| Category 3 Hurricane (SSHS) | |
|---|---|
| Hurricane Hernan near peak intensity | |
| Formed | August 6, 2008 |
| Dissipated | August 12, 2008 |
| Highest winds | 1-minute sustained: 120 mph (195 km/h) |
| Lowest pressure | 956 mbar (hPa; 28.23 inHg) |
| Fatalities | None Reported |
| Damage | None |
| Areas affected | Hawaii |
| Part of the 2008 Pacific hurricane season | |
Hurricane Hernan was the ninth tropical depression, eighth named storm, fifth hurricane, and first major hurricane of the 2008 Pacific hurricane season. Hernan developed out of a tropical wave that formed off the east coast of Africa on July 24. Over the next week, the wave traversed the Atlantic without development and entered the Eastern Pacific basin on August 2. The wave became better organized over the next several days and was declared Tropical Depression Nine-E on August 6. The depression quickly became Tropical Storm Hernan later that day. Hernan steadily intensified over the next two days and was upgraded to a hurricane on the morning of August 8. Hernan continued to intensify and became the first major hurricane—a storm with winds of 111 mph (178 km/h) or higher—of the season on August 9. After reaching major hurricane status, Hernan steadily weakened to a minimal hurricane. The weakening continued, and Hernan was further downgraded to a tropical storm on August 11. As Hernan moved over cold waters, the convection associated with the storm dissipated, leaving only a swirl of clouds. By the morning of August 12, almost all of the convection associated with Hernan had dissipated and the system was declared a remnant low-pressure area despite retaining tropical storm-force winds. The low moved towards the west-southwest over the next several days before dissipating 460 mi (740 km) southeast of the Island of Hawaii. The remnants of the hurricane caused light rain to Hawaii.
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On July 24, a tropical wave emerged from the east coast of Africa, near the Cape Verde Islands. The wave was disorganized and failed to develop convection as it traversed the Atlantic over the next several days. The wave eventually entered the Eastern Pacific basin on August 2, and interacted with a broad area of cyclonic flow located a few hundred miles south of Mexico.[1] The wave became better organized throughout the day, and cyclonic turning was noted.[2] This was due to the interaction between the wave and the area of cyclonic turning, which had produced an area of low pressure 660 mi (1060 km) south of Manzanillo, Mexico on August 5.[1] Strong convection began to develop and slow development was forecast over the next 36–49 hours.[3] On August 5, a banding feature formed as convection continued to persist around the center of the wave.[4] On the morning of August 6, a Tropical Cyclone Formation Alert was issued, as development of the system was anticipated over the next several days.[5] Later in the day, the National Hurricane Center determined that the system had developed sufficient convection to be declared Tropical Depression Nine-E while located 775 mi (1230 km) to the south-southwest of the southern tip of Baja California.[1] However, the storm was operationally believed to have become a depression several hours later.[6]
Tropical Depression Nine-E was influenced by a high pressure area located over Mexico, causing it to move 16 mph (26 km/h) to the north-west. The depression was upgraded to Tropical Storm Hernan overnight as the storm became more organized.[7] Although Hernan was located over warm waters, moderate wind shear prevented the storm from intensifying quickly, and the storm continued towards the west-northwest at a slower pace.[8] Hernan continued to slowly become better organized throughout the night, but was still being impaired by northeasterly vertical wind shear during the morning on August 7.[9] Wind shear continued to impair Hernan through the morning; as a result, none of the forecast models, excluding the GFDL, forecast Hernan to become a hurricane.[10]
In the early afternoon, an eye feature began to form, prompting the National Hurricane Center to state that Hernan was nearing hurricane status.[11] However, by the nighttime hours, a microwave satellite found that the center of Hernan was located to the west-southwest of the eye feature, and the intensification ceased for the rest of the day and into the morning of August 8.[12][13] Later in the morning, the center of Hernan was determined to be located underneath the eye and was determined to have become a hurricane, the fifth of the season, at 8 a.m. (PST) on August 8.[14] Throughout the day, the eye became better defined, indicating that Hernan was intensifying. The moderate northeasterly wind shear had already begun to diminish, leading forecasters to show Hernan peaking as a Category 2 hurricane overnight.[15] That night, the eye was determined to be 30 mi (50 km), though cloud-filled. The convection surrounding the eye also became more intense.[16]
Early on August 9, Hernan was upgraded to a Category 2 hurricane, with winds of 100 mph (155 km/h). Convection around the eye continued become more symmetrical, though the National Hurricane Center had stated that Hernan had most likely reached its peak intensity or was very close to doing so.[17] Hernan continued to become better organized throughout the morning, and the eye suddenly became better defined and deep convection formed around the eye. Due to the sudden increase in organization, Hernan was upgraded to a major hurricane.[18][19] The intensity of Hernan was uncertain, as there was some difference between intensity estimates.[20] Hernan maintained its appearance through the early afternoon, and it was stated that the peak intensity of the storm may have been 125 mph (205 km/h).[21] However, this was not supported by the post-storm report on Hernan.[1] In the late afternoon, Hernan moved over cooler waters and started to weaken. The eye contracted slightly to 25 miles (35 km). Although the eye remained well defined, outflow to all the southwestern semicircle became poor.[22]
Hernan rapidly weakened overnight and was barely a Category 2 in the early hours of August 10 as it moved over cooler waters. However, the eyewall remained intact throughout the night.[23] Later in the morning, weakening slowed, and Hernan was downgraded to a strong Category 1. The western portion of the eyewall had begun to erode due to the cooling waters.[24] The erosion of the eyewall was later found to be caused by an eyewall replacement cycle that rapidly completed itself during the afternoon. Continuing to slowly weaken,[25] The newly formed eye began to shrink and deteriorate through the early afternoon, but Hernan briefly stopped weakening.[26] The weakening trend continued again during the night of August 10 as the cloud tops warmed.[27] Once more, the weakening stopped as a strong burst of convection around the center of the storm appeared. Hernan's strong circulation allowed it to maintain hurricane status over 24°C waters.[28]
Early on August 11, Hernan was downgraded to a tropical storm.[29] Slow weakening continued through the morning and afternoon as convection diminished around the center of the storm.[30] By the early morning of August 12, almost all of the deep convection dissipated as Hernan continued to weaken.[31] Convection continued to diminish as Hernan traveled over 23°C water and was barely holding on to tropical storm status.[32] By the afternoon of August 12, only a small area of tropical storm force winds remained with Hernan. Only a swirl of clouds remained and the storm was barely a tropical system.[33] Later that night, Hernan had degenerated into a remnant low, and the final advisory was issued by the National Hurricane Center.[34] The remnant low still retained tropical storm-force winds for a short while before weakening further by the next morning. The low moved towards the west-southwest over the next several days before dissipating 460 mi (740 km) southeast of the Island of Hawaii on August 16.[1] The remnant low-pressure area of Hernan later brought moisture to the island of Hawaii, causing cloud and shower activity. The associated rainfall was light and insignificant.[35]