Hurricane Irma Update

By Paul Homewood

This is the latest news from ABC on Hurricane Irma:

Hurricane Irma passed over Barbuda during the early morning hours on Wednesday as Floridians prepared for the worst ahead of the record-breaking storm.

The monster storm maintained winds of 185 mph — with gusts topping 200 mph — even as it made landfall in Barbuda at about 2 a.m. on Wednesday. The storm was moving west-northwest at about 15 mph with St. Martin in its crosshairs as of 5 a.m. on Wednesday.

There was a hint of good news for Floridians in Wednesday’s 5 a.m. update from the National Weather Service. The hurricane’s path is now forecast to ride up the middle of Florida, keeping the worst side of the storm in the Atlantic Ocean. But many projections for the storm now show it could move east of Florida and make landfall near Georgia or the Carolinas.

Irma is the strongest hurricane ever in the Atlantic basin outside of the Caribbean Sea and Gulf of Mexico. But the storm is closing in on the record set by Hurricane Allen in 1980, which reached maximum sustained winds of 190 mph.

FORECAST

The "potentially catastrophic" storm, as described by the NWS, is expected to skirt the northern parts of Puerto Rico, the Dominican Republic and Cuba over the next three days. New hurricane warnings were issued for the Bahamas and Turks and Caicos on Wednesday morning, with the storm expected to move over Puerto Rico on Wednesday afternoon.

Preparations are already underway in Florida, where landfall is expected in south Florida on Sunday afternoon. The shifting projections of the storm, as of 5 a.m. on Wednesday, show Irma will travel up the middle of Florida and even possibly to the east.

So-called "spaghetti models," which project possible paths for the storm, show Irma could threaten the Carolinas and East Coast of the United States.

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There is still a lot of uncertainty about where Irma will head next, but it is slightly encouraging that it will gradually weaken as it heads towards the US.

There will time for proper analysis later, But there have been claims that Irma is now the most powerful Atlantic Ocean hurricane in recorded history, for instance here, with sustained winds of 185 mph.

But, as Fox News reports, this is slightly misleading:

Four other storms have had winds as strong in the overall Atlantic region but they were in the Caribbean Sea or the Gulf of Mexico, which are usually home to warmer waters that fuel cyclones.

Hurricane Allen hit 190 mph in 1980, while 2005’s Wilma, 1988’s Gilbert and a 1935 great Florida Key storm all had 185 mph winds.

There is also the issue of “recorded history”. Over the years, methods of measuring hurricane strength have changed radically.

Nowadays of course we have comprehensive satellite coverage, backed up in the Atlantic particularly with modern hurricane hunter aircraft.

Leading hurricane expert Chris Landsea, of the National Hurricane Center, published a study in 2012, “On the Classification of Extreme Atlantic Hurricanes Utilizing Mid-Twentieth-Century Monitoring Capabilities”.

His report included this chart, illustrating how much things have changed.

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Even though the US began using hurricane hunters in 1944, they often did not fly into the centre of the strongest storms, as Landsea explained:

“The analyses indicate that all of the hurricanes in the study that did not reach Category 5 strength would have been classified as a Category 4. The reader is reminded that the methodology employed is somewhat conservative. For example, many times during the late 1940s the aircraft often did not penetrate the center of hurricanes with central pressures in the 950s or even the 960s. If this criteria of, say, a 960-mb threshold were utilized, many of these cyclones would have been listed with a peak intensity of only Category 3 strength.”

Landsea concluded that

An investigation is conducted to determine how improvements in observing capabilities and technology may have affected scientists’ ability to detect and monitor Saffir–Simpson Hurricane Wind Scale Category 5 hurricanes in the Atlantic Ocean basin during the mid-twentieth century. Previous studies state that there has been an increase in the number of intense hurricanes and attribute this increase to anthropogenic global warming. Other studies claim that the apparent increased hurricane activity is an artifact of better observational capabilities and improved technology for detecting these intense hurricanes. The present study focuses on the 10 most recent Category 5 hurricanes recorded in the Atlantic, from Hurricane Andrew (1992) through Hurricane Felix (2007). These 10 hurricanes are placed into the context of the technology available in the period of 1944–53, the first decade of aircraft reconnaissance. A methodology is created to determine how many of these 10 recent Category 5 hurricanes likely would have been recorded as Category 5 if they had occurred during this period using only the observations that likely would have been available with existing technology and observational networks. Late-1940s and early-1950s best-track intensities are determined for the entire lifetime of these 10 recent Category 5 hurricanes. It is found that likely only 2 of these 10—both Category 5 landfalling hurricanes—would have been recorded as Category 5 hurricanes if they had occurred during the late-1940s period. The results suggest that intensity estimates for extreme tropical cyclones prior to the satellite era are unreliable for trend and variability analysis.

Below are the hurricanes he reanalysed. we can see, as an example, that Hurricane Wilma, which peaked at 160 kt in 2005, (185 mph – the same as Irma), would likely have been recorded as only a Cat 4 storm in the 1940s and 50s, with sustained winds of 125 kt or 144 mph.