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Karl was a category 4 hurricane on the Saffir-Simpson Hurricane Scale that traveled across the open central North Atlantic.

a. Synoptic History

Karl formed from a strong tropical wave that moved westward from the coast of Africa on 13 September. The system showed increasing shower activity on 14 September, and Dvorak satellite intensity estimates began the next day. The wave spawned a tropical depression around 0600 UTC 16 September about 340 n mi southwest of the southern Cape Verde islands. 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 initially moved westward south of a subtropical ridge and strengthened into a tropical storm later that day. Karl turned northwestward on 17 September, then moved west-northwestward while becoming a hurricane the next day. The hurricane continued west-northwestward on 19 September, then turned northwestward on 20 September and north-northwestward on 21 September towards a weakness in the ridge. Maximum sustained winds reached an estimated 115 kt on 20 September and an estimated 125 kt on 21 September. Karl continued moving north-northwestward until 22 September when it turned northeastward in response to a deep-layer baroclinic trough developing north of the hurricane. This motion continued through 23 September. The intensity fluctuated during this time due to a concentric eyewall cycle, with maximum sustained winds decreasing to an estimated 90 kt on 22 September and increasing to an estimated 110 kt the next day. Karl turned northward east of the trough on 24 September and weakened as transition to an extratropical cyclone began. The transition was complete early on 25 September about 510 n mi east of Cape Race, Newfoundland.

As an extratropical low, Karl moved northeastward and eastward across the North Atlantic Ocean and the North Sea, eventually reaching Norway before being absorbed into another extratropical low late on 28 September.

b. Meteorological Statistics

Observations in Karl (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 QuikSCAT, the NASA Aqua, the Department of Defense Coriolis/Windsat satellite, and Defense Meteorological Satellite Program (DMSP) satellites were also useful in tracking Karl.

Shipping avoided the core of Karl, with reports of winds of tropical storm force from the periphery of the cyclone given in Table 2. The highest reported wind was from the Rotterdam, which reported a sustained wind of 45 kt at 1800 UTC 24 September. Two drifting buoys encountered the core of Karl on 24 September. Buoy 41600 reported a pressure of 958.1 mb at 0100 UTC, while buoy 44617 reported a pressure of 964.2 mb at 2100 UTC.

c. Casualty and Damage Statistics

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

d. Forecast and Warning Critique

Average official track errors (with the number of cases in parentheses) for Karl were 37 (31), 65 (29), 84 (27), 101 (25), 118 (21), 125 (17), and 147 (13) 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] (44, 78, 112, 146, 217, 248, and 319 n mi, respectively), (Table 3) - about 20% lower at 12-36 h increasing to 50-60% lower at 96 and 120 h. Some of the track forecast models had average errors lower than the official. These include the GUNA consensus model, which was better at all times except 12 h, and the GFS global model, which was better at all times except 12 and 120 h.

Average official intensity errors were 11, 11, 11, 13, 16, 13 and 11 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.

Watches and warnings were not necessary for Karl.

Acknowledgements

Portions of the track of Karl as an extratropical low were provided by the Ocean Prediction Center and the United Kingdom Meteorological Office.

Figure 1: Best track positions for Hurricane Karl, 16 - 24 September 2004. Track during the extratropical stage is based on analyses from the NOAA Ocean Prediction Center and the United Kingdom Meteorological Office.

Figure 2: Selected wind observations and best track maximum sustained surface wind speed curve for Hurricane Karl, 16 - 24 September 2004. Estimates during the extratropical stage are based on analyses from the NOAA Ocean Prediction Center and the United Kingdom Meteorological Office.

Figure 3: Selected pressure observations and best track minimum central pressure curve for Hurricane Karl, 16 - 24 September 2004. Estimates during the extratropical stage are based on analyses from the NOAA Ocean Prediction Center and the United Kingdom Meteorological Office.