Tropical Cyclone Report

Hurricane Gabrielle

Gabrielle made landfall on the Florida west coast as a tropical storm with 60-knot sustained winds and then became a category one hurricane (on the Saffir-Simpson Hurricane Scale) as it moved across the northwest Atlantic Ocean. It produced major river floods over west-central Florida and at the lower St. Johns River and also caused heavy rain over southeastern Newfoundland.

a. Synoptic History

Gabrielle's origin was non-tropical. On 5 September, a weak low- to mid-level trough, and its associated cloudiness and showers, was nearly stationary and located just east of the southeastern U.S. coast. This feature persisted and gradually resulted in a low- to mid-level cut-off low over Florida by the 9^th. Late on the 11^th, a surface low formed over the southeastern Gulf of Mexico in association with the cut-off low and convection was sufficiently well-organized to classify the system as Tropical Depression Eight. The "best track"of the tropical cyclone begins at 1800 UTC on the 11^th. The track is shown in and six-hour center positions, wind speeds, and central pressures are listed in Table 1. Time series curves of best track wind speed and pressure are shown in and .

Under weak steering currents, the cyclone moved in a small counterclockwise loop over the southeastern Gulf of Mexico for two and one-half days and gradually strengthened. It reached tropical storm strength mid-day on the 13^th while located about 175 n mi southwest of Venice, Florida. By this time, a mid-level trough in the westerlies was moving into the eastern United States and Gabrielle began moving northeastward with increasing forward speed. Gabrielle's center made landfall on Florida's west coast near Venice at about 1200 UTC on the 14^th. The storm strengthened to 60 knots just before landfall, even though a SHIPS analysis, based on the Aviation model, calculated about 25 knots of westerly vertical shear affecting the storm at this time.

The storm decelerated and its winds decreased to 40 knots during the center's 18-hour traverse across central Florida. The center moved offshore on the Florida east coast near Titusville and accelerated northeastward. Although the cyclone was highly sheared, winds strengthened to 70 knots on the 17^th while it was located about 200 n mi northwest of Bermuda.

Continuing northeastward, Gabrielle weakened to 60 knots and, at 0600 UTC on the 19^th, lost all deep convection near the center while located about 300 n mi south of Newfoundland. Gabrielle is assigned extratropical status at this time. Still moving northeastward, Gabrielle passed near southeastern Newfoundland and merged with another extratropical low on the 21^st over the far north Atlantic Ocean.

b. Meteorological statistics

Data from reconnaissance aircraft and satellites that were used to estimate the maximum 1-minute surface wind speed and minimum central surface pressure are plotted in and . Selected surface observations from Florida are listed in Table 2 and reports from ships encountering wind speeds greater than 33 knots are listed in Table 3.

On the 14^th, when Gabrielle was nearing the west coast of Florida, aircraft 700-millibar flight-level winds suggest that surface winds were near 65 knots. But the Venice C-MAN station, almost beneath the location of the aircraft strong winds, reported a maximum surface wind of 50 knots (Table 2). At nearby New Pass, the highest measured surface wind was 51 knots and this was a 15-minute average (Table 2). The aircraft reported a dropsonde central pressure of 980 mb at 1009 UTC and 983 mb at 1132 UTC (). The aircraft weather officer also reported a 972-mb value extrapolated from flight level at 0850 UTC and associated with a temperature spike. The time scale of this pressure fluctuation is not represented in the best-track Table 1, which gives intensity and track values only every six hours.

The best-track wind speed of 60 knots at 1200 UTC is a compromise between these observations and represents an estimate of the strongest sustained winds that occurred on the southwest Florida coast. There is an uncertainty associated with official surface wind speed estimates and it is possible that Gabrielle was briefly a hurricane while making landfall. Inland winds ranged from 35-45 knots as the storm moved northeastward across Florida. A few tornados were reported from Brevard and Volusia Counties in northeast Florida.

