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Hurricane Irma continues her push through the Caribbean on her way to the US mainland.

For current information, please check with local and national sources for updates.

British territories are among the islands hit. Prime Minister Theresa May continues to be briefed while the Ministry of Defence sends relief and personnel:

The British Virgin Islands, which saw houses reduced to their foundations and many roads impassable in the wake of Irma, has declared a state of emergency.

Images posted on social media showed entire structures razed to the ground, with debris scattered across the streets.

A spokesman for the Ministry of Defence said aircraft carrying around 230 personnel, made up of engineers, marines and medical specialists, will take rations and medical supplies to places affected including Barbados and the British Virgin Islands …

Theresa May’s spokeswoman dismissed criticism that the UK lagged behind France and the Netherlands in taking care of its territories in the path of the hurricane.

The spokeswoman told a Westminster briefing: “We believe our response was swift. We had a ship pre-positioned. We are getting lifesaving aid now to those who need it.”

True. I read that earlier this week.

The Caribbean

As a Category 5 hurricane, Irma caused ‘catastrophic’ damage to the US Virgin Islands. Three people have died.

Irma pummelled the Turks and Caicos Islands, which had never before experienced a Cat 5 hurricane:

Irma is now downgraded from a Category 5 to a Category 4 ‘storm’. Be in no doubt that the downgrade means nothing much happened or will happen.

Bahamians were evacuated (mandatory) to Nassau. Irma is expected on Friday.

Here’s Haiti:

Eastern Cuba, including Guantanamo — is expected to be hit. Hmm. Finding out what happens to the prison could make for interesting reading.

This is the projection from Thursday:

More information on hurricanes

Vox has an excellent article explaining hurricanes.

I wrote yesterday that most North American hurricanes originate in Africa. That post is useful reading, explaining exactly how the different weather systems in the Sahara and West Africa create hurricane conditions.

Vox‘s ‘How do hurricanes form? A step-by-step guide’ is excellent and comes with illustrations. It relates to Irma in particular and will help to explain the news updates in the next section of this post. Excerpts follow, emphases mine.

Irma:

started out as a wave off the African coast …

This shows how Irma has been plotted by various trackers:

Vox explains that tropical waves originate off the West African coast:

A hurricane in the Atlantic Ocean typically begins life as a lowly “tropical disturbance” — defined as organized thunderstorm activity that stretches at least 100 miles across and maintains its identity for more than 24 hours.

During the summer, these disturbances often start as storms moving westward off the coast of Africa in what are known as “tropical waves.”

Many times, nothing much develops. However, if the tropical disturbance needs to be tracked, meteorologists designate it as an ‘investigative area’ and refer to it as an ‘invest’:

Irma became a disturbance off the Cape Verde Islands in late August, with forecasters keeping close watch as it headed west.

A tropical disturbance that develops further spins around a low pressure centre. It is then reclassified as a tropical depression or tropical cyclone. This is what happens:

The warm ocean heats the air above

Rising warm air evaporates and starts to spin

The air then cools and condenses to form a towering cumulonimbus cloud

Intense low pressure then sucks in air, causing very strong winds

For this to occur, the following conditions must all be in place. Emphases in the original below:

The water has to be warm enough to fuel the system, with temperatures of 80°F or hotter. There needs to be enough moisture in the lower and middle part of the atmosphere. Local winds also have to be arranged so that they allow the depression to spin — too much wind shear can tear an aspiring tropical cyclone apart.

This is why hurricanes develop during the summer into early autumn. The water has to be warm enough.

If the tropical depression or tropical cyclone continues to pick up strength, it is reclassified as a tropical storm and is given a name:

That’s what happened to Irma on August 30, as it picked up speed in the far Eastern Atlantic and intensified.

The US National Hurricane Center makes the call for when a tropical depression officially becomes a tropical storm. It relies on data from islands and buoys as well as from reconnaissance aircrafts that fly into the storms to measure wind speed.

