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|Category 4 major hurricane (SSHWS/NWS)|
|Formed||September 25, 2018|
|Dissipated||October 3, 2018|
|(Remnant low after October 2)|
|Highest winds||1-minute sustained: 150 mph (240 km/h) |
|Lowest pressure||936 mbar (hPa); 27.64 inHg|
|Damage||$50.5 million (2018 USD)|
|Areas affected||Baja California Peninsula, Northwestern Mexico, Southwestern United States|
|Part of the 2018 Pacific hurricane season|
Hurricane Rosa was the first tropical cyclone to make landfall in the Mexican state of Baja California since Nora in 1997. The seventeenth named storm, tenth hurricane, and seventh major hurricane of the 2018 Pacific hurricane season, Rosa originated from a broad area of low pressure that the National Hurricane Center began monitoring on September 22. The disturbance moved westward and then west-northwestward for a few days, before developing into a tropical depression on September 25. Later that day, the depression strengthened into Tropical Storm Rosa. One day later, Rosa became a hurricane. On September 27, Rosa began a period of rapid intensification, ultimately peaking as a Category 4 hurricane with maximum sustained winds of 150 mph (240 km/h) and a minimum central pressure of 936 mbar (27.64 inHg) on the next day. Over the next couple of days, Rosa turned towards the northeast. By September 29, Rosa had weakened into a Category 2 hurricane due to ongoing structural changes and less favorable conditions. Later on the same day, Rosa re-intensified slightly. On September 30, Rosa resumed weakening as its core structure eroded. Early on October 1, Rosa weakened into a tropical storm. On October 2, Rosa weakened to a tropical depression and made landfall in Baja California. Later in the day, Rosa's remnants crossed into the Gulf of California, with its surface and mid-level remnants later separating entirely. The mid-level remnants of Rosa continued to travel north, reaching northeast Arizona late in the day. On October 3, Rosa's remnants were absorbed into an upper-level low situated off the coast of California.
Rosa prompted the issuance of tropical storm watches and warnings along the coast of Baja California as well as various flood related watches and warnings throughout the Southwestern United States. Rainfall from Rosa affected a large geographical area, due to the remnants having split apart in the Gulf of California. Rosa caused significant flooding throughout northwestern Mexico, which has resulted in the deaths of one person. Rosa also caused flash-flooding in Arizona, with several inches of rain falling in areas, which indirectly resulted in the deaths of two individuals. Additionally, remnant moisture from Rosa led to rainfall throughout the Southwestern United States. Flood damage from Rosa totaled about US$50.5 million.
Hurricane Rosa originated from a vigorous tropical wave that departed from the west coast of Africa on September 6. The wave spawned Hurricane Helene on September 7, and then travelled across the tropical Atlantic with minimal convective activity. On September 19, the National Hurricane Center (NHC) began monitoring the wave for tropical development, anticipating that an area of low pressure would form a few hundred miles south of the Gulf of Tehuantepec over the weekend. On September 22, a broad area of low pressure formed approximately 200 mi (320 km) south of Mexico. The NHC continued to track the disturbance for a few more days as it moved westward to west-northwestward. On September 25 at 06:00 UTC, the NHC reported that a tropical depression had formed approximately 350 mi (565 km) south-southwest of Manzanillo, Mexico. Around that time, the NHC stated that the system had a well defined center and deep convection that was increasing in both coverage and intensity. Additionally, the depression was located in an environment with warm sea surface temperatures and minimal vertical wind shear. Six hours later, the depression strengthened into a tropical storm and was assigned the name Rosa.
Over the next day, Rosa continued to strengthen, becoming the tenth hurricane of the season at 12:00 UTC on September 26. Around that time, the NHC indicated that Rosa had a solid mid-level ring and strong, well developed banding in the southern half of the system. Rosa's intensity leveled off for about eighteen hours before strengthening resumed, resulting in the system attaining Category 2 status on September 27 at 12:00 UTC, and major hurricane status six hours later. On September 28, at 03:00 UTC, Rosa peaked with maximum sustained winds of 150 mph (240 km/h) and a minimum central pressure of 936 mbar (27.64 inHg), becoming the seventh Category 4 hurricane of the season. Shortly after, the NHC stated that Rosa's eye had warmed considerably and the clouds in the inner ring had warmed considerably, which marked the commencement of an eyewall replacement cycle. By the afternoon of September 28, Rosa began to track northwest in response to an approaching mid- to upper-level trough. As the eyewall cycle took place and Rosa began to track towards cooler water, the storm steadily weakened and was dwongraded to Category 2 status at 00:00 UTC on September 29. After completing its eyewall replacement cycle, the storm briefly re-intensified, following a major increase in organization and the expansion of its northeastward outflow channel. However, westerly wind shear caused the circulation to become displaced, as it tracked due north under the increasing influence of the trough. Rosa resumed weakening, falling to Category 1 status at 12:00 UTC on September 30.
