Meteorologist Explains Interaction Between Cyclone Pam, Bavi

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South Pacific cyclone helped create tropical storm in the north

By Jasmine Stole

HAGÅTÑA, Guam (Marianas Variety, March 18, 2015) – Had it not been for Cyclone Pam, Guam and the Mariana Islands might not have ever experienced Tropical Storm Bavi, according to meteorologists.

"If there were no Pam there’d have been no Bavi," said Derek Williams, a meteorologist with the National Weather Service operating out of Tiyan.

Cyclone Pam formed in the South Pacific first, followed by Bavi forming in the Northern Pacific basin.. "It’s a complex interaction. In some instances you can say Pam actually started Bavi but in others it inhibited the intensification Bavi. So it was a really complex flow pattern," Williams said.

Williams said Cyclone Pam and Tropical Storm Bavi are called "twin cyclones."

"It was what we call twin cyclones. There was a cyclone in the southern hemisphere and a twin cyclone in the northern hemisphere, which would’ve been Bavi," he said.

Bavi in the northern hemisphere rotated counterclockwise and Pam, in the southern hemisphere, rotated clockwise. Bavi and Pam were two of four weather systems in the Pacific region that developed around the same time. Tropical cyclones Olwyn and Nathan were also active, near Australia, at the same time Bavi and Pam were progressing across the Pacific.

With twin cyclones, the two weather systems are not always equal in intensity, Williams said. The environment surrounding the Mariana Islands was too hostile for Tropical Storm Bavi to develop, in part because of Pam.

"In this case, Bavi was weaker for a lot of reasons. Pam intensified quicker. It’s the southern hemisphere’s summer, so (Pam) had warmer water to deal with, so it just intensified more rapidly," he said.

Bavi, on the other hand, developed during Guam’s "dry season," an unusual time for storms to develop in this area. Williams said it’s essentially winter in the northern hemisphere now, so water temperatures are a few degrees cooler.

Another factor that inhibited Bavi from getting stronger is the cooler ocean temperatures, since storms need warmer waters to intensify.

"We’re pretty fortunate in that respect," Williams said. As of 1 p.m. yesterday, Bavi was 610 miles west-northwest of Guam and sustained winds had decreased to 40 mph. The NWS bulletin issued yesterday said it could be downgraded to a tropical depression and is expected to weaken over the next day.

Tropical Storm Bavi was forecast to pass north of Guam but late Sunday night, the system split and passed directly over Guam while another part of it passed over Saipan and Tinian.

Guam experienced wind gusts of about 45 mph, while some parts of Saipan had gusts that reached 72 mph.

A couple of hours after Bavi passed over Saipan and Guam, the weather service posted information about the strange occurrence on Facebook. "The system was looking to move just north of Rota when it suddenly separated from most of its heavy showers and moved quickly toward and virtually over Guam and out to west of Apra Harbor," the post read. "This was a bit of a surprise."

The separated part of Bavi passed over Guam around 10 p.m. Sunday night, according to the weather service.

Cyclone Pam ripped through the island nation of Vanuatu in the South Pacific over the weekend and is still going strong, heading toward New Zealand. Pam is classified as the strongest tropical cyclone on record. The Weather Underground reported Pam was bringing maximum sustained winds of 135 mph as of yesterday afternoon. Vanuatu is an archipelago located about 1,100 miles east of north Australia. According to the United Nations, at last count yesterday, 24 people died from damaged caused by Pam. The Weather Underground reported that at the height of Cyclone Pam’s strength, it produced winds upwards of 165 mph.

Bavi is a Vietnamese name that refers to a mountain chain in northern Vietnam.

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