The 2012 Alaska hurricane season is an annual event in tropical cyclone formation in the Alaska Hurricane Center's (AHC) area of responsibility. The season began on May 1, 2012 and ended on November 30, 2012. However, the season's first tropical storm formed on April 17. This year, the AHC began using the Saffir-Simpson Hurricane Scale. It was an average season with 14 tropical depressions, 11 of them reached tropical storm strength.
Initial forecasts indicated an above average season, with the AHC forecasting 13-16 named storms, 4-7 hurricanes and 1-2 major hurricanes. The National Weather Service Forecast Office in Anchorage, however, was only forecasting 11 named storms.
The AHC's forecast area extends from 35°N to 80°N and 160°E to 120°W.
A stationary low pressure area persisted in the northern Pacific Ocean, about 1,300 miles north of Hawaii. This low gradually lost frontal characteristics and was given a 'high' chance of formation by the AHC early on April 17. It became Tropical Depression One-K later that day, and a special advisory was issued several hours later noting that the depression had strengthened. At that time, One-K was named 'Arianna'. The low strengthened further to 60 mph the following day. Arianna defied all forecasts and became the first hurricane of the season early on April 18. Later in the day, the hurricane weakened back to tropical storm status as cooler sea surface temperatures and dry air weakened the storm. While 300 miles south of the Aleutian Islands, the wind field of Arianna expanded, bringing strong winds to the islands. Late on April 21, Arianna weakened to a tropical depression due westerly 40-45 mph wind shear. The shear continued, and Arianna became a post-tropical remnant low on April 22. The remnant became a complex low with multiple fronts and brought up to 10 inches of rain to the Alaska Panhandle and the Cook Inlet area.
It was noted that Alaskan hurricanes typically move west to east south of the Aleutian Islands, not north towards them. The last hurricane to do this was Estelle last year.
On May 29, a tropical storm formed in the Gulf of Alaska without warning. Almost immediately, the storm was named Brandon and tropical storm warnings were issued from Kinak Bay to Controller Bay, Alaska. The storm rapidly intensified into a major hurricane within twelve hours. Brandon made landfall south of Anchorage with winds of 120 mph.
A state of emergency was declared for Alaska as the storm approached on June 3. Mandatory evacuations were ordered in Anchorage. Major flooding occurred, and preliminary damage totals were near $2 billion.
On October 1, an area of strong thunderstorms developed 450 miles west of Seattle, Washington, and the AHC gave it a 30% chance of formation. Moving gradually west, it strengthened and advisories were initiated on Tropical Depression Twelve. Twelve continued westward and showed little sign of additional strengthening. Upper level wind shear ripped the system apart and dissipated on October 6 while 730 miles west of Juneau, Alaska.