Tropical Cyclone Report

Hurricane Nora

a. Synoptic History

Nora developed from a tropical wave that moved off the coast of Africa on 13 September. There were various attempts for this wave to develop as it moved westward across the Atlantic and the Caribbean Sea. The wave crossed Central America accompanied by cloudiness and numerous thunderstorms on 25 September. The main shower activity continued westward very near the southern coast of Mexico. However, it was not until 1800 UTC 1 October that the activity became organized enough for the system to be classified as a tropical depression. It was then located about 525 nautical miles south of the southern tip of Baja California. The cloud pattern continued to organize and the system became a tropical storm at 0600 UTC 2 October. Nora moved slowly westward and then west-northwestward and reached hurricane status at 0000 UTC 4 October, reaching its peak intensity of 90 knots and a minimum pressure of 969 mb 12 h later. Nora maintained hurricane status for two days as it continued moving slowly toward the northwest. The cyclone then made a sharp turn to the east and northeast ahead of a strong middle-level trough. This trough, and the outflow from Hurricane Olaf, which was located to the southeast of Nora, produced strong southwesterly shear over the cyclone, resulting in weakening. The poorly-defined center of Tropical Depression Nora reached the coast of Mexico just north of Mazatlan early on the 9^th and rapidly dissipated over the high terrain.

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 Nora (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). Operationally, scatterometer wind data were used to downgrade Nora to tropical depression status at 0600 UTC 7 October.

c. Casualty and Damage Statistics

According to reports from the Mexican Weather Service, the impact of Nora on the coast of Mexico was minimal, and there were no reports of damages or casualties. Heavy rains affected the state of Sinaloa.

d. Forecast and Warning Critique

Average official track errors for Nora were 32, 62, 88, 108, 139, 195 and 293, n mi for the 12, 24, 36, 48, 72, 96 and 120 h forecasts, respectively¹. For comparison, the average errors for the 10-yr period of 1993-2002² are 39, 72, 103, 131, 186, 197 and 223 n mi, respectively. The performance of the available track models is given in Table 2. Note that the UK model was better than the official forecast for the 24 through the 120 h periods and the consensus GUNA was better than the official at all times. However, the model consensus GUNA and GUNS were not available at the 120-h period.

Average official intensity errors were 6, 11, 15, 20, 22, 16 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 1993-2002² are 6, 11, 15, 17, 20, 18 and 19 kt, respectively. Nora was forecast to intensify further when it reached 90 knots, based on low shear indicated by SHIPS model guidance. Instead, the hurricane began to weaken due to the shear caused by Olaf.

Since Nora was expected to reach land as a tropical depression, there were no watches and warnings issued.

¹ All 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 Nora,1- 9 October 2003.

Figure 2: Best track maximum sustained surface wind speed curve for Hurricane Nora, 1-9 October 2003.

Figure 3: Best track minimum central pressure curve for Hurricane Nora, 1-9 October 2003.