Hurricane Olivia Preliminary Report

Excerpts courtesy of the
National Hurricane Center (NHC)
--Written by Richard J. Pasch

Synoptic History

On 18 September, deep convection increased along the ITCZ between 90W and 100W, well to the south of Mexico. By 1900 UTC on the 19th, visible satellite imagery suggested a weak disturbance centered near 10N 98W. This disturbance moved westward, and, about 24 hours later, appeared to become better organized a little over 500 nmi. to the south of Manzanillo, Mexico. The system received its first Dvorak classifications, as a T1.0, from the NHC Tropical Satellite Analysis and Forecast (TSAF) unit and from the NESDIS Synoptic Analysis Branch (SAB), at 0600 UTC 21 September. Visible satellite pictures on the 21st showed that deep convection was sheared mostly to the west of the low-level circulation center. However, during the following night, convection associated with the system increased and became better organized. It is estimated that it formed into a tropical depression at 0600 UTC 22 September, about 625 nmi to the south of the southern tip of Baja California.

By 1800 UTC on the 22nd, banding measurements on visible satellite pictures indicated that the cyclone was of tropical storm intensity. The storm was moving west-northwestward, and this general direction of motion (at around 10 knots) continued for the next couple of days. Strengthening took place, as the upper-tropospheric environment favored adequate outflow from the tropical cyclone. Olivia became a hurricane around 0600 UTC on the 24th, based on intensity estimates of 65 knots from both TSAF and SAB, and a report of a "warm spot" surrounded by 60 percent eyewall coverage on 85 GHz data from the Air Force Global Weather Central (AFGWC) near that time. After Olivia became a hurricane, it strengthened rapidly; maximum winds were near 95 knots by 1800 UTC on the 24th. A distinct eye was evident on satellite images at that time, and, a few hours later, radar observations from a NOAA research aircraft investigating Olivia showed a well-defined eye (about 15 nmi in diameter) as well. Central pressure was falling while the plane was in the hurricane, and was down to 949 mb on the last fix on the 24th, at 2224 UTC. Around this time, Olivia turned toward the northwest, as the hurricane moved around the periphery of a deep-layer mean anticyclone.

Later on the 24th and early the next day, the hurricane continued to strengthen, and reached its estimated peak sustained wind speed of 130 knots around 1200 UTC on the 25th. A mid- to upper-tropospheric cyclone, located a few hundred miles to the west of extreme southern California, induced a turn to the north by 1800 UTC on the 25th. After 0000 UTC on the 26th, under the continuing influence of the mid- to upper level low, Olivia veered toward the north-northeast, while gradually weakening. Southwesterly shearing was having a noticeable effect on the hurricane's cloud pattern. Later on the 26th, with Olivia's maximum winds reduced to below 100 knots, the forward motion slowed and became northeastward. Located a little over 500 nmi west-southwest of the southern tip of Baja California, the weakening, drifting hurricane made a small clockwise loop. Just after completing the loop, Olivia diminished to tropical storm intensity, when most of the deep convection disappeared. Continuing to weaken, and steered by the lower- to mid-tropospheric flow, the tropical cyclone moved west-northwestward, then westward. Olivia degenerated to a tropical depression around 1200 UTC on the 28th, and was dissipating around 1800 UTC on the 29th.

Meteorological Statistics

The peak intensity, 130 knot-maximum sustained winds with a minimum central pressure of 923 mb at 1200 UTC 25 September, is based on objective Dvorak numbers and the fact that a NOAA flight obtained a 925 mb pressure, by dropsonde, about 12 hours later, when the satellite data suggested the hurricane was just slightly less intense. The three-hour average objective T-number of 6.9 at the estimated time of peak intensity corresponds to a wind speed of 135 knots. However, comparison of objective T-numbers with contemporaneous flight-level wind speeds suggests that satellite-derived winds are slightly too high for this case. The maximum flight-level wind reported by the NOAA aircraft missions was 122 knots, at 10000 feet, at 0015 UTC 26 September.

There were no ship reports received at the NHC of tropical storm force winds associated with Olivia.

Casualty and Damage Statistics

No reports of casualties or damage due to Olivia have been received at the NHC.