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|Category 4 major hurricane (SSHWS/NWS)|
|Formed||September 29, 2018|
|Dissipated||October 13, 2018|
|Highest winds||1-minute sustained: 140 mph (220 km/h) |
|Lowest pressure||942 mbar (hPa); 27.82 inHg|
|Damage||$402.1 million (2018 USD)|
|Areas affected||Baja California Peninsula, Northwestern Mexico, Southwestern United States, Texas|
|Part of the 2018 Pacific hurricane season|
Hurricane Sergio was a powerful and long-lived tropical cyclone that affected the Baja California Peninsula as a tropical storm. Sergio became the eighth Category 4 hurricane in the East Pacific for 2018, breaking the old record of seven which was set in 2015. The twentieth named storm, eleventh hurricane, and ninth major hurricane of the season, Sergio originated from a broad area of low pressure that formed a few hundred miles south-southeast of the southern coast of Mexico on September 26. The National Hurricane Center monitored the disturbance for a few days until it organized into a tropical storm, after which it was assigned the name Sergio. The system gradually strengthened for the next couple of days, becoming a hurricane on October 2. Sergio then began a period of rapid intensification, becoming a major hurricane later that day. Intensification then halted for about twelve hours before resuming on October 3. The next day, Sergio peaked as a Category 4 hurricane with maximum sustained winds of 140 mph (220 km/h) and a minimum central pressure of 942 mbar (27.82 inHg). Sergio maintained peak intensity for six hours before beginning to weaken. On October 5, the system bottomed out as a low-end Category 3 hurricane. Sergio then began another period of intensification, achieving a secondary peak on October 6. The next day, Sergio began to weaken again, falling below major hurricane strength. At the same time, Sergio unexpectedly assumed the structure of an annular tropical cyclone. By October 9, Sergio had weakened into a tropical storm. On October 12, Sergio made landfall as a tropical storm on the Baja California Peninsula, and later in northwestern Mexico as a tropical depression before dissipating early on October 13.
On October 10, Sergio's approach warranted the issuance of tropical storm watches and warnings along the western and eastern coasts of Baja California. Sergio made landfall in western Baja California Sur and Sonora on October 13 as a weak tropical storm, causing over US$2 million in damages, over a thousand school closures, and a few hundred evacuations due to severe flooding. The remnants of Sergio also brought heavy rainfall to Arizona, resulting in the closure of its state fair. Throughout Mexico and Arizona, no injuries or deaths were reported. Heavy rainfall also occurred in Texas, resulting in around US$400 million in damage and one death. Multiple tornadoes also spawned as a result of the increased moisture.
The origins of Hurricane Sergio can be traced to a system that was located over northwestern South America on September 24. On September 25, the National Hurricane Center (NHC) forecasted that an area of low pressure would form a few hundred miles south or southwest of the Gulf of Tehuantepec in a couple days. On September 26 at 12:00 UTC, a broad area of low pressure formed a few hundred miles south-southeast of the southern coast of Mexico. The NHC continued to track the disturbance for a couple more days as it moved west-northwest. On September 29 at 12:00 UTC, Tropical Storm Sergio formed approximately 345 miles (555 km) south-southwest of Acapulco, Mexico. The NHC later discussed that Sergio was indeed a tropical storm, but without an inner wind core. Instead, the maximum sustained winds were located in a band approximately 125 mi (205 km) east of the center.
Sergio continued to gradually intensify over the next couple of days, becoming a hurricane on October 2 at 00:00 UTC. At this time, Sergio had a well defined eye underneath a deep convective overcast with cloud tops around −85 °C (−121 °F). Sergio then began a period of rapid intensification, becoming a major hurricane at 18:00 UTC. Soon after, the intensification halted as northwesterly shear affected Sergio's inner core, with microwave data indicating an open eyewall on the north side of the hurricane. Sergio maintained its intensity for 18 hours before resuming intensification. At the same time, the mid-level ridge to the north had weakened, resulting in Sergio travelling to the northwest. On October 4 at 00:00 UTC, Sergio became the eighth Category 4 hurricane of the season, breaking the old record of 7 which was set in 2015. Six hours later, Sergio peaked with maximum sustained winds of 140 mph (220 km/h) and a minimum central pressure of 942 mbar (27.82 inHg). Sergio maintained peak intensity for 12 hours before beginning to weaken. At 03:00 UTC on October 5, it was noted that Sergio's structure had deteriorated, with cloud tops warming and the eye becoming ragged and less distinct. At 09:00 UTC, Sergio began to turn due to a mid-level ridge that was developing to the northwest. Over the next couple of days, Sergio turned from the northwest to the southwest. At 06:00 UTC, Sergio bottomed out as a low-end Category 3 hurricane.
