Ex-Hurricane Oho Going Where Few Hurricanes Have Gone Before: Alaska

Alaska and British Columbia are on alert to receive a very unusual dose of tropical weather: the remains of Hurricane Oho, which are on track to hurtle into the Alaska Panhandle on Friday evening. Oho completed the transition from a hurricane to an extratropical storm with 70 mph winds on Thursday morning, and after short period of weakening, is expected to interact with a powerful jet stream over the Gulf of Alaska and intensify on Friday afternoon off the coast of Alaska into a powerful 960 mb low pressure system with near-hurricane-force winds and heavy rain. A High Wind Warning is up for Sitka, Alaska for sustained winds of 40 to 55 mph with gusts of 65 to 75 mph on Friday. Sustained winds of 70 mph--just below hurricane-force--accompanied by 26-foot seas are expected over the offshore waters of the Alaska Panhandle from Cape Decision to Clarence Strait. The Juneau, Alaska NWS discussion from Thursday morning noted that 2 - 4" of rain is expected on Friday from the unusual storm, and "the dry week we just had has allowed rivers to fall...but risk of mudslides cannot be ruled out due to the extreme/unusual nature of this system."


Figure 1. MODIS image of Hurricane Oho as seen from NASA's Aqua satellite on Wednesday, October 7, 2015. At the time, Oho was a Category 2 storm with 105 mph winds. Image credit: NASA.

Hurricane Oho goes where few hurricanes have gone before
This is the second year in a row for the extratropical remains of a Central Pacific hurricane to affect the northwestern coast of North America. As I blogged about last year, Hurricane Ana, which took an extended tour of the Hawaiian Islands, then turned northeast and maintained hurricane strength to a latitude of 36.3°N--approximately the latitude of Monterey, California--transitioned to a powerful extratropical storm that brought strong winds and heavy rain to the Pacific Northwest and British Columbia on October 28, 2014. Only one other time since 1949 have the remains of a Central Pacific hurricane directly impacted the northwest coast of North American--when the extratropical remains of an unnamed 1975 storm that maintained hurricane strength remarkably far north, to 46.8°N (the latitude of the Oregon/Washington border), hit the region.

What's going on? Well, record-warm sea surface temperatures near the Hawaiian Islands the past two years have helped fuel highly unusual tropical cyclone activity in the waters surrounding the islands. The warmest water temperatures surrounding Hawaii are usually just below the threshold where a tropical cyclone can form and maintain itself, 26°C (79°F.) Water temperatures in these waters have been 27 - 28°C in much of the summer of 2014 and 2015, and that extra bump in temperature has pushed the Central Pacific past a threshold which allows more tropical storms and hurricane to form. (Note: it is more complicated that just a simple temperature influence, since low wind shear is also needed to increase tropical cyclone activity; however, the increased ocean temperatures have helped create a feedback in which wind shear has been lower over the Central Pacific in 2015.) In 2014, a record three hurricanes--Iselle, Julio, and Ana--came within 300 miles of the Hawaiian Islands, and two of these storms--Julio and Ana--tracked far to the north of the islands, where very few tropical cyclones have ever been observed. In September 2015, Hurricane Ignacio joined the club, penetrating to 32.4°N latitude as a hurricane, and 34.8°N as a tropical storm. Hurricane Oho maintained Category 2 strength on Thursday over a region of ocean well to the northeast of Hawaii where no hurricane that strong had ever been observed (Figure 2.) Since 1949, just ten Central Pacific tropical cyclones have pushed north of 34°N, and four of those storms occurred in 2014 and 2015--and latest model runs point to the possibility of a fifth storm joining the club, tropical disturbance Invest 95E, which is organizing in the waters to the east of Hawaii today.


Figure 2. Tracks of the ten Central Pacific tropical depressions, tropical storms, and hurricanes since 1949 to make it far to the north of the Hawaiian Islands (at least 34°N latitude.) An unusual number of these storms have occurred in the past two years--four out of the ten storms. Another one--currently dubbed Invest 95E--could join the club next week. Image credit: NOAA.

Next member of the Far-North-of-Hawaii Club: Eastern Pacific disturbance 95E?
The far-north of Hawaii hurricane club may get a new member next week: an area of disturbed weather in the Eastern Pacific located about 1350 miles southwest of the southern tip of the Baja California Peninsula (Invest 95E) that has the potential to develop into a tropical depression this weekend. Satellite loops show 95E has a modest amount of spin and heavy thunderstorms, and is slowly growing more organized. Our top three models for predicting tropical cyclone genesis all develop the system, and in their 8 am EDT Thursday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day odds of development of 40% and 70%, respectively. Models show 95E taking a west to west-northwest track over the next week, then turning northwards and missing Hawaii by at least 500 miles late next week.

Climate change may increase the number of Hawaiian hurricanes
The amazingly active Hawaiian hurricane seasons of 2014 and 2015 could well be a harbinger of the future, as I discussed in a 2014 blog post, Climate Change May Increase the Number of Hawaiian Hurricanes. A 2013 modeling study published in Nature Climate Change, "Projected increase in tropical cyclones near Hawaii", found that global warming is expected to increase the incidence of tropical storms and hurricanes in Hawaii. Lead author Hiroyuki Murakami, from the International Pacific Research Center at the University of Hawaii at Manoa, commented in a press release accompanying the paper: "Historically, only every four years on average did a tropical cyclone come near Hawaii. Our projections for the end of this century show a two-to-three-fold increase for this region."

Joaquin dies
Hurricane Joaquin is no more. The storm has become an extratropical storm over the cold waters of the north central Atlantic, ending its ten-day rampage. At its peak at 00 UTC October 2, Joaquin stood just below Category 5 status, with 155 mph winds and a central pressure of 931 mb. By the time Joaquin's remnant reaches Portugal on Saturday, the ex-hurricane should have top winds of about 35 mph.

Jeff Masters