By September 30, a combination of strong southwesterly wind shear, dry air entanglement, and cooler sea surface temperatures caused the core of Rosa to quickly erode, resulting in the collapse of its eye and convection in the storm's southern semicircle. On the morning of October 1, at 00:00 UTC, Rosa weakened into a tropical storm. Shortly after, Rosa began travelling towards the northeast. Twenty-four hours later, Rosa weakened into a tropical depression, with the NHC reporting that the remaining convection was displaced to the northeast of the system's center and that the circulation was becoming elongated. At 11:00 UTC, Rosa made landfall approximately 70 mi (115 km) southeast of Punta San Antonio in Baja California. The NHC issued its last advisory on Rosa at 16:00 UTC as it began to cross into the Gulf of California, after which the Weather Prediction Center (WPC) began issuing storm summaries on the system. On the same day, the WPC noted that Rosa's low-level and mid-level remnants had split, with the surface low of Rosa located in the Gulf of California while its mid-level remnants were over northeast Arizona. On October 3, the remnants of Rosa were absorbed into a developing upper-level low off the coast of California, and the WPC issued their last advisory on the system.
On September 29, the Government of Mexico issued a tropical storm watch for the Pacific Coast of the Baja California peninsula from Punta Abreojos to Cabo San Quintin. On the next day, the Government of Mexico changed the tropical storm watches on the west coast of Baja California to tropical storm warnings and issued watches for the east coast of Baja California from Bahia de los Angeles to San Felipe. All the watches and warnings were discontinued after Rosa weakened to a tropical depression.
On October 2, a woman drowned in Caborca after being swept away by flood waters. The governor of Baja California, Francisco Vega de la Madrid, issued a State of Emergency for the cities of Ensenada and Mexicali. Schools were closed on October 1 in several communities as Rosa approached the area. Classes were also suspended in the neighboring state of Sonora, as floodwaters inundated roads and highways across the state. Damage from the port of San Felipe totaled about MX$10 million (US$531,000). On October 3, an emergency declaration was approved for Puerto Peñasco after Rosa passed by. Dozens of homes and businesses experienced flooding after a total of 3.94 in (100 mm) of rain fell. Multiple road closures occurred as a result of the flooding.
After Rosa made landfall, its remnants tracked northward, spawning rain showers and thunderstorms in the Four Corners region. On September 30, in anticipation of severe rainfall from the remnants of Rosa, flood watches and warnings were issued for Southern California, Arizona, southwest Colorado, Utah, central Nevada, and a small portion of southeast Idaho. At that time, rainfall was causing flooding in Arizona and Southern California. In San Bernardino County, the remnants of Rosa and a Pacific low produced thunderstorms over the Mojave Desert. On October 1, portions of U.S. Route 95 were flooded out, with floodwaters depositing rocks and other objects on the road. Additionally, portions of State Route 62 and State Route 127 were also flooded out.
By 09:00 UTC on October 3, a preliminary total of 6.89 in (175 mm) was reported at Towers Mountain, Arizona, with other areas reporting 1 to 5.5 in (25 to 140 mm) of rain. After more than 2 in (51 mm) of rain fell, the National Weather Service issued a flash flood warning for the Phoenix area and firefighters were forced to rescue several people from flooded vehicles. Additionally, over two dozen road closures and 80 car crashes occurred. Multiple schools and businesses were closed due to flooding At the intersection of 35th Avenue and Cactus Road, it was reported that a sinkhole had formed. Rosa also caused flash flooding in the communities of Guadalupe, Glendale, Scottsdale, Fountain Hills, Deer Valley, and Sun City. Rosa caused thousands of power outages in Yuma. On October 3, a 26-year-old French woman was killed just north of Cameron after being struck by a vehicle. Portions of U.S. Route 89 were washed out after flash flooding affected the area. In Pioche, Nevada, flash flooding left several buildings inundated and water and debris on Main Street.
At Menager Lake near Sells, rainfall from Rosa filled the Menagers Dam above maximum capacity, raising concerns about its structural integrity. This prompted the National Weather Service in Tucson to recommend immediate evacuation for the village of Ali Chuk on October 2, stating that dam failure was imminent. On October 2, 162 people were evacuated from Ali Chuk, 32 from Kohatk, and 23 others from the Menegar's Dam community. Despite the water level having receded, there were still concerns that the dam could fail. On October 4, the Tohono O’odham Nation announced that they were assembling an engineering team to inspect the dam before more rain arrives. On the same day, a 34-year-old man was found deceased after the flooding in Golden Valley. Damage from flooding totaled about US$50 million.
person reportedly drowned in Mexico after being swept away by floodwaters.
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