It’s Official – Gonzalo was a Hurricane when it Hit Antigua!

25 01 2015

By Dale C. S. Destin|

GonzaloFromSpace

Hurricane Gonzalo as seen from the International Space Station on Oct 16, 2014.
At the time, Gonzalo was at peak strength, with 145 mph winds. Picture courtesy NOAA.

It turns out that we were all right or, at least, most of us. Gonzalo was indeed a hurricane when it hit Antigua on Monday morning, October 13, 2014. This has been confirmed in a just released report by the United States National Hurricane Center (USNHC), written by Daniel Brown, Senior Hurricane Specialist. The USNHC has retroactively upgraded Gonzalo to a hurricane about 13 miles east of Antigua based on images (courtesy Observer Media Group) and data shared by the Antigua and Barbuda Meteorological Service (ABMS) Climate Section.

Best track positions for Hurricane Gonzalo, 12-19 October 2014, show the system becoming a hurricane with centre about 13 miles east of Antigua

Best track positions for Hurricane Gonzalo, 12-19 October 2014. Gonzalo became a hurricane with centre about 13 miles east of Antigua. Graphic courtesy USNHC.

Gonzalo developed rapidly into a hurricane just before it slammed into Antigua. The development of the system was poorly forecast; thus, many persons were caught by surprise. The system cost the country around US $40 million in damage and lost revenue.

What is a hurricane?

For a tropical cyclone (the generic name for depressions, storms, hurricanes) to be classified as a hurricane, in our part of the world, it must have maximum sustained 1-minute wind speed of 64 kt or 74 mph or greater. Hurricanes are further categorized on a scale of 1 to 5.

What did we get?

Gonzalo's best estimated winds.

Gonzalo’s best estimated winds. Dashed vertical lines correspond to 0000 UTC (or 8 pm local time) and solid vertical lines correspond to landfalls. The first one indicate landfall on Antigua. Graphic courtesy USNHC.

Gonzalo blew hurricane-force winds across Antigua from around 8:37 am to 8:44 am, October 13. Data from the automatic weather station (AWS) at the V. C. Bird International Airport show that Gonzalo had sustained winds of 67 kt or 77 mph. Further, maximum wind gusts were around 78 kt or 90 mph. This makes Gonzalo a Category 1 hurricane (on the Saffir-Simpson Hurricane Wind Scale) at the time of impacting Antigua.

Officially, the strongest winds were measured around 8.44 am. However, unofficial reports suggest that sustained winds, over some parts of the island, could have reached as high as 85 mph with gusts near 105 mph.

Gonzalo's radar image

Radar image from Guadeloupe at 10 am, 13 Oct, 2014, around the time the centre of Gonzalo passed over Antigua. Image courtesy of Meteo France.

Immediately after the passage of Gonzalo, the majority of residents were of the firm view that the cyclone had winds of hurricane strength and was not a tropical storm as was designated by the USNHC and adopted by the Antigua and Barbuda Meteorological Service (ABMS). The AWS data have proven them correct beyond a shadow of a doubt, if there were ever any. After experiencing many tropical cyclones over the past 20 years, the residents of Antigua have developed innate anemometers (instrument for measuring wind speed) that proved spot on.

What went wrong?

The USNHC has admitted that “Gonzalo was not well forecast…the timing of genesis was not well predicted”. According to the report, the cyclone was not forecast to develop until is passed the Leeward Islands.

Having formed, the forecast continued to be less than desired. Gonzalo was forecast by USNHC to have sustained winds of 40-60 mph at the time of impacting Antigua. The ABMS adopted these ranges as is almost always the case. However, the Daniel Brown report now shows that the USNHC forecasts were 5 to 15-kt below the actual wind speed of the system.

USNHC is the World Meteorological Organization’s regional specialized meteorological centre task with the sole responsibility to issue tropical cyclone bulletins for our region. They are especially skilled in determining tropical cyclones wind speeds, either indirectly with the use of the Dvorak Technique or directly by way of flying aircrafts into the cyclones. The latter method is most accurate and was used in determining the maximum sustained winds of Gonzalo at various times.

The error most likely came from the undetected significant intensification of Gonzalo between reconnaissance (recon) flights into the system and conservative estimation of the winds on the Sunday afternoon, just prior to landfall. There were two recon flights on that Sunday and the next one was not until the Monday morning during the onslaught on Antigua. Between the latter two flights, the strength was mainly based on Dvorak technique, models and trend, all of which apparently underestimated the winds; hence, the discrepancy between what was forecast and what we actually got.

The ABMS also played a part in the underforecasting of Gonzalo’s wind strength. Unfortunately, the AWS wind data that formed the basis for the upgrade of the cyclone to a hurricane, just east of Antigua, are not currently available operationally. [If the data were available operationally, USNHC would have more than likely done a better job of estimating the winds and upgraded Gonzalo to a hurricane by the time it was over us the Monday morning, as opposed to 5 pm the Monday afternoon. Of course, this would not have mattered much from a preparation standpoint, but it would have prevented any debate about the strength of Gonzalo when it hit us.]

What now?

The record has now been officially corrected to reflect that Gonzalo impacted Antigua and Barbuda as a Category 1 hurricane. The tropical cyclone community is very pedantic about the recording of tropical cyclones; thus, it was always anticipated that once our findings were made known to the USNHC, the correction would have been made. It is hope that the upgrade has not come too late for those who could not claim for damages from Gonzalo because it was not officially classified as a hurricane.

Of course, we would have liked to have known that Gonzalo was going to become a hurricane, at least, the Saturday afternoon, prior to it almost “breaking hell loose” over us. While track forecasting of tropical cyclones continues to improve, intensity forecasting continues to be of great challenge to the community and there have been very little improvements over the recent years. However, the work to improve this aspect of tropical cyclone forecasting continues earnestly.

As for the ABMS, all of the AWS wind data need to be available operationally for optimal service to the public. Anything less is unacceptable. The hurricane season runs from June 1 to November 30. On average, the season produces 12 named storms, 6 hurricane and 3 major hurricanes. Follow me on anumetservice.com/wordpress.com to be among the first to read my blogs. Also, follow me on social media, including @anumetservice and facebook.com/anumetservice for the latest on weather and climate.

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