its strength or even intensify after making landfall.
For the brown ocean effect to occur, according to a NASA funded study by Theresa Andersen and Marshall Shepherd of the University of Georgia, three criteria need to be met:
The soil needs to contain copious amounts of moisture.
Atmospheric conditions near the ground must have tropical characteristics with minimal variation in temperature.
Evaporation rates must be high enough to provide the storm with sufficient latent heat that it uses for fuel, at least 70 watts averaged per square meter.
Although this process supplies less energy than the ocean, it is enough to sustain a storm for a longer period than normal over land. It was first noticed in 2007 after Tropical Storm Erin made landfall in Texas and then intensified as it traveled inland. It formed an eye over Oklahoma and unleashed a massive amount of rainfall.
Storms that are impacted by the brown ocean effect maintain a warm-core and are known as Inland Tropical Cyclone Maintenance and Intensification events (TCMIs). While rare, they are most common in the US, China, and Australia.
The Hurricane Season officially began in June, but August is when things typically start to ramp up. It is also when the word “invest” (short for an investigative area) becomes more prevalent in weather forecasts.
When the National Hurricane Center (NHC) wants to take a closer look at an area of disturbed weather that could possibly develop into a tropical cyclone, it designates it as an invest. This opens up specialized resources, such as computer models and satellites that provide forecasters with additional data on the area in question. That said, an invest does not always become a tropical system.
More than one invest can exist at any given time, so the NHC has a special way to identify them. They are numbered from 90 to 99, followed by a letter. If the invest is in the Atlantic, the letter will be “L” and if it is in the Eastern Pacific, it will be an “E”. The numbers can be reused throughout a season, as necessary.
The Central Pacific Hurricane Center and the Joint Typhoon Warning Center follow a similar system. Letters for the basins they cover include “C” for the Central Pacific, “W” for the Western Pacific, “A” for the Arabian Sea, and “B” for the Bay of Bengal.
Tropical Storm Emily, the fifth named storm of the 2017 Atlantic Hurricane Season, made landfall at Anna Maria Island in Tampa Bay, FL on Monday morning. It slammed the Sunshine state’s west coast with heavy rain and winds measured up to 45 mph.
According to the NWS, Valrico, FL, a few miles east of Tampa, saw 8.19 inches of rain. Widespread flooding was reported across the region and Gov. Rick Scott declared a state of emergency for 31 counties.
Most land-falling tropical storms come with a few days warning, as they develop over the ocean before moving toward populated areas. Emily, however, sprung up very quickly. It formed as the result of an out-of season cold front stalling out over the extremely warm waters of the Gulf of Mexico near Florida’s west coast.
Moving across the Florida peninsula, away from the warm waters that fueled it, the storm weakened quickly and was downgraded to a tropical depression. It is forecast to travel northeast out into the Atlantic Ocean. No major impacts are expected along the Eastern Seaboard, but rip currents will be an issue for beachgoers over the next few days from Florida to the Carolinas.
TS Emily moves over central Florida. Credit: NOAA/NASA GOES