Tropical Cyclone Report

Tropical Storm Bonnie

Tropical Storm Bonnie made landfall in the vicinity of St. Vincent Island, Florida as a weak tropical storm.

a. Synoptic History

Bonnie developed from a tropical wave that crossed Dakar, Senegal on 29 July, and moved westward for several days accompanied by cloudiness, thunderstorms and a well-defined cyclonic rotation at the mid-levels. The shower activity became concentrated and the system developed a few convective bands as it moved westward. Data from QuikSCAT suggested that a small surface circulation had developed, and it is estimated that a tropical depression formed at 1200 UTC 3 August when the system was located about 360 n mi east of Barbados in the Lesser Antilles. The depression moved westward about 20 knots and lost its surface circulation when it entered the eastern Caribbean Sea. As a tropical wave, it continued moving rapidly to the west and the west-northwest producing intermittent convection. Once the system reached the western Caribbean Sea, it developed significant convection and regenerated a surface circulation. It is estimated that the tropical depression re-developed about 100 n mi southeast of the western tip of Cuba at 1200 UTC 8 August. The depression move toward the west-northwest across the Yucatan Channel and became Tropical Storm Bonnie near the northeastern tip of the Yucatan Peninsula. Bonnie moved north and northeast, reaching its maximum intensity of 55 knots and a minimum pressure of 1001 mb at 1800 UTC 11 August. Strong southwesterly wind shear became established over Bonnie and the cyclone began to weaken. It made landfall near Saint Vincent and Saint George Islands just south of Apalachicola, Florida as a tropical storm. These winds were confined to coastal sections to the east of the center. As a depression, Bonnie continued to move northeastward, across the eastern United States. It finally became a weak remnant low just south of Cape Cod at 0000 UTC 14 August.

The "best track" chart of the tropical cyclone's path is given in Figure 1, with the wind and pressure histories shown in Figure 2 and Figure 3, respectively. The best track positions and intensities are listed in Table 1.

b. Meteorological Statistics

Observations in Bonnie (Figure 2 and Figure 3) include satellite-based Dvorak technique intensity estimates from the Tropical Analysis and Forecast Branch (TAFB), the Satellite Analysis Branch (SAB) and the U. S. Air Force Weather Agency (AFWA), as well as flight-level and dropwindsonde observations from flights of the 53^rd Weather Reconnaissance Squadron of the U. S. Air Force Reserve Command. Microwave satellite imagery from NOAA polar-orbiting satellites, the NASA Tropical Rainfall Measuring Mission (TRMM), the NASA QuikSCAT, and Defense Meteorological Satellite Program (DMSP) satellites were also useful in tracking Bonnie. In addition, radar from Cancun, Mexico, and from the U.S. NWS network near the Gulf coast were used to analyze the cyclone when it was located near these sites. Selected ship and surface observations from land stations and data buoys are given in Table 2 and Table 3. Bonnie had a very small circulation when it regenerated near the Yucatan Peninsula, and initial data from a reconnaissance plane indicate that it had a closed eyewall of 8 n mi in diameter. The small center was also observed from the Cancun radar shown in Figure 4. A reconnaissance plane reported a minimum pressure of 995 mb at 0941 UTC 12 August when the cyclone was on a steady weakening trend. This minimum pressure was assumed to be associated with a meso-cyclone and this value is not representative of Bonnie's central pressure. A tornado outbreak over the southeastern United States was associated with Bonnie.

c. Casualty and Damage Statistics

There were three deaths in Pender County, NC, from a tornado spawned by Bonnie.

d. Forecast and Warning Critique

Average official track errors (with the number of cases in parentheses) for Bonnie were 43 (16), 75 (14), 87 (11), 78(9), 106(5), 595 (2), and 1038 (5) n mi for the 12, 24, 36, 48, 72, 96, and 120 h forecasts, respectively. For the short range, these errors are lower than average official track errors for the 10-yr period 1994-2003^[1] (44, 78, 112, 146, 217, 248, and 319 n mi, respectively). However, the 4 and 5 day errors are much large than the average. The model errors are displayed in Table 4. Note that dynamical models also had very large errors at the 120-h period. Models related to climatology produced the smallest errors.

Average official intensity errors were 8, 10, 12, 11, 8, 8, and 36 kt for the 12, 24, 36, 48, 72, 96, and 120 h forecasts, respectively. For comparison, the average official intensity errors over the 10-yr period 1994-2003 are 6, 10, 12, 15, 19, 20, and 21 kt, respectively. Note that in general the intensity errors were similar to the average with the exception of the large error at 120 hours.

^[1]Errors given for the 96 and 120 h periods are averages over the three-year period 2001-3.

Figure 1: Best track positions for Bonnie, 3-13 August, 2004. Track after landfall stage is based on analyses from the NOAA Hydrometeorological Prediction Center (HPC).

Figure 2: Selected wind observations and best track maximum sustained surface wind speed curve for Tropical Storm Bonnie, 3-13 August 2004. 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). Objective Dvorak estimates represent linear averages over a three-hour period centered on the nominal observation time. Estimates after landfall are based on analyses from the NOAA Hydrometeorological Prediction Center.

Figure 3: Selected pressure observations and best track minimum central pressure curve for Bonnie, 3-13 August, 2004. Estimates after landfall are based on analyses from the NOAA Hydrometeorological Prediction Center.

Figure 4: Radar image from Cancun, Mexico at 0516 UTC 9 August, showing the small center of the developing cyclone.