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Hurricane Irma Update

September 6, 2017

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.


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.


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.



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.

Meanwhile, the official report into Hurricane Camille found that wind speeds reached 201 mph.



So any claims about “most powerful hurricane” need to be regarded with extreme suspicion.

  1. September 6, 2017 12:25 pm

    On 10-16 October 1780, a Great Hurricane struck Barbados, Saint Vincent, Granada, Saint Lucia, Martinique, Hispaniola, Dominica, Guadeloupe, Antigua, Saint Kitts, Sint Eustatius [now called Statia], Puerto Rico and Bermuda killing over 27,500 people. The hurricane produced wind speeds (gusts) in excess of 200 miles per hour (320 kilometers per hour). In Barbados, “The winds stripped the bark off trees before the hurricane downed every tree on the island.” In Barbados, the winds and seas moved heavy cannons about 100 feet (30 meters). The hurricane destroyed 19 Dutch ships at Grenada; the British fleet of Admiral Rodney at Saint Lucia; a fleet of 40 French ships off Martinique; many ships washed ashore at Saint Kitts; and grounded 50 ships near Bermuda.

    Click to access A_Chronological_Listing_of_Early_Weather_Events.pdf

  2. Bloke down the pub permalink
    September 6, 2017 12:55 pm

    One comment I’ve seen on a number of occasions in hurricane reports is along the lines of ‘wind speeds reached X knots before the anemometer broke’.

  3. Broadlands permalink
    September 6, 2017 1:24 pm

    It seems that the point of asserting a record this or record that (irrespective of it’s validity) is to imply that us humans were able to make it happen by adding some CO2 to the atmosphere. And we can stop it but we better act soon.

    • dave permalink
      September 6, 2017 4:35 pm

      If ‘their’ intent is to confuse history, the success is perfect.

      When the ‘hurricane season’ of this year in the Southern Hemisphere was so unusually subdued, the MSM in the world – so far as I could ascertain – breathed NOT ONE WORD.

      • nigel permalink
        September 6, 2017 5:00 pm

        Suppress contradictory facts, throw in a bit of emotional confirmation bias, and – voila! – most of the people are fooled most of the time – which is all it takes.

  4. The Old Bloke permalink
    September 6, 2017 4:05 pm

    The use of the web sites Wunderground and Flightradar 24 at the same time will tell you exactly what the wind speeds are at ground level and not at 48,000 ft. At this precise moment Irma is showing a wind speed of, at best, 40 mph with the occasional gust of 48/50 mph. Wunderground is good for 1 minute observations, Flightradar24, good for 5 seconds.

  5. Athelstan permalink
    September 6, 2017 4:10 pm

    al beeb relentlessly playing footage of it “second biggest evah” and for all its ‘value’….. we know what is the subtext.

    Sincerely, I hope that, the bugger [Irma] sidles off north and east, away from the US seaboard into the Atlantic, fills and doesn’t cause to much damage. Though, the islands now feeling its wrath – hope and prayers for those people. In having said that, the Caribbean is infamous, hardly unknown for suffering ferocious storms.

    A Beautiful climate most of the time – “most of”, presently it will be of no consolation however people will, should, must understand the quid pro quo, the risk, the threat of a major storm its brooding presence is always there.

  6. September 6, 2017 6:30 pm

    The President of Antigua says (in this extract):
    ‘We in Antigua have weathered the most powerful hurricane ever to storm its way through the Caribbean.
    And we have done so with stunning results.
    The forecast was that Antigua would be devastated, our infrastructure demolished, people killed and our economy destroyed.
    In the light of day, the picture is very different.
    In Antigua, no life has been lost – all the people survived.
    The guests in our hotels are all well.
    Even our animals were protected from this massive storm.
    Our airport will be open for flights into and out of Antigua by 2pm today.’
    [see Facebook link]

  7. September 6, 2017 6:53 pm

    Excellent research Paul. I’ve been tracking Atlantic hurricanes and tropical storms every year since 1963 when I was 11 and trying to gauge the strength of past storms is often highly uncertain. There is also a fair amount of uncertainty even today where we have a lot more information about the storms. There is no way to directly measure the peak sustained wind speed in a storm. It is always estimated based on a variety of information that is often subject to interpretation, like satellite imagery or SFMR measurements that are really proxies for storm strength. We can certainly tell when a storm is very intense, like Irma, but determining the exact intensity is not possible. That makes comparing storms quite subjective in most cases, and subject to different interpretations, even based on the same evidence.

    • September 6, 2017 7:40 pm

      The lowest pressure is often quoted (especially if close to being the lowest evah), could that be a good estimate of strength? Maybe (surrounding – lowest) pressure might be better, but there is also the issue of the background wind speed, which can add to that from the hurricane.

      • September 6, 2017 10:51 pm

        Lowest pressure is a somewhat crude indicator of strength. It is the pressure gradient near the center that generates the highest winds so storms can have the same minimum pressure but substantially different peak wind speeds because of differing pressure gradients near the center. The size of the storm also comes into play regarding the pressure gradient. Large storms will generally have weaker gradients and winds than small tight wound storms with the same central pressure.

  8. Dermot Flaherty permalink
    September 7, 2017 8:42 am

    Sept 7 8:40am and a few minutes into an interview with Gaston Browne (PM of Antigua and Barbuda) on the “Today” programme, Justin Webb asked him “Do you think Climate Change is to blame ?”. Even I was stunned by the question given the undoubted devastation IRMA is causing his islands and (you would have thought) most normal people’s immediate focus on supplying practical help.
    Plus the fact that I didn’t realise that the PM had a background in “Climate Change” such that he could give a considered reply. (I checked and as far as I can tell, he doesn’t).
    I am afraid I didn’t concentrate on the PM’s extensive and fairly dignified reply as I should have done but I think it was more on the lines of “if there was anything that could be done to reduce the likelihood of such events in future then we should do it” – a sentiment with which no one could disagree but which is definitely NOT answering “Yes” to JW’s question.

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