Tropical Cyclone Report

Hurricane Hernan

Hurricane Hernan attained category 5 hurricane intensity on the Saffir-Simpson Hurricane Scale. It did not affect land.

a. Synoptic History

Hernan became a tropical cyclone on August 30^th, when a closed circulation and organized convection formed about 340 n mi south-southeast of Manzanillo, Mexico. The pre-existing intertropical convergence zone disturbance coincided with a weak tropical wave that moved over the eastern Atlantic from Africa on the 16^th.

The "best track" the tropical cyclone's path and intensity begins on the 30^th. The path of the tropical cyclone center is shown in Figure 1 and the wind and pressure histories are shown in Figure 2 and Figure 3. The best track positions and intensities are listed in Table 1.

Hernan's track was basically toward the west-northwest for five days and then northwestward for three days followed by dissipation. This track follows the southwest periphery of the geopotential height contours of the mean subtropical high pressure ridge centered over Mexico.

Hernan's wind history is equally straightforward. The 1-min, 10-m maximum wind speed (Figure 2) increased from 30 kt on the 30^th to 140 kt on September 1^st, an increase of 110 kt in 54 hours. The wind then decreased back to 30 kt in the next 96 hours. Within 24 h of the peak wind of 140 kt, Hernan encountered progressively cooler waters. Two days later on the 4^th, increasing vertical shear continued the weakening process. By the 6^th, Hernan was devoid of significant convection and reduced to a remnant low, while located about 780 n mi west of the southern tip of Baja California. The remnant low drifted southwestward until the 9^th when it dissipated.

b. Meteorological Statistics

Observations of intensity plotted in 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). Satellite subjective Dvorak estimates of 140 kt are the basis for the best-track maximum wind speed of the same value. An objective Dvorak 3-h average estimate was 135 kt at the same time. Ship reports of winds of tropical storm force associated with Hernan are given in Table 2.

The center passed about 90 n mi south of Socorro Island on September 1^st, when the intensity was near its maximum value. Although no reports have been received from the island, it is likely that hurricane conditions remained to its south.

c. Casualty and Damage Statistics

There were no reports of damages or casualties.

d. Forecast and Warning Critique

Table 4 lists the average official track errors for Hernan, with the number of cases in parentheses, along with the errors for a selection of track guidance models. Official errors averaged 27 (22), 44 (20), 72(18), 99(16), and 115(12) n mi for the 12, 24, 36, 48, and 72 h forecasts, respectively. These errors are 21 to 37 percent lower than the average official track errors for the 10-yr period 1992-2001, also listed in Table 4. Several models out-performed the official forecasts, including the NWS Global Forecast System (AVNI), the GFDL model embedded in the NOGAPS model (GFNI), the NOGAPS model (NGPI), some of the BAM models, and the GUNS and GUNA consensus forecasts.

Average official intensity errors were 10, 21, 27, 28, and 26 kt for the 12, 24, 36, 48, and 72 h forecasts, respectively. For comparison, the average official intensity errors over the 10-yr period 1992-2001 are 7, 12, 16, 18, and 21 kt, respectively. These rather large official errors are partially the result of the first few forecasts under-forecasting the rapid intensification. Also, some fairly large errors occurred from incorrectly under-forecasting the rate of weakening. The SHIPS intensity guidance model had similarly large errors.

Figure 1: Best track positions and minimum pressure for Hurricane Hernan, 30 August-6 September 2002.

Figure 2: Selected wind observations and best track maximum sustained surface wind speed curve for Hurricane Hernan, 30 August-6 September 2002. Objective Dvorak estimates represent linear averages over a three-hour period centered on the nominal observation time.

Figure 3: Selected pressure observations and best track minimum central pressure curve for Hurricane Hernan, 30 August-6 September 2002. Objective Dvorak estimates represent linear averages over a three-hour period centered on the nominal observation time.