The southern extension of the tropical wave that triggered Tropical Storm Emily in the Atlantic moved westward through the Caribbean Sea for several days and crossed Central America between 31 August and 1 September (Fig. 1). Satellite imagery revealed a middle-level circulation with a large area of thunderstorms associated with the wave moving westward partially inland over southern Mexico. A large area of disturbed weather associated with a cyclonic monsoon-type flow had prevailed over the eastern Pacific for several days. However, it was not until the tropical wave arrived in the area that tropical cyclone formation began. The thunderstorm activity became concentrated while cloud banding features were developing. Dvorak T-numbers suggested the formation of a 30-knot tropical depression very near Manzanillo, Mexico at 1200 UTC 5 September. It appears that the same tropical wave combined with the strong southwest monsoonal-type flow which extended northward into the Bay of Campeche, led the formation of the Atlantic Tropical Depression Seven in the Gulf of Mexico during that period.
The eastern Pacific depression continued to become better organized, and both satellite images and ship reports indicated that the tropical cyclone reached tropical storm strength by 1800 UTC on the same day. Greg was then moving on a general northwest track very close to the southwestern coast of Mexico. A large area of very deep convection formed near the center of circulation and radar from Los Cabos, Mexico suggested the formation of an ragged eye during the morning of 6 September. Greg was upgraded to hurricane status at 1800 UTC on the same day while approaching the southern portion of Baja California. Figure 2 display the radar presentation of Greg and a visible satellite image of the hurricane near the time of its peak intensity. No significant change in strength occurred for the next 24 hours, and thereafter, Greg weakened to tropical storm status as its center moved over the area of Cabo San Lucas around 2100 UTC 7 September. Greg turned toward the west-northwest and west over cooler waters and weakening began. Greg's track is shown in Fig. 3. Table 1 is a listing, at six-hourly intervals, of the best-track position, estimated minimum central pressure and maximum 1-minute surface wind speed.
b. Meteorological Statistics
The best track pressure and wind curves as a function of time are shown in Fig. 4 and are primarily based on satellite intensity estimates from the Tropical Analysis and Forecast Branch (TAFB), the Satellite Analysis Branch (SAB) and the Air Force Weather Agency, (AFGWC in figures). Operationally, Greg was upgraded to tropical storm based on data from the ship 3EJO6, which reported winds of 230 degrees with 42 knots and a pressure of 1006.5 mb at 1800 UTC 5 September. Greg produced torrential rains over portions of southwestern Mexico. Nearly 9 inches were measured in Manzanillo, about 8 inches in Colima and 5 inches in Islas Marias as Greg moved nearby. San Jose del Cabo, on the southern tip of Baja California, reported sustained winds of 35 knots with gusts to 40 knots and a minimum pressure of 995 mb at 2100 UTC 7 September while the center of Greg was crossing the southwestern tip of Baja California.
c. Casualty and Damage Statistics
Reports from the El Nuevo Herald indicate that torrential rains caused extensive flooding over the states of Colima, Jalisco, Michoacan, Nayarit and Sinaloa. Nine people were killed as a consequence of the rains.
d. Forecast and Warning Critique
Because Greg formed very close to the southwest coast of Mexico, tropical storm warnings were required immediately and were issued in the first advisory. Additional watches and warnings were issued for portions of Baja California. A summary is included in Table 2. Both official forecasts and the Statistical Hurricane Intensity Prediction Scheme (SHIPS) indicated that Greg would peak at 65 knots.
The NHC average official track errors (in n mi) for Greg (excluding the tropical depression stage) were 35 (10 cases), 59 (8 cases), 98 (6 cases), 132 (4 cases), respectively, for the 12-, 24-, 36-, 48-hour forecast periods. There were no 72-hour forecast to verify. These errors for 12, 24, 36 and 48 hour periods are very near the 1989-1998 average official forecast errors.
Figure 1. Sequence of daily GOES 8 satellite images at 1200 UTC from 27 August to 4 September 1999. Dashed line marks the westward propagation of the tropical wave which eventually triggered Hurricane Greg (G).
Figure 2a. View of Hurricane Greg at 1917 UTC September 6 from San Jose del Cabo radar.
Figure 2b. Visible satellite image of Hurricane Greg at 0000 UTC 7 September, near the time of peak intensity.
Figure 3. Best track positions for Hurricane Greg, 5-9 September 1999.
Figure 4. Best track minimum central pressure and maximum speed curves for Hurricane Greg.
Table 1. Best track, Hurricane Greg, 5-9 September, 1999 Date/Time
Stage Lat. (°N) Lon. (°W) 5/1200 18.6 105.1 1008 30 tropical depression 1800 19.2 105.4 1005 40 tropical storm 6/0000 19.9 106.0 1000 45 " 0600 20.6 106.7 997 50 " 1200 21.2 107.6 994 55 " 1800 21.5 108.4 987 65 hurricane 7/0000 21.9 109.0 986 65 " 0600 22.2 109.5 987 65 " 1200 22.5 109.5 987 65 " 1800 22.8 109.7 994 55 tropical storm 8/0000 23.1 110.3 1000 45 " 0600 23.1 110.8 1005 40 " 1200 23.0 111.1 1006 35 " 1800 23.0 111.3 1008 30 tropical depression 9/0000 23.2 111.5 1008 30 " 0600 23.2 111.8 1008 25 " 1200 23.2 112.0 1008 25 " 1800 23.2 112.3 1009 20 dissipating 7/0000 21.9 109.0 986 65 minimum pressure 7/2100 22.9 110.0 994 50 landfall near Cabo San Lucas
Table 2. Tropical Cyclone watch and warning summary for Hurricane Greg. Date/Time
Action Location 5/1600 Tropical Storm Warning for the coast of Mexico from Manzanillo to Cabo Corrientes including Islas Marias. 5/2100 Tropical Storm Warning extended southeastward to include Lazaro Cardenas. 6/0300 Tropical Storm Warning issued for the east coast of Baja California south of La Paz. 6/0300 Tropical Storm Watch issued for the west coast of Baja California south of Punta Abreojos. 6/0900 Tropical Storm Warning issued for the west coast of Baja California south of Cabo San Lazaro. 6/1500 Hurricane Warning for the west coast of Baja California from arroyo Seco southward and for the east coast from La Paz southward. 6/2100 Tropical Storm Watch for the east coast of Baja California fron north of La Paz to Loreto. 7/2100 change hurricane warning to tropical storm warning for Baja California from Arroyo Seco southward and from La Paz southward. 8/0900 Tropical Storm Warning and Watch discontinued for the east coast of Baja California 8/1500 Tropical Storm Warning and Watch discontinued all areas.