Hurricane Olivia Could Angle Toward Hawaii Next Week

After a burst of intensification on Tuesday took it to Category 3 status, Hurricane Olivia settled down a bit on Wednesday. As of 5 pm EDT Wednesday, Olivia’s top winds had decreased to 100 mph, making it a Category 2 storm. Olivia is currently far from land—positioned between Mexico and Hawaii, close to 1000 miles west-southwest of Cabo San Lucas—but it is possible that Olivia’s track will bring it near the Hawaiian Islands about a week from now.

Olivia is the ninth hurricane and the sixth major hurricane of this year’s hyperactive Northeast Pacific season. Five other Northeast Pacific seasons have also had at least six major hurricanes by September 4, according to Phil Klotzbach (Coloardo State University). Those are 1978, 1992, 1993, 2014, and 2015. Only two seasons prior to this year—2014 and 2015—managed to produce nine hurricanes by September 3, Klotzbach tweeted.

The 2018 Northeast Pacific (to 180°) #hurricane season has generated its most Accumulated Cyclone Energy through September 5 on record. #Olivia pic.twitter.com/UvW9yyZNj1

— Philip Klotzbach (@philklotzbach) September 5, 2018

The outlook for Olivia

Solid steering currents will keep Olivia moving generally west-northwest, with a subtle bend toward the west by this weekend. Olivia’s burst of intensification on Monday and Tuesday was aided by sea surface temperatures (SSTs) that topped 28°C (82°F), or about 1.5 – 2.0°C above average for the location and time of year. Olivia will soon be encountering cooler waters, with SSTs closer to 25 – 26°C (77° – 79°F) by Friday. That’s still about 1°C warmer than average, but near the minimum threshold for supporting tropical cyclones. Olivia will also be encountering much drier air aloft, with relative humidities dropping from the 50% range to less than 40% by Saturday. Together, these factors will likely take an increasing toll on Olivia’s power, even with vertical wind shear becoming quite low. The HWRF model, one of our most reliable for intensity, brings Olivia below hurricane strength by Saturday, and the NHC forecast follows suit by Sunday.

The GFS and European (ECMWF) global models show the steering currents for Olivia growing more complex after this weekend. A strong surface and upper-level low is predicted to form this weekend around 40°N and the International Date Line. East of this low, the models agree in predicting a very strong upper-level ridge to develop. Such a ridge could bend Olivia’s track toward the west-southwest and push it across or near the Hawaiian Islands by the middle of next week, as depicted by the 12Z Wednesday runs of the GFS and ECMWF models. There’s plenty of time between now and then to see how these steering features evolve. Even if Olivia does end up taking aim at Hawaii, it would most likely be as a tropical storm or perhaps a Category 1 hurricane, as the SSTs northeast of Hawaii would be marginal for supporting any increase in strength (unlike the warmer SSTs southeast of Hawaii traversed by hurricanes Hector and Lane).

Even stronger than Olivia on Wednesday was Hurricane Norman, back up to Category 3 strength with top sustained winds of 115 mph at 5 pm EDT. Norman was located about 435 miles east of Hilo, heading west at 8 mph. Fortunately, models agree that Norman will turn to the northwest on Thursday, sparing Hawaii from another hurricane scare. Large swells from Norman will batter the islands over the next couple of days.

NOAA’s Gulfstream-IV jet has been sampling the environment around Norman so that computer models will have the best possible handle on the huricane’s track. The G-IV is tentatively scheduled to carry out a similar mission with Olivia starting on Saturday.

Figure 1. Visible-wavelength high-resolution image of Hurricane Florence as of 1935Z (3:35 pm EDT) Wednesday, September 5, 2018. Image credit: tropicaltidbits.com.

Florence rockets to Category 4 strength

One of the most unexpected and striking cases of rapid intensification seen over the open Atlantic in recent years continued to unfold on Wednesday. Hurricane Florence—still just a tropical storm as recently as Tuesday morning, with uncertain prospects for strengthening—became a Category 4 hurricane at 5 pm EDT Wednesday, with top sustained winds howling at 130 mph. Florence has managed to fend off wind shear and make the most of marginally warm water, perplexing forecasters.

Offering any outlook for Florence seems like a fraught task at this point, but it’s safe to say Florence will remain over the open Atlantic for the next few days, heading generally west-northwest to northwest. The latest NHC forecast is for Florence to be a Category 3 hurricane on Sunday night, when it will be within a couple of days of Bermuda. There are still huge unresolved questions about Florence’s possible track next week, including whether or not Florence might approach the U.S. East Coast.

See today’s post from Jeff Masters for an in-depth discussion of Florence’s future. Jeff also has a separate post on a new disturbance over the eastern tropical Atlantic, which could threaten the Caribbean next week, and yet another system that may emerge from Africa on Friday.

A side note: Both Florence and Norman (formerly a Category 4 storm) are among the furthest north Category 4 storms on record at their respective longitudes (see embedded tweet below). A 2014 paper in Nature led by James Kossin (NCEI/CIMMS) found "a pronounced poleward migration in the average latitude at which tropical cyclones have achieved their lifetime-maximum intensity over the past 30 years." These trends may be related to reduced wind shear as well as warmer SSTs and greater atmospheric instability. Jeff Masters discussed this paper in a Cat 6 post.

I dug up a bit more into the HURDAT2 dataset, plotting max wind speed in each 1º bin (position & wind were interpolated in between 6-hr time steps). Both Florence & Norman are near the northern historic edges of their respective categories. pic.twitter.com/129A7485jA

— Tomer Burg (@burgwx) September 5, 2018