Hilary briefly became a 65-knot hurricane while located about 200 n mi southwest of the southern tip of Baja California, but did not affect land.a. Synoptic history
Hilary originated from a tropical wave that moved from west Africa to the Atlantic on 29 August. It was a weak wave with minimal organized convection as it moved across the tropical Atlantic and the Caribbean. The wave acquired some organized convection on the 10th and 11th of September while over Central America. Visible satellite imagery on the 17th showed a low-level circulation, along with some organized deep convection, and a tropical depression is estimated to have formed later that day, while centered about 475 n mi south-southeast of the southern tip of Baja California, Mexico. The best track begins on the 17th, as indicated in Table 1 which is a listing, every six hours, of best track positions, maximum one-min surface wind speeds, and minimum central surface pressure. A map of the best track positions is shown in Figure 1.
The depression was upgraded to a tropical storm on the 18th, based on the low-level center being located under deep convection. Hilary had been moving generally west-northwestward during this time as it slowly strengthened. A deepening mid- to upper-level trough approached Hilary from the northwest and the motion turned sharply toward the north-northwest late on the 19th. A banding-type eye feature, increased symmetry, and an enlarging of deep convection are the bases for upgrading Hilary to a 65-knot hurricane for a short time early on the 20th, while centered some 200 n mi southwest of the southern tip of Baja California. Later that day, the low-level center became exposed to the south of the deep convection and Hilary weakened to a tropical storm. The cyclone moved over cool water and the weakening continued. Hilary was reduced to a swirl of low clouds, devoid of deep convection on the 21st, and the best track ends about 100 n mi west of Cabo San Lazaro, Baja California.
b. Meteorological statistics
The best track pressure and wind speed time series curves are shown in Figure 2 and Figure 3, along with plots of the data on which the curves are based. Satellite-based intensity estimates are the only available data, except that the ship SALUS reported a 37-knot wind speed while located about 120 n mi east of the center at 0000 UTC on the 20th.
c. Casualties and damages
Hilary did not affect land and there are no known deaths or damages.
d. Forecast and warning critique
There were 12 forecasts made while Hilary was a tropical storm and only four of these verified at 48 hours and none verified at 72 hours. The average track forecast errors were 45 n mi at 12 hours (10 cases), 81 n mi at 24 hours (8 cases), 119 n mi at 36 hours (6 cases) and 194 n mi at 48 hours (4 cases). These errors are somewhat higher that the previous 10-year averages for the eastern Pacific basin. This is probably related to the sharp right turn described above and the resultant left bias of the official forecasts. Wind speed forecast errors were modestly small as the intensification and weakening of Hilary were well-forecast.
Figure 1. Best track positions for Hurricane Hilary, 17-21 September 1999.
Figure 2. Best track maximum 1-min sustained wind speed curve for Hurricane Hilary.
Figure 3. Best track minimum central surface pressure curve for Hurricane Hilary.
Table 1. Preliminary Best Track, Hurricane Hilary, 17-21 September 1999. Date/Time
(UTC)Position Pressure
(mb)Wind Speed
(kt)Stage Lat. (°N) Lon. (°W) 17/0600 15.2 107.1 1006 25 tropical depression 1200 15.6 107.9 1006 25 " 1800 16.0 108.8 1005 25 " 18/0000 16.2 109.7 1004 30 " 0600 16.2 110.5 1003 30 " 1200 16.2 111.3 1002 35 tropical storm 1800 16.4 111.8 1001 40 " 19/0000 16.8 112.2 999 45 " 0600 17.5 112.5 995 50 " 1200 18.5 112.8 992 55 " 1800 19.7 113.1 989 60 " 20/0000 20.7 113.6 988 65 hurricane 0600 21.6 114.0 987 65 " 1200 22.4 114.3 988 60 tropical storm 1800 23.0 114.5 991 55 " 21/0000 23.6 114.6 995 45 " 0600 24.3 114.6 999 40 " 1200 24.9 114.6 1003 30 tropical depression 1800 25.3 114.5 1006 25 " 22/0000 dissipated 20/0600 21.6 114.0 987 65 minimum pressure