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Darby reached category 3 status on the Saffir-Simpson Hurricane Scale over the open eastern North Pacific ocean.

a. Synoptic History

Darby formed from a tropical wave that moved westward across the coast of Africa on 12 July. The wave crossed the Atlantic and Caribbean with no development and reached the eastern North Pacific on 20 July. Moving westward, the system first showed signs of organization on 23 July. Continued slow development resulted in the formation of a tropical depression around 1200 UTC 26 July about 660 n mi south-southwest of Cabo San Lucas, Mexico. 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.

The depression moved westward on the south side of the subtropical ridge and became a tropical storm early on 27 July. It turned west-northwestward later that day and continued to strengthen. Darby became a hurricane early on 28 July, and then reached an estimated peak intensity of 105 kt on 29 July. The hurricane started to weaken later that day due to a combination of cooler sea-surface temperatures and increasing westerly vertical shear. It became a tropical storm again on 30 July as it turned westward, and it weakened to a depression on 31 July. The depression crossed 140°W into the central North Pacific hurricane basin later that day. Darby dissipated as a tropical cyclone on 1 August about 740 n mi east of the Hawaiian Islands. The remnants of Darby continued westward in the low-level trade winds and caused heavy rains and flooding over portions of the Hawaiian Islands on 3-4 August.

b. Meteorological Statistics

Observations in Darby (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). Microwave satellite imagery from NOAA polar-orbiting satellites, the NASA Tropical Rainfall Measuring Mission (TRMM), the NASA Aqua, the NASA QuikSCAT, and Defense Meteorological Satellite Program (DMSP) satellites were also useful in tracking Darby.

There were no surface observations of winds of tropical storm force from Darby.

c. Casualty and Damage Statistics

There were no reports of damages or casualties associated with Darby.

d. Forecast and Warning Critique

Average official track errors (with the number of cases in parentheses) for Darby were 19 (21), 36 (19), 58 (17), 80 (15), 127 (11), 206 (7), and 333 (3) n mi for the 12, 24, 36, 48, 72, 96, and 120 h forecasts, respectively. These errors are lower than the average official track errors for the 10-yr period 1994-2003^[1] (38, 70, 100, 127, 180, 210, and 247 n mi, respectively) through 96 h, but much greater than the average 120 h error. The large errors at 120 h resulted from the early track forecasts being too far south and too slow.

Average official intensity errors were 7, 11, 13, 15, 21, 31 and 40 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, 11, 15, 17, 20, 18, and 19 kt, respectively. The large errors at 96 and 120 h resulted from early intensity forecasts making Darby too strong.

Watches and warnings were not required for Darby.

Acknowledgements

The best track west of 140°W was provided by the Central Pacific Hurricane Center in Honolulu, Hawaii.

Figure 1: Best track positions for Hurricane Darby, 26 July - 1 August 2004. The best track west of 140°W was provided by the Central Pacific Hurricane Center in Honolulu, Hawaii.

Figure 2: Selected wind observations and best track maximum sustained surface wind speed curve for Hurricane Darby, 26 July - 1 August 2004. Objective Dvorak estimates represent linear averages over a three-hour period centered on the nominal observation time. The best track west of 140°W was provided by the Central Pacific Hurricane Center in Honolulu, Hawaii.

Figure 3: Selected pressure observations and best track minimum central pressure curve for Hurricane Darby, 26 July - 1 August 2004. Objective Dvorak estimates represent linear averages over a three-hour period centered on the nominal observation time. The best track west of 140^oW was provided by the Central Pacific Hurricane Center in Honolulu, Hawaii.