Late on October 5, Sergio began another period of intensification, reaching a secondary peak with winds of 125 mph (205 km/h) on October 6 at 00:00 UTC. Twelve hours later, a shortwave trough weakened the ridge to the northwest, resulting in the commencement of another turn. Over the next few days, Sergio turned from the southwest to the northeast. After maintaining secondary peak intensity for eighteen hours, Sergio began to weaken due to upwelling at 00:00 UTC on October 7. On October 7, after having weakened to a Category 2 hurricane, Sergio unexpectedly acquired the structure of an annular hurricane. At the same time, it was reported that the eye had doubled in size overnight. At 00:00 UTC the next day, Sergio weakened to a Category 1 hurricane. On October 9, at 18:00 UTC, Sergio weakened to a tropical storm, due to deteriorating conditions. Around 12:00 UTC on October 12, Sergio made landfall near Los Castros, Baja California Sur, as a tropical storm, with winds of 50 mph (85 km/h). Shortly after 18:00 UTC, Sergio made landfall near Guaymas, Sonora, as a tropical depression. At 00:00 UTC on October 13, Sergio dissipated over northwestern Mexico.
On October 10, the Government of Mexico issued a Tropical Storm Watch along the western coast of the Baja California Peninsula from Punta Eugenia to Cabo San Lazaro and along the eastern coast from Bahia San Juan Bautista to San Evaristo. Early on the next day, as Sergio rapidly approached the peninsula, tropical storm watch on the west coast was upgraded to a tropical storm warning. The watches on the east coast were also upgraded to Tropical Storm Warnings later on the same day. All watches and warnings were discontinued as Sergio moved inland.
Sergio made landfall in western Baja California Sur and Sonora, bringing heavy rains and strong winds to northern Mexico. In Baja California Sur, multiple roads and schools located in Loreto were damaged. In Sonora, almost 400 people had to evacuate to a temporary shelter due to the threat of flooding. Strong winds up to 43 mph (70 km/h) downed trees and utility poles in Guaymas. Damage in the city was estimated at approximately MX$40 million (US$2.12 million), with houses and businesses suffering damages. In Punta de Aqua II, over 5.05 inches (128 mm) of rain was reported. Additionally, Mazatlán and Plutarco Elías Calles reported rainfall totals of 4.59 inches (117 mm) and 4.53 inches (115 mm), respectively. More than 1,000 schools were closed in Chihuahua and classes in eight municipalities of Sinaloa were suspended. However, there were no deaths or injuries reported.
Sergio's remnants bought powerful thunderstorms to Southern California on the evening of October 12. Rain extended into October 13, with scattered showers in the morning, followed by cloudiness in the afternoon and night. One of the thunderstorms triggered by the remnants of Sergio caused a blackout that left thousands in the dark in Los Angeles County. The thunderstorms were also blamed for starting fires in Santa Ana.
The remnants of Sergio also moved into Arizona and Texas, bringing heavy rains and spawning multiple tornadoes. In Arizona, some areas in Phoenix recorded rainfall totals of 3 inches (76 mm), and water ankle deep. The Arizona State Fair was closed due to flooding, the first time in "recent memory". In Texas, heavy rainfall in Llano County caused severe flooding. A rainfall total of 9.27 inches (235 mm) was reported at Castell. Additionally, Llano saw 6.26 inches (159 mm) of rain. The water level of Llano River rose about 25 feet (7.6 m) in 12 hours, causing a bridge in Kingsland to collapse. The cost to replace the bridge was estimated at US$20 million. In Llano, a 58-year-old woman drowned in her vehicle. Additionally, telephone and internet service went down after wires underneath the Llano River Bridge were swept away by floodwaters. Throughout the county, 38 homes were destroyed and over 600 sustained damages from flooding. Flood damages were estimated at US$50 million. Near Lake Travis in Travis County, 176 homes sustained damages from floods and over 200 more were affected. In Kerr County, a rainfall total of 8.40 inches (213 mm) was recorded at Mountain Home. At Lake Lyndon B. Johnson and Lake Marble Falls in Burnet County, 17 homes were destroyed, 232 were damaged, and 31 were affected. Damages to houses at both lakes are estimated at US$30 million total. In total, one person was killed and the economic loss of the flooding was about US$400 million. Multiple tornadoes spawned as a result of the increased moisture. An EF0 tornado uprooted a tree and caused roof damage in Brady. Doppler radar detected another tornado near Hillsboro and Bynum. Multiple tornadoes were sighted on Interstate 45 near Angus in Navarro County and north-northwest of Buffalo in Freestone County. Another tornado downed trees and caused power flashes near Henderson.
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