Hurricane Guillermo (1997) - Meteorological History

Meteorological History

Hurricane Guillermo began its extensive track as a tropical wave that moved off the coast of Africa on July 16, 1997. Initially disorganized and weak, the system tracked westward across the Atlantic Ocean for several weeks. The National Hurricane Center (NHC) stated that they had issues monitoring the system as it moved through the Caribbean; however, they interpreted through satellite data that the wave crossed Central America and entered the Pacific Ocean between July 27 and 28. Once in the Pacific, convection, areas of thunderstorm activity, and banding features began to form. Additionally, a low pressure system developed within the disturbance by July 29. The following day, the system became sufficiently organized for the NHC to classify it as Tropical Depression Nine-E; at this time the depression was situated roughly 345 mi (555 km) south of Salina Cruz, Mexico. In response to a deep-layer ridge to the north, the depression tracked at a steady pace towards the west-northwest, and this motion persisted through the first week of August. Within a day of being classified, the system intensified into Tropical Storm Guillermo, the seventh named storm of the 1997 season.

By the afternoon of August 1, a strong central dense overcast, an area of high, thick clouds, developed over the center of circulation, prompting the NHC to upgrade it to a Category 1 hurricane on the Saffir–Simpson Hurricane Scale. Throughout the next day, the system gradually developed an eye within the central dense overcast, leading to further strengthening. Operationally, the Guillermo was thought to have briefly leveled out in intensity on August 2; however, in post-storm analysis, the NHC discovered that a steady period of rapid intensification took place. Unlike most hurricanes in the eastern Pacific, Guillermo was investigated by Hurricane Hunters reconnaissance aircraft during its rapid intensification stage. The aircraft released several dropsondes into the storm to gather meteorological data. This mission marked the first time the Hurricane Hunters recorded high-resolution wind data from flight level to within several meters of the ocean surface inside the eyewall of a major hurricane.

Radar was also employed to determine the size of the hurricane's eye, stated to be roughly 13 mi (20 km) in diameter, following a 6-mile (10 km) decrease due to the strengthening. Light wind shear surrounding the hurricane allowed further strengthening to take place. Late on August 2, the system attained winds of 135 mph (215 km/h), making it a Category 4 storm. During the afternoon of August 3, Guillermo nearly attained Category 5 intensity as it reached its initial peak intensity with winds of 155 mph (250 km/h) along with a barometric pressure estimated at 925 mbar (hPa; 27.32 inHg). A brief period of weakening took place later that day before further strengthening ensued. Late on August 4, Guillermo intensified into a Category 5 hurricane, attaining peak winds of 160 mph (260 km/h). Operational analysis of satellite information indicated a minimum pressure of 921 mbar (hPa; 27.2 inHg); however, post-storm studies stated that the pressure was more likely around 919 mbar (hPa; 27.14 inHg).

At its peak, cloud temperatures within the eyewall were estimated to be as low as −79 °C (−110 °F). Using the Dvorak technique, a method used to estimate the intensity of tropical cyclones, a value of 7.6 was obtained. This indicated that maximum winds at the surface could have been as high as 181 mph (291 km/h); however, this was not used as the reported intensity as six- to twelve-hour averages indicated sustained winds around 160 mph (260 km/h). After maintaining this intensity for roughly 18 hours, the system began to weaken as it moved into a less favorable environment with moderate wind shear. Cloud temperatures within the eyewall also began to increase, indicating that the hurricane was losing intensity.

Steady weakening took place over the following several days, and the storm dropped below major hurricane status on August 6. By August 8, Guillermo moved over colder waters and was downgraded to a tropical storm as sustained winds dropped below 75 mph (120 km/h). Around this time, the storm started to move along the western edge of the ridge that previously steered it towards the west-northwest, causing Guillermo to turn northwest. On August 9, the storm crossed longitude 140°W, denoting a shift in warning responsibility from the NHC to the Central Pacific Hurricane Center (CPHC). Not long after crossing into the CPHC's area of responsibility, the storm further weakened to a tropical depression as it moved over 24 °C (75 °F) waters.

Although most tropical cyclones generally weaken as they increase in latitude, warm waters of 26 °C (79 °F) allowed Guillermo to re-intensify into a tropical storm on August 11. Gradually, the storm attained winds of 65 mph (100 km/h) before succumbing to cooler waters further north. The storm weakened to a tropical depression once more on August 15, situated well to the north of the Hawaiian islands. Later that day, Guillermo transitioned into an extratropical cyclone at an unusually high latitude of 41.8°N, roughly 850 mi (1,370 km) south-southeast of Unalaska, Alaska. Over the following days, the remnants of the hurricane turned northeastward towards the Pacific coast of North America. On August 19, the system moved within 575 mi (925 km) of Vancouver Island, British Columbia before Guillermo tracked southward. Over the following few days, the system slowed significantly and turned towards the east. By August 20, moisture from the remnants of Tropical Storm Ignacio were entrained into the eastern portion of Guillermo's circulation. On August 24, the low was finally absorbed by a larger extratropical system while situated roughly 345 mi (555 km) off of the northern California coastline.