EXPLAINER: How a subtropical ridge is steering Typhoon Inday away from Philippines

Typhoon Inday was steered northwestward by a subtropical ridge, or a high-pressure system, keeping it from making landfall in the Philippines, according to the state weather bureau PAGASA on Thursday.

PAGASA Assistant Weather Services Chief Chris Perez said the weather system is expected to continue influencing Inday’s movement over the next few days, with the typhoon forecast to reach southeastern China on Saturday.

“Ang orientation po ng ridge ay pahilagang kanluran at ‘yan po ang sinusundan ng bagyo,” Perez said.

(The ridge is oriented toward the northwest, and the typhoon is following that direction.)

Australia’s Bureau of Meteorology describes a subtropical ridge as a band of high-pressure systems, or anticyclones, that generally bring stable and dry conditions. In Inday’s case, the subtropical ridge acted like a barrier, steering the typhoon—a low-pressure system—northwestward and preventing it from moving westward toward the Philippines.

Inday has weakened from a super typhoon to a typhoon, with maximum sustained winds of 165 km/h near the center and gusts of up to 205 km/h as of 4 p.m. Thursday.

“Ang paghina naman nito ay dulot ng tinatawag na vertical wind shear o pagbabago ng direksyon at lakas ng hangin sa iba’t ibang level ng atmosphere,” Perez said.

(Its weakening was caused by what is known as vertical wind shear, or changes in the direction and speed of winds at different levels of the atmosphere.)

“Kapag lumakas ang wind shear o malaki ang pagbabago sa direksyon ng hangin, maaaring maapektuhan ang sirkulasyon ng hangin ng bagyo at maging dahilan ng paghina nito,” he added.

(When wind shear strengthens, or when there is a significant change in wind direction, it can disrupt the cyclone’s circulation and cause it to weaken.)

Despite remaining offshore, Inday will enhance the Southwest Monsoon (Habagat), bringing rains and strong winds to parts of the country.

Perez advised the public to remain vigilant.

“Dapat po maging handa pa rin tayo at maging updated tungkol sa Typhoon Inday at sa pinaiigting nitong Habagat,” he said.

(We should still remain prepared and stay updated on Typhoon Inday and the enhanced Southwest Monsoon.) — VBL, GMA News