|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|
|Fatalities||1 direct, 2 indirect|
|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 brought widespread flooding to northwestern Mexico and the Southwestern United States as the first tropical cyclone to make landfall in 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 an Atlantic tropical wave that crossed the West African coast on September 6. The wave proceeded westward across the Atlantic, traversing Central America before entering the Gulf of Tehuantepec on September 22. There, the weather system acquired cyclonic features and became a tropical storm on September 25. Within a favorable atmosphere, Rosa entered a period of rapid intensification on September 27, peaking as a Category 4 hurricane on the Saffir-Simpson scale with maximum sustained winds of 150 mph (240 km/h) a day later. Over the next few days, Rosa turned north and then northeast while steadily weakening, making landfall in Baja California as a tropical depression on October 2. Crossing into the Gulf of California, the remnant system split apart before its absorption into an upper-level low off the coast of California by October 3.
Rosa prompted the issuance of tropical storm watches and warnings along the coast of Baja California, as well as various flood watches and warnings throughout the Southwestern United States. The storm and its remnants caused widespread flooding throughout northwestern Mexico, mainly in Sonora and Baja California, which led to one drowning and minor damage. In Arizona, rainfall peaked at 6.89 in (175 mm) and caused flash floods that killed two people. Flood damage from Rosa and its remnants totaled $50 million (2018 USD) in the Southwestern United States and $530,000 in Baja California.
Hurricane Rosa originated from a vigorous tropical wave that departed from the west coast of Africa on September 6. The wave traveled across the tropical Atlantic with minimal associated weather and became difficult to track after interacting with a mid-level trough in the Caribbean Sea. The National Hurricane Center (NHC) issued a Tropical Weather Outlook on September 19, anticipating that an area of low pressure would form in the Gulf of Tehuantepec over the weekend. The wave entered the gulf on September 22, where it produced a surface circulation with convective activity aloft. Though broad in structure, the system consolidated as it proceeded slightly north of west. It was officially classified as Tropical Depression Twenty-E on September 25, at 06:00 UTC, located 350 mi (565 km) south-southwest of Manzanillo, Mexico.
At the moment of its formation, the depression was located within a favorable tropospheric environment of warm sea surface temperatures and minimal vertical wind shear, featuring a well-defined center of circulation under an expanding area of strong convection. The depression maintained a trend of steady strengthening over the next couple of days: it was upgraded to Tropical Storm Rosa six hours after being classified, and became the tenth hurricane of the season a day later, at 12:00 UTC on September 26. The NHC remarked that Rosa's structure was well developed at the middle levels of the troposphere, with distinct rainbands wrapped around the southern semicircle of the cyclone. The hurricane leveled in intensity for eighteen hours before proceeding into another phase of rapid intensification; it reached major hurricane status at 18:00 UTC on September 27, peaking in intensity with maximum sustained winds of 150 mph (240 km/h) and a minimum central pressure of 936 mbar (27.64 inHg). This made Rosa the seventh Category 4 hurricane of the year's season.
After Rosa reached its peak, the hurricane's eyewall – an inner ring of clouds around the eye – began to warm considerably, signaling the start of an eyewall replacement cycle. Rosa turned to the northwest on the afternoon of September 28 in response to an approaching mid- to upper-level trough, which would continue to influence the remainder of the hurricane's development. Now tracking over cooler seas, Rosa steadily weakened down to Category 2 strength by 00:00 UTC, September 29, while undergoing its eyewall replacement. Once the replacement cycle was completed, the storm briefly restrengthened because of its much-improved structure, with expanding outflow to the northeast of the eye. However, Rosa began to experience impinging wind shear from the developing trough, causing a misalignment between the upper and lower levels of the hurricane and coinciding with a final weakening phase.
Rosa turned to the north on September 29, ahead of the trough. The unrelenting wind shear, combined with progressively cooler seas and drier air, quickly eroded Rosa’s core, disrupting the eye and convection over the southern half of the hurricane. At 12:00 UTC on September 30, the diminishing hurricane dropped to Category 1 while being steered towards the northeast between the trough and a subtropical ridge. Rosa lost its hurricane status twelve hours later, proceeding towards the Baja California Peninsula as a tropical storm. It further weakened to a tropical depression on October 2, after the convection became displaced from the elongating center. At 11:00 UTC on October 2, Rosa made landfall about 70 mi (115 km) southeast of Punta San Antonio in Baja California, becoming the first tropical cyclone to move over the state since Nora of 1997. During its approach towards the Gulf of California, Rosa exhibited an increasingly unwound and diffuse structure, prompting the NHC to declassify it as a tropical cyclone at 15:00 UTC. Shortly after, forecasters at the Weather Prediction Center (WPC) noted that the low- and mid-level circulations of Rosa's remnant had decoupled; the mid-level remnants proceeded into northeast Arizona, while the lower segment traced behind it over the Gulf of California. On October 3, in their final advisory on the system, the WPC reported that the remnants had been absorbed into a deepening non-tropical low off the coast of California.
