| Above: Himawari-8 infrared image of Typhoon Wutip taken at 11:40 am EST Thursday, February 21, 2019. At the time, Wutip was a Category 2 storm with 100 mph winds headed northwest at 8 mph on a track that would take it 150 miles southwest of Guam on Saturday. Image credit: NOAA/RAMMB. |
Typhoon season is off to an early start in the Northwest Pacific, where Category 2 Typhoon Wutip is gathering strength in the waters to the southeast of Guam. Wutip is expected to pass 150 miles to the southwest of Guam as a Category 3 typhoon on Saturday night local time, bringing tropical storm conditions to Guam and Rota in the Mariana Islands.
The 10 am EST Thursday advisory from the Joint Typhoon Warning Center put Wutip’s top winds at 100 mph, making it a low-end Category 2 storm. With wind shear a moderate 15 – 20 knots and sea surface temperatures a warm 29°C (84°F), expect intensification into a Category 3 typhoon with 120 mph winds by 12Z Saturday (10 pm local time), when Wutip will be making its closest pass to Guam—about 150 miles to the island’s southwest. By Sunday, when Wutip will be swinging to the north-northwest, passing about 200 miles to the west of the Mariana Islands, the storm will experience increasing wind shear and decreasing SSTs, which should cause slow weakening.
A Tropical Storm Watch is up for Guam, which is likely to experience a twelve-hour period of sustained winds of tropical storm-force beginning near 0Z February 23 (10 am Saturday local time). Storm surge inundation of 2 – 4 feet is possible, with waves at sea potentially exceeding 20 feet on Saturday and Saturday night. Heavy rains of 3 – 6 inches will pose a flood threat, and a Flash Flood Watch is posted for the islands of Guam and Rota. Guam is used to strong typhoons, and the close brush by Wutip should not cause a large amount of damage.
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Figure 1. Crowds watch huge waves from Tropical Cyclone Oma at Snapper Rocks on February 21, 2019 in Gold Coast, Australia. Image credit: Chris Hyde/Getty Images. |
Tropical Storm Oma weakening off the east coast of Australia
Earth’s only other active tropical cyclone, Tropical Storm Oma, was located on Thursday morning about 500 miles east of the Australian coast, on the other side of the equator from Wutip. Oma was headed south at about 8 mph, parallel to the coast. Oma peaked as a Category 1 storm with 85 mph winds on Tuesday, and its winds had dropped to just 50 mph by Thursday morning as the storm encountered marginal SSTs near 26°C (79°F). Oma is expected to dissipate well offshore from the Australian coast this weekend, when high wind shear in combination with cool SSTs will create unfavorable conditions.
Oma is predicted to bring gale-force winds to the northeast coast of Australia on Friday and Saturday, and the storm’s high waves and surge may bring coastal inundation up to a meter (3.3 feet) above high tide.
A typhoon when Guam least expects it
Of all the states and territories in the United States, Guam has the highest risk of being hit by a typhoon or hurricane, and in fact one of the highest risks of any densely populated area on Earth, as noted in a University of Guam report. Unlike the North Atlantic, the Northwest Pacific has a virtually year-round typhoon season, as the waters of the tropical Northwest Pacific are warm enough to produce tropical cyclones throughout the calendar year.
If you were going to pick a calm period in Guam based on climatology, it would be right about now, though. The main factor suppressing typhoons in late winter is strong wind shear that often extends well into lower latitudes. One study (see PDF) by the U.S. Navy examined 162 tropical cyclones that came within 180 nautical miles (about 205 miles) of Guam between 1945 and 1990. Of those 162 events, just one (a tropical storm) occurred in February, making it the most placid month of the year on average.
El Niño and the twin cyclones
Twin cyclones straddling the equator, like we are seeing today with Wutip and Oma, typically occur two or three times a year, most often in the Indian Ocean. In the western Pacific, they’re a bit more likely to form during El Niño events, because the usual east-to-west trade winds tend to be weaker across the equatorial Pacific during El Niño.
Occasionally the trades even switch direction, in assocation with westerly wind bursts (WWBs) that can propagate along the equator. These winds lead to enhanced spin on both sides of the equator, as they help to foster counterclockwise rotation in the Northern Hemisphere and clockwise rotation in the Southern Hemisphere. If twin cyclones do develop, their circulations can feed back into the very WWB that helped them to form, until the cyclones migrate to higher latitudes.
A strong westerly wind burst developed in mid-February near the International Date Line, hand in hand with the gradual development of Wutip and Oma. Mike Ventrice (The Weather Company) analyzed the situation in a series of tweets on Wednesday.
Remember my equatorial Rossby wave tweet about the tropical W Pacific? Well we now how twin tropical cyclones on both sides of the equator. The N. Hemisphere TC is Tropical Storm Wutip (on its way to becoming the first Typhoon of 2019). The S. Hemisphere TC is Tropical Storm Oma. pic.twitter.com/UWtupIVlei
— Michael Ventrice (@MJVentrice) February 20, 2019
Twin cyclones Pam and Bavi developed in the western Pacific in March 2015 (see the Tom Di Liberto writeup at climate.gov) following a winter of marginal El Niño conditions similar to what’s been observed in the last few months, as confirmed in February by NOAA’s El Niño Advisory. The year 2015 went on to produce the blockbuster El Niño of 2015–16. The latest consensus forecast from NOAA and the International Research Institute for Climate and Society (IRI) calls for a less-than-50% chance of El Niño conditions extending into summer and autumn 2019, but El Niño forecasts are notoriously difficult across the “spring predictability barrier".
A westerly wind burst can feed into a slow-moving, large-scale oceanic pulse called a downwelling Kelvin wave that can enhance El Niño conditions. Such a wave is now entering the eastern Pacific, and as a result, Ventrice said he expects the Niño3.4 value (the most common index of El Niño strength) to increase over the next month or so. “I think the atmosphere and ocean are aligned for an El Nino boost,” he told me. However, he cautioned, “I'd be very surprised if we had another Super El Nino birth here.”
See the article by Jonathan Belles at weather.com for more on Wutip and Oma and the mechanisms behind them.
Bob Henson co-wrote this post.
