Category 4, the second-highest classification on the Saffir–Simpson scale,[nb 1] is used for tropical cyclones that have winds of 130–156 mph (209–251 km/h; 113–136 kn).
The North Pacific High and Aleutian Low, usually present between January and April, cause strong wind shear and unfavorable conditions for the development of hurricanes.
On the Saffir–Simpson Hurricane Scale, "Category 4" is the second-most powerful classification, with winds ranging between 130 and 156 mph (209 and 251 km/h; 113 and 136 kn).
Following the move of the hurricane center to Redwood City in 1976, track files were created and altered by Arthur Pike and were later re-modified following the release of a study in 1980.
The National Hurricane Center (NHC) extended its authority to the EPHC in 1988, and subsequently began maintaining the tracks.
It was estimated that if sea-surface temperatures ascended by 2 to 2.5 degrees, the intensity of tropical cyclones would increase by 6–10% internationally.
[8] Contrary to El Niño, La Niña increases wind shear over the eastern Pacific and reduces it over the Atlantic.
A second factor preventing tropical cyclones from forming during the winter is the occupation of a semi-permanent low-pressure area designated the Aleutian Low between January and April.
Its presence over western Canada and the northwestern United States contributes to the area's occurrences of precipitation in that duration.
In addition, its effects in the central Pacific near 160° W causes tropical waves that form in the area to drift northward into the Gulf of Alaska and dissipate.
Its retreat in late-April allows the warmth of the Pacific High to meander in, bringing its powerful clockwise wind circulation with it.
The Intertropical Convergence Zone departs southward in mid-May permitting the formation of the earliest tropical waves,[4] coinciding with the start of the eastern Pacific hurricane season on May 15.