The Government of Mexico issued a tropical storm watch on September 29 for the Pacific Coast of the Baja California Peninsula from Punta Abreojos to Cabo San Quintín. The watches on the west coast of Baja California were changed to tropical storm warnings and watches were issued for the east coast of Baja California from Bahia de los Angeles to San Felipe on the next day. All the watches and warnings were discontinued after Rosa weakened to a tropical depression. The State Unit of Civil Protection of Sonora reported on September 30 that a yellow rain alert (imminent impact) had been issued for 11 municipalities and a green rain alert (possible impact) for 19 municipalities in Sonora in anticipation of severe weather conditions. A red alert was issued for San Felipe on October 1 as Rosa approached Baja California. On the same day, schools were closed in several communities throughout Baja California as well as in the neighboring state of Sonora. The Marine Plan, an evacuation and rescue plan, was activated in the states of Baja California, Baja California Sur, Sonora, and Sinaloa.
Hurricane Rosa produced heavy rainfall in Baja California, peaking at 6.54 in (166 mm) in Percebu. A woman drowned in Caborca after being swept away by floodwaters. Rosa dropped 5.39 in (137 mm) of rain in San Felipe, where floods collapsed part of a highway and opened up a sinkhole in the port. The port of San Felipe suffered MX$10 million ($530,000 USD) worth of losses following a five-day shutdown of operations. In Los Cabos, heavy rainfall triggered a power outage and caused street floods that swept away vehicles. In Puerto Peñasco, dozens of homes and businesses suffered from flooding after an estimated 4 in (100 mm) of rain fell. Multiple road closures occurred and four bridges were impassable. In Manzanillo, Colima, floodwaters caused sinkholes, ruptured underground pipes, and inundated homes and businesses. Landslides in and around the city blocked roads and buried three vehicles in mud. In Michoacán, the combined effects from Rosa and nearby Tropical Storm Sergio had destroyed 86,000 acres (35,000 ha) of crops by September 30. After Rosa's passage, the Governor of Baja California issued a state of emergency for the cities of Ensenada, Mexicali, and Puerto Peñasco.
After Rosa made landfall, its remnants tracked northward, spawning rain showers and thunderstorms in the Four Corners region. Damage from flooding in the Southwestern United States totaled about $50 million (USD). In anticipation of severe rainfall from the system's remnants, flood watches and warnings were issued for Southern California, Arizona, southwest Colorado, Utah, central Nevada, and a small portion of southeast Idaho on September 30. At that time, rainfall was causing flooding in Arizona and Southern California. In the latter state, the remnants of Rosa and a Pacific low produced thunderstorms in San Bernardino County. Floodwaters swept rock and other objects across portions of U.S. Route 95 on October 1. Portions of State Route 62 and State Route 127 were caked with mud and debris as floodwaters surged across the roads. In Pioche, Nevada, flash flooding inundated several buildings and deposited debris on Main Street.
By the time of Rosa's absorption on October 3, a total of 6.89 in (175 mm) was reported at Towers Mountain, Arizona, which is approximately 85 mi (135 km) north of Phoenix, with other areas reporting up to 5.5 in (140 mm) of rain. After more than 2 in (50 mm) of rain fell, the National Weather Service issued a flash flood warning for the Phoenix area; over two dozen road closures and 80 car crashes occurred. Multiple schools and businesses were also closed. Rosa caused flash flooding in the communities of Guadalupe, Glendale, Scottsdale, Fountain Hills, Deer Valley, and Sun City as well as thousands of power outages in Yuma. A 26-year-old woman was struck by a vehicle and killed just north of Cameron on October 3 after portions of U.S. Route 89 washed out. In Kingman, a 34-year-old man was killed on October 4 after his vehicle was swept off a road by floodwaters; the vehicle was discovered entirely submerged in water.
At Menagers Dam near Sells, Arizona, rainfall from Rosa brought the water level to within a foot of maximum capacity, raising concerns about the dam's structural integrity. Stating that dam failure was imminent, the National Weather Service in Tucson recommended immediate evacuation for the village of Ali Chuk on October 2. Later that day, 162 people were evacuated from Ali Chuk, 32 from Kohatk, and 23 others from the Menegars Dam community. Despite the water level having receded, there were still concerns that the dam could fail. The Tohono O’odham Nation announced on October 4 that they were gathering engineers to assess the dam before the arrival of more rain later in the week. Over the next two weeks, officials monitored the water level of the dam before allowing residents to return to their homes on October 17.
person reportedly drowned in Mexico after being swept away by floodwaters.
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