A tropical storm can become a hurricane if the water it passes over is very warm (around 28°-30° C or low- to mid-80°s F) and there isn’t much wind shear (i.e. change in speed or direction). If those two conditions occur simultaneously, the development of a tropical storm into a hurricane is relatively quick. The tropical storm becomes rounded and develops an eye:

A tropical storm with winds of 74 mph or higher is reclassified as a hurricane, according to the Saffir-Simpson Hurricane Wind Scale:

Irma was a Category 5 as of Tuesday with wind speeds of 185 miles per hour. That’s serious — major hurricanes can do structural damage to buildings, take down trees, and cause widespread flooding.

Irma is now a Category 4.

Two things can happen with a hurricane. They can weaken to return to tropical cyclone status as a result of hitting land or cooler water. Sometimes, though, they can weaken then strengthen if they go out to sea and hit warm water again (emphases in purple mine):

Hurricane Hermine in 2016 was downgraded to a tropical storm not long after it made landfall in Florida in September. But then Hermine moved back over the Atlantic Ocean and hit record-warm ocean temperatures there, gathering to hurricane strength again.

It’s worth emphasizing that even tropical cyclones that aren’t hurricanes can still do a great deal of damage by bringing torrential rain, dangerous surf, beach erosion, high winds, and flooding. In 2012, “superstorm” Sandy was technically no longer a hurricane when it hit the East Coast, but it still proved devastating to the New York and New Jersey coasts.

Record-setting Irma

Irma is likely to set a few records or be in the top list of perfect storms:

Latest Irma news

From what I have read anecdotally on Friday, some Americans who potentially lie in Irma’s projected path are frustrated and angry with news reports. Do they evacuate or stay put?

The sad truth is that no one knows definitively what Irma’s path is.

New technologies developed over the past few years are giving people the best images and projections possible.

As of Thursday, September 7:

Mention has been made of Irma’s eyewall:

Eyewall replacement cycles occur in stronger tropical cyclones (winds greater than 185 km/h or 115 mph) and Category 3 – 5 hurricanes. Wikipedia explains (emphases mine):

Since the strongest winds are in a cyclone’s eyewall, the tropical cyclone usually weakens during this phase, as the inner wall is “choked” by the outer wall. Eventually the outer eyewall replaces the inner one completely, and the storm may re-intensify.

The next two illustrations show unusual movement. Notice how Irma might not affect the Carolina coastline as previously thought. However, Irma will probably go to the centre of the country, towards Tennessee, possibly Nashville. This is unheard of. With regard to Florida, Irma is likely to cut a south to north swathe, which is very rare, as hurricane movement is normally east to west there. This will be bad:

The water in the Atlantic and Gulf of Mexico is very warm, therefore, favourable to Irma — disastrous for those who lie in her path:

These are the weather alerts:

Here are more specifics on storm surges (see point 3), which seriously endanger life:

Much of southern Florida could be devastated. Let’s hope not:

This is the potential for storm surge damage along Florida’s southeastern coastline:

These are the potential wave heights:

Florida

Meteorologist James Spann issued a detailed forecast on Thursday.

In Florida:

The most severe damage should be on the Atlantic coast from Miami up through Daytona Beach and Jacksonville late Saturday night and into Sunday.

*The Florida west coast (cities like Naples, Fort Myers, Sarasota, St Petersburg, and Tampa) are on the west side of the system, but due to it’s strength wind damage is very possible there Sunday along with widespread power outages and some flooding.

*Hurricane force winds are likely in Orlando Sunday. Potential for structural damage there. I would not want to be in Orlando Sunday; the weather will be much better by Monday.

The whole of Florida is under a state of emergency with mandatory evacuations in certain counties. Mar-a-Lago has also been evacuated.

Relief shelters are open. Broward County is home to Fort Lauderdale:

Petrol is in short supply as people attempt to drive out of state:

People are stocking up and battening down the hatches.

Traffic is crazy. This is from Thursday:

Airports in evacuation areas are closing, including on the west Coast.

People are preparing for the worst:

In closing, this is no time to party:

Prayers and Godspeed to all.

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