Was Hurricane Flora really a Category 3 Hurricane at Landfall?

Hurricane Flora is widely regarded as the strongest hurricane to make landfall across Trinidad and Tobago in recent recorded history. Hurricane Flora made landfall on September 30^th, 1963, across Southwestern Tobago as a major Category 3 Hurricane with maximum sustained winds of 105 knots (195 KM/H) and gusts to 125 knots (232 KM/H).

However, recent reanalysis has led to significant changes in this system’s intensity at landfall as part of The Atlantic Hurricane Re-analysis Project.

The Hurricane Re-analysis Project

The Hurricane Research Division of NOAA has led the Atlantic Hurricane Database Re-analysis Project. The project is an effort to extend and revise the National Hurricane Center’s North Atlantic hurricane database (or HURDAT and HURDAT2).

Scientists have gone back to 1851 and revisited storms in more recent years to maintain HURDAT2 and to provide the most accurate database possible based on all available data. Information on tropical cyclones is revised using an enhanced collection of historical meteorological data in the context of today’s scientific understanding of hurricanes and analysis techniques.

Data from Hurricane Flora (1963) was reanalyzed and published in November 2019. Revisions to the hurricane database were accomplished by obtaining the original observations collected – mainly by ships, weather stations, the Hurricane Hunter Navy, Air Force, and Environmental Science Services Administration (ESSA) aircraft reconnaissance planes, and the earliest available satellite images – and assessing the storms based upon our understanding of hurricanes today.

Hurricane Flora’s Meteorological History (Until Entering the Caribbean Sea)

Flora’s Formation

Initially, the disturbance that spawned Hurricane Flora was thought to have been spotted as a very weak depression within the Intertropical Convergence Zone. In the Monthly Weather Review of the 1963 Hurricane Season, the report cited that Flora developed from a “second cloud mass located at about 8.0°N, 32.5°W., which was associated with a very weak depression in the Intertropical Convergence Zone.”

In the reanalysis, based on satellite images between September 26th (when the formation was initially thought to have occurred) and September 28th, a tropical cyclone formed. Tropical Depression Seven was initialized at 8:00 AM AST on September 28^th, 1963, with maximum sustained winds of 30 knots (56 KM/H) at 9.2°N, 46.3°W. This is two days later than previously reported.

Flora’s Rapid Strengthening

Tropical Storm Flora formed 24 hours later, at 8:00 AM AST September 29^th, 1963, with sustained winds of 74 KM/H at 10.0°N, 52.7°W, and was rapidly strengthening. Twelve hours later, by 8:00 PM, Hurricane Flora had formed at 10.3°N, 56.0°W.

On the 29^th, the San Juan Hurricane Center requested surface observations from all ships in the area, and a Navy hurricane reconnaissance flight was arranged for daybreak on September 30^th.

The SS Del Alba forwarded all observations made at 1:00 AM AST, indicating winds from the northeast of 35 knots (65 KM/H) and a barometer reading of 1006.8 millibars with a fall of 5 millibars in the past 3 hours. At 10:00 AM, the complete report came in, much too late for adequate warning to Tobago. The San Juan Weather Bureau office issued a bulletin at 10 AM AST and the first formal hurricane advisory on Flora at 12:00 PM AST. A Hurricane Warning was issued for Trinidad and Tobago, Grenada, and the Grenadines south of St. Vincent.

Flora’s Landfall & Reanalysis

The eye of Hurricane Flora passed over Tobago at 1:40 PM AST, with the lowest pressure at 974 millibars (uncorrected). In most literature, Hurricane Flora made landfall on Tobago as a Major Category 3 Hurricane with maximum sustained winds at 105 knots (195 KM/H) and gusts of 125 knots (232 KM/H). In reanalysis, this was downgraded to a Category 2 Hurricane with maximum sustained winds at 90 knots (167 KM/H) with gusts to 110 knots (205 KM/H).

At 9:07 AM on September 30^th, reconnaissance aircraft reached Flora measuring a central pressure of 994 millibars, estimated surface winds of 110 knots, and an eye diameter of 25 nautical miles. However, A central pressure of 994 millibars suggests maximum surface winds of 58 knots due to relationships in pressure and winds south of 25°N latitude. This wind speed is well below the initial estimated value. The reanalysis, using the forward speed (17 knots), the small size of circulation, and satellite imagery, an intensity of 70 knots is analyzed at 8:00 AM on the 30^th, down from 100 knots originally in HURDAT, a major intensity change.

Around 2:00 PM (18Z) AST on the 30^th, the center of Flora made landfall in Tobago, and a central pressure of 974 millibars was measured. Though the barometer was noted as “uncorrected,” Crown Point’s elevation is 10 meters, so adjusting to sea level would not be a significant uncertainty. The central pressure of 974 millibars suggests maximum surface winds of 86 knots from the south of 25N based on the established pressure-wind relationship. The landfall intensity was revised downwards. Flora made landfall with maximum sustained winds of 90 knots (167 KM/H) at 18Z (2:00 PM) AST, down from 105 knots originally in HURDAT. This change in intensity was supported by the forward speed of about 15 knots and small circulation.

Flora Moves to Grenada and into the Caribbean Sea

On October 1st, Flora was located over the eastern Caribbean Sea, moving west-northwest, and became slightly more intense late in the day. Reconnaissance aircraft made center fixes at 16Z, 19Z, and 2110Z, measuring a central pressure of 974 mb, 967 millibars, and 970 millibars, respectively. An intensity of 90 knots is analyzed at 00Z, 06Z, and 12Z and 95 knots at 18Z, down from 110 knots at 00Z and 06Z and down from 115 knots at 12Z and 18Z, as initially shown in HURDAT. Ship data over the southeastern Caribbean Sea is sparse, and only one ship reported gale-force winds on the 1^st.

Hurricane Flora eventually made landfall across Haiti and the Dominican Republic at Category 4 strength, with another eventual landfall across Eastern Cuba at Category 3 strength and moved out to sea after passing over the Southeastern Bahamas.

Why does this matter?

Flora was a historic hurricane for Trinidad and Tobago. To date, the following are records that Hurricane Flora holds (as of September 2023):

• The strongest tropical cyclone (maximum sustained wind speed) to landfall across the country (T&T).
• The most intense tropical cyclone (minimum central pressure) to make landfall across the country.
• The strongest (and only) hurricane to landfall across Tobago.
• The deadliest tropical cyclone (and hurricane) for T&T.
• The most costly tropical cyclone (and hurricane) for T&T.
• The first hurricane warnings for T&T in history..

There is also a notable difference in impacts from a Category 2 Hurricane and a Category 3 Hurricane. While this change makes no difference to the damage caused and lives lost, these revisions and analyses are used by research scientists, operational hurricane forecasters, insurance companies, emergency managers, and others. Thus, its accuracy is essential.