The strengthening to 70 knots on the 17^th is based on aircraft dropsonde and flight-level wind speeds. The maximum flight-level wind speed was 85 knots at 850 mb at about 1700 UTC. The standard reduction to the surface under deep convection is 90 percent, which yields a 77-knot surface wind. A few hours earlier, a GPS-dropsonde indicated a surface wind speed of 60 knots.

Florida rainfall totals were generally in the 4 to 7 inch range over a swath along the storm track. More than 12 inches fell on Volusia and Lake Counties in northeast Florida. This was from a combination of rainfall from Gabrielle and from a strong onshore flow prior to the storm. This flow, combined with the winds of the storm, high astronomical tides and rainfall, caused near record flooding of the lower St. Johns River. The rain over west-central Florida resulted in major floods on the Manatee River, Little Manatee River, Myakka River, Peace River, and Horse Creek. A river gage at Arcadia on Horse Creek reported a crest of 16.8 feet. Flood stage for this location is 12 feet. Minor river and urban flooding occurred elsewhere along the path of the storm across Florida.

Eighteen tornados were reported in Florida.

After becoming extratropical, the storm brought over 6 inches of rain in 12 hours or less to the Avalon Peninsula of Newfoundland. An impressive report was from Cape Race, where an automatic station reported 1.9 inches in 1 hour.

c. Casualty and damage statistics

The insured loss total of $115,000,000 in Florida from wind and rain was reported by the Property Claim Services of the American Insurance Services Group. The total damage estimate is $230,000,000, pending additional information on flood damage.

A fifteen-year-old boy drowned in Gee Creek near Winter Springs in Seminole County, Florida. Gabrielle's swells contributed to a rip current death on September 13th at the Alabama coastline. One indirect death occurred in the Florida Keys when a person fell off a boat and drowned. This death is assigned an indirect status, since local officials determined that intoxication was more of a factor than high winds or seas.

d. Forecast and warning critique

Table 4 lists the watches and warnings issued for Gabrielle. Tropical storm warnings were issued for the Florida west coast at 2100 UTC on the 13^th and landfall occurred at 1200 UTC on the 14^th, or 15 hours later.

Thirty official track and wind speed forecasts were verified. The average track errors were 12, 49, 87, 128, 167, and 235 nautical miles for 0, 12, 24, 36, 48, and 72 hours. These errors are near or slightly above the previous ten year average official errors of 11, 44, 82, 118, 150, and 226 nautical miles. The guidance models used operationally had errors somewhat larger than the official errors, except for the UKMET model, whose errors were slightly smaller. The official forecasts with the largest track errors were made early on the 13^th when the storm was finishing its slow loop in the southeastern Gulf of Mexico.

The official average wind speed forecast errors were 1, 6, 9, 8, 10, and 14 knots for 0, 12, 24, 36, 48, and 72 hours, considerably smaller than the 3, 7, 11, 14, 16, and 20 knots for the previous 10-year averages. These errors are also smaller than the SHIPS statistical-dynamical intensity forecast model errors for Gabrielle.

Figure 1: Best track positions for Hurricane Gabrielle, 11-19 September 2001. Track during the extratropical stage (after 19/0000 UTC) is based on analyses from the NOAA Marine Prediction Center. Inset is an enlargement of loop over southeastern Gulf of Mexico.

Figure 2: Best track maximum sustained surface wind speed curve for Hurricane Gabrielle, 11-19 September 2001, and the observations on which the best track curve is based. Aircraft observations have been adjusted for elevation using 90%, 80%, and 80% reduction factors for observations from 700 mb, 850 mb, and 1500 ft, respectively. Dropwindsonde observations include actual 10 m winds (sfc), as well as surface estimates derived from the mean wind over the lowest 150 m of the wind sounding (LLM), and from the sounding boundary layer mean(MBL). Estimates during the extratropical stage are from the NOAA Marine Prediction Center.

Figure 3: Best track minimum central pressure curve for Hurricane Gabrielle, 11-19 September 2001, and the observations on which the best track curve is based. Estimates during the extratropical stage are based on analyses from the NOAA Marine Prediction Center.