Tropical Cyclone Report

Hurricane Kate

Kate had a long, rather unusual track over the east-central Atlantic. It became a powerful hurricane at subtropical latitudes.

a. Synoptic History

A tropical wave crossed the coast of western Africa on 21 September and moved slowly westward, passing near the Cape Verde Islands on 23 September. Although the system was not well organized, meteorologists at the Tropical Analysis and Forecast Branch (TAFB) and the Satellite Analysis Branch (SAB) identified a low-level circulation center several hundred miles west-southwest of the Cape Verde Islands on 24 September. Cloudiness and deep convection gradually became better organized into curved bands, and initial Dvorak T-numbers were assigned around 1200 UTC 25 September. It is estimated that a tropical depression formed, roughly 800 n mi west-southwest of the Cape Verde Islands, at 1800 UTC 25 September. 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. Table 1 lists the best track positions and intensities.

The tropical cyclone moved generally northwestward for a couple of days toward a weakness in the subtropical ridge over the central Atlantic. Meanwhile, south-southwesterly upper-level flow caused strong vertical shear over the depression, and the low-cloud center was intermittently exposed to the southwest of the main area of deep convection. Around 1800 UTC 27 September, deep convection increased near the center, and satellite-based estimates indicate that the cyclone became Tropical Storm Kate. Even though the system continued to experience moderate southwesterly shearing, it strengthened further over the next two days while turning northward and northeastward. Kate briefly became a hurricane, from about 1800 UTC 29 September to 0000 UTC 30 September. Then, while weakening, the cyclone took a hairpin turn around the eastern side of a mid-tropospheric cyclonic circulation. By 1 October, Kate was moving west-southwestward on the northern side of the cyclonic circulation. It also regained hurricane strength that day. Kate continued west-southwestward for the next couple of days, guided by the steering flow between an anticyclone to its north and the cyclonic circulation centered to its south. There was a relaxation of vertical shear over the area, and as Kate moved over progressively warmer waters, it strengthened significantly. By 2 October, a well-defined eye was apparent on geostationary satellite images. The hurricane moved westward to west-northwestward over the next couple of days while strengthening to its peak wind intensity of 110 kt at 1800 UTC 4 October. Kate was centered about 565 n mi east-southeast of Bermuda at that time.

Shortly after reaching maximum intensity, the western portion of the hurricane's central dense overcast became partially eroded, signifying the beginning of a weakening trend. Also, the forward speed slowed from 10-11 kt to 6-7 kt as Kate neared the western periphery of the deep-layer anticyclone that had been anchored over the north Atlantic for several days. On 5-6 October, the cyclone turned northward, then accelerated north-northeastward ahead of a deep-layer trough. Kate was slow to weaken as an eye was occasionally visible on geostationary satellite imagery. However, the cyclone finally weakened below hurricane strength on 7 October while it continued to accelerate north-northeastward over cooler waters. Kate also began losing tropical characteristics as cold air clouds wrapped around the center over the southern semicircle, and the remaining central convection weakened and became disorganized. The system completed its extratropical transition by 0000 UTC 8 October, but remained a formidable extratropical storm for a couple more days while moving northeastward to eastward across the northern Atlantic. It merged with another extratropical low near Scandinavia on 10 October.

b. Meteorological Statistics

Observations in Hurricane Kate (Figure 2 and Figure 3) include satellite-based Dvorak technique intensity estimates from TAFB, SAB and the U. S. Air Force Weather Agency (AFWA). QuikSCAT data indicated that Kate had winds of hurricane force from 1800 UTC 29 September to 0000 UTC 30 September. Kate's peak intensity, 110 kt, at 1800 UTC 4 October, is based on a 3-hour average objective Dvorak T-number of 5.8.

Ship reports of winds of tropical storm force associated with Hurricane Kate are given in Table 2.

c. Casualty and Damage Statistics

There were no reports of damage or casualties associated with Kate.

d. Forecast and Warning Critique

Forecasters had a difficult time predicting the path of this tropical cyclone. Average official track errors (with the number of cases in parentheses) for Kate were 48 (47), 91 (45), 133 (43), 179 (41), 288 (37), 363 (33), and 406 (29) n mi for the 12, 24, 36, 48, 72, 96, and 120 h forecasts, respectively¹. These errors are greater than the average official track errors for the 10-yr period 1993-2002² of 45, 81, 116, 150, 225, 282, and 374 n mi, respectively. Table 3 shows a comparison of the track errors for various models and the official forecast. Several models, notably GFDI, AVNI (GFS), and NGPI had lower errors than the official forecast at practically every forecast period. Interestingly, UKMI, which is normally an excellent performer, had very large average errors for Kate. This was due, in part, to a number of UKMI forecasts thatwhich failed to predict Kate's sharp turn to the west, and took the system erroneously eastward into the eastern Atlantic.

Wind speed forecasts for Kate were mostly better than the long-term means. The average official intensity forecast errors were 4, 7, 9, 10, 12, 17, and 23 kt for the 12, 24, 36, 48, 72, 96, and 120 h forecasts, respectively, compared to the average 1993-2002 errors of 6, 10, 13, 15, 19, 21, and 22 kt, respectively. There was, however, a significant negative bias at 96 and 120 h.

No watches or warnings were required for Kate.

¹All forecast verifications in this report include the depression stage of the cyclone. National Hurricane Center verifications presented in these reports prior to 2003 did not include the depression stage.

² Errors given for the 96 and 120 h periods are averages over the two-year period 2001-2002.

Figure 1: Best track positions for Hurricane Kate, September/October 2003. Track during the extratropical stage is based on analyses from the NOAA Ocean Prediction Center.

Figure 2: Selected wind observations and best track maximum sustained surface wind speed curve for Hurricane Kate, 25 September - 7 October 2003. Objective Dvorak estimates represent linear averages over a three-hour period centered on the nominal observation time. Estimates during the extratropical stage are based on analyses from the NOAA Ocean Prediction Center.

Figure 3: Selected pressure observation s and best track minimum central pressure curve for Hurricane Kate, 25 September - 7 October 2003. Objective Dvorak estimates represent linear averages over a three-hour period centered on the nominal observation time. Estimates during the extratropical stage are based on analyses from the NOAA Ocean Prediction Center.