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Tropical Cyclone Harold impacts the Solomon Islands and Vanuatu

Tuesday, 7th April 2020

Severe Tropical Cyclone Harold is the strongest cyclone to strike Vanuatu since 2015?s tropical cyclone Pam, which also produced wind gusts of 265km/h sustained and resulted in an estimated $380 million in damage.

The hardest hit parts over the past 24-48 hours have been parts of Pentecost Island near the community of Bunlap. The island has a population of nearly 17,000 and suffered a direct hit from the cyclonic eyewall which featured winds topping 257 km/h, a potentially devastating storm surge from surrounding waters, torrential rainfall and a frequent lightning within the eyewall/core.

The rapid intensification of Severe Tropical Cyclone Harold, Category 5, was partly due to unusually warm sea surface temperatures and a reduction in the amount of wind shear in the atmosphere.

Wind shear is a change of wind speed or direction with height, therefore an abundance of shear can tear apart or disrupt a tropical storm or tropical cyclones cyclonic circulation, while the absence of shear can encourage episodes of significant strengthening. Water temperatures in and around the Solomon Island and Vanuatu are up to 1.5 above average at time of the event.

After exiting Vanuatu, Severe Tropical Cyclone Harold is forecast to remain over mainly open waters of the South Pacific though Tuesday, after which the cyclone could skirt Fiji. Severe Tropical Cyclone Harold is expected to retain a Category 4 status over the next 24 hours, however a downgrade to a Category 3 system is possible by tomorrow night, with a Category 2 storm forecast into Thursday and Friday.

What are the categories and what are the wind speeds associated with Tropical Cyclones?

Category 1 cyclones produce mean winds to 34-47 knots with a central pressure greater than 985hPa. Category 2 cyclones produce mean winds to 48-63 knots with a central pressure between 985-970hPa. Category 3 tropical cyclones produce mean winds between 64-85 knots with a central pressure between 970-945hPa. Category 4 cyclones produce mean winds speeds between 86-107 knots with a central pressure of between 945-910hPa. The most damaging and dangerous of all are Category 5 cyclones which produce mean winds greater than 107 knots with a central pressure less than 910hPa.

As tropical cyclones are 'a ring' of tall persistent thunderstorms in circulation, many lightning strikes were recorded associated with Severe Tropical Cyclone Harold, with the GLD360 detecting more than 23,000 lightning events, 21,470 intercloud pulses and 2,396 cloud-to-ground lightning strikes.In the images provided below, intracloud pulses are denoted with black dots, with the cloud-to-ground lightning stokes recorded with red squares.

Image 1: Vaisala Group GLD360 lighting detection over 24 hours near Vanuatu - April 6th, 2020


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