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WEATHER REPORT
Perfect storms: hurricanes and typhoons
by Staff Writers
Paris (AFP) Sept 13, 2018

Why a weaker, Category 2 hurricane is still dangerous
Wilmington, United States (AFP) Sept 13, 2018 - Hurricane Florence was downgraded to Category 2 storm on Thursday, but forecasters cautioned that the mammoth storm still poses a major threat to millions of people. Here's why.

- Wind speeds -

"There's more to the story than the category," said Brian McNoldy, a researcher at the University of Miami's Rosenstiel School of Marine and Atmospheric Science.

"While the peak winds have fallen, they are still strong enough to be destructive to houses and trees, and the water-related threats (storm surge and rain) remain a very big concern," he wrote in a blog post Thursday.

Hurricanes are categorized on a scale of 1 to 5, with 5 being the strongest. These categories are based only on the speed of sustained winds, not other deadly factors like storm surge, rainfall or flooding.

A Category 2 storm packs sustained wind speeds of 96-110 miles per hour (154-177 kilometers/hour) and is no longer considered a "major" storm -- a class belonging to hurricanes Cat 3 and up.

However, a Cat 2 storm's wind speed is "extremely dangerous," according to the National Hurricane Center, capable of ripping trees from the ground, wreaking major roof damage on homes and causing power outages that may last weeks and affect three million households.

The storm has a wide reach, too. Tropical storm force winds extend up to 200 miles (320 kilometers) from the center.

- Storm surge -

Storm surge can be the deadliest part of a hurricane, and is only partially affected by wind speeds.

The term "storm surge" refers to rising seas that are whipped up by a storm.

"I don't care if this goes down to a Category 1," said CNN meteorologist Chad Myers.

"We're still going to have a Category 4 storm surge."

High tides and increasing sea levels from climate change can magnify the effects of storm surge, and surge doesn't necessarily weaken in step with wind speeds.

The coastal areas of North Carolina are expected to bear the brunt of the storm surge, with a nine to 13-foot (four-meter) wall of water pushing inland.

Any "flooding will be worse around high tide, and a slow-moving storm will stick around for at least one tide cycle," explained McNoldy.

- Devastating floods -

Florence is moving slowly, about 12 miles per hour (19 km/h) on a meandering track along the coast.

A crawling pace can be even more punishing than a fast one, because it lets the storm stall in one area as it dumps more and more heavy rain.

Forecasters say 40 inches (one meter) of rain are possible along the coast of North Carolina.

"This rainfall would produce catastrophic flash flooding and significant river flooding," the National Hurricane Center said.

Florence's flooding is expected to reach far inland, with more than one foot (30 centimeters) expected in some areas.

As Hurricane Florence looms off the eastern United States and Typhoon Mangkhut threatens the Philippines, here are some facts about monster storms and what to expect as climate change supercharges our weather.

- A cyclone by any other name -

Hurricanes, cyclones, and typhoons are different names for the same type of giant tropical storms that form in oceans near the Americas and Asia.

Bringing torrential rains, high winds, storm surges, and giant waves, the storms can be deadly and wreak havoc once they make land.

At their most fearsome, these low-pressure weather fronts pack more power than the energy released by the atomic bomb that levelled Hiroshima.

In the Atlantic and northeast Pacific, they are known as hurricanes, while typhoon is the term used in Pacific Asia. The same weather phenomenon in the South Pacific and Indian Ocean is a cyclone.

- Humble beginnings -

Cyclones start life as simple thunderstorms. But at certain times of the year, when sea temperatures are high enough to create evaporation, the storm fronts begin to suck up vast quantities of water.

In the northern hemisphere they are pulled into an anti-clockwise spiral as they make their way across the ocean by the rotation of the Earth. Cyclones in the southern hemisphere rotate clockwise.

The water they hold is then deposited as rainfall, bringing catastrophic flooding, property damage and loss of life.

The storms themselves -- with a calm "eye" at their centre -- can measure up to 1,000 kilometres (600 miles) across.

But they weaken rapidly when they travel over land or colder ocean waters.

- Size doesn't (always) matter -

Scientists rank cyclones from Category 1-5. Category 5 storms have sustained winds of at least 252 kilometres (157 miles) per hour or higher.

Recent Category 5s include Hurricane Irma, which battered the Caribbean and the southern United States in September 2017.

Hurricane Katrina, which killed over 1,800 people across the US Gulf Coast in 2005, was also a Category 5.

In contrast, Florence weakened on Thursday to a Category 2 event, although that is not to say it is not still extremely dangerous.

Florence is forecast to dump up to 100 centimetres (40 inches) of rain in some areas after making landfall in North and South Carolina Thursday night or Friday.

"We're expecting 500-1,000 millimetres in Jacksonville, where the average monthly rainfall is 180-200 mm," Emmanuel Bocrie, forecaster at the Meteo France weather service, told AFP.

"So a lower category storm can still be dangerous and do a lot of damage."

- A summer of violence -

Last year saw a string of catastrophic storms batter the west Atlantic -- including Irma, Maria and Hurricane Harvey -- causing a record-equalling $125 billion (107 billion euros) in damages when it flooded the Houston metropolitan area.

Bocrie said 2017 was exceptional for Atlantic superstorms as surface water temperatures were on average two-to-three degrees Celsius warmer than normal.

For this hurricane season, the NOAA forecasting service and Britain's Met Office predict between five and nine storms of Category 3 or stronger.

"That's a relatively normal season," according to Bocrie. "But be careful. That's not to say we can rule out a disaster, it only takes one."

- Worse to come? -

Scientists have long predicted that global warming will make cyclones more destructive, and some say the evidence for this may already be visible.

Warmer oceans add to the raw fuel on which cyclones feed, and higher sea levels boost storm surges that may overcome coastal defences.

Cyclones "are going to be far stronger, more violent and destructive, and we expect more rain as well," said Bocrie.


Related Links
Weather News at TerraDaily.com


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WEATHER REPORT
Monsoon, landslides kill 20 in southern India
New Delhi (AFP) Aug 9, 2018
At least 20 people were killed Thursday in landslides triggered by heavy rains in southern India, an official said, pushing the nationwide monsoon death toll for this year to over 700. The monsoon, which lasts roughly from June to September, is heralded by millions of Indian farmers but the relentless rain wreaks death and destruction every year. Last year around 1,200 people perished. In the coastal state of Kerala, where more than 70 people have died so far this year, heavy rains hit overnight ... read more

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