Powerful Typhoon Sinlaku strikes the Northern Marianas

After rapidly intensifying over the western Pacific, Super Typhoon Sinlaku bore down on the Northern Marianas with the US Joint Typhoon Warning Center (JTWC) reporting maximum sustained winds of around 175 mph before eventually striking the islands of Saipan and Tinian as a Category 4 typhoon.

Sinlaku began in the West Pacific Ocean from a broad westward-moving area of disturbed weather containing abundant convective (thunderstorm) activity that approached Chuuk (formerly known as Truk), Micronesia from the east. This area of disturbed weather was part with a large cyclonic gyre north of the Equator that resulted from anomalous westerly winds near the Equator due to the Madden-Julian Oscillation or MJO, a large-scale tropical feature that enhances convective activity and propagates eastward in the vicinity of the Equator; the cyclonic gyres are favorable for tropical cyclone formation. The persistent thunderstorm activity eventually resulted in the formation of a low-pressure center north of Chuuk, and at 00:00 UTC (10:00 am LST) April 9th JTWC declared the formation of a tropical depression with maximum sustained winds estimated at 30 mph. Over the next day and a half, the system slowly gathered strength, becoming a minimal tropical storm early on the morning of the 10th (local), as the center drifted slowly south then west. The storm moved very little on the 10th, remaining centered roughly 50 miles (80 km) northwest of Chuuk, but continued to strengthen, becoming a minimal Category 1 typhoon on the morning of the 11th.

At this point, Sinlaku took on a generally northward track and steadily gained strength due to favorably warm sea surface temperatures (SSTs) of 28 to 29 oC (~82 to 84 oF) and relatively low vertical wind shear with maximum sustained winds reaching 100 mph by 15:00 UTC on the 11th (1:00 am April 12th LST). Sinlaku then underwent a period of rapid intensification, becoming a super typhoon with maximum sustained winds increasing to 150 mph by 6:00 UTC (4:00 pm LST) on the 12th, before reaching a peak intensity estimated at 175 mph by JTWC at 18:00 UTC on the 12th (4:00 am April 13th LST).

It was during this period of rapid intensification that the GPM Core Observatory flew over the center of Super Typhoon Sinlaku at 12:08 UTC (10:08 pm LST) April 12th, providing a detailed look into the structure and intensity of the precipitation within Sinlaku. Surface rainfall estimates from the GPM Microwave Imager (GMI) and Dual-frequency Precipitation Radar (DPR) show a ring of very intense rain (magenta) surrounding a well-defined eye in the storm’s center. The complete, symmetric and intense eyewall is both reflective of a powerful storm but also shows there is ongoing intense heating occurring near the core, which can sustain or further strengthen the storm’s intensity. Further out from the center, strong bands of heavy rain (red bands) that also contain areas of intense rain spiral counterclockwise around Sinlaku’s center. The intensity of the two most prominent rainbands, northeast and especially southwest of the center, reveal the early stages of a potential eyewall replacement cycle, wherein a second eyewall forms outside the original, weakening the original, before then replacing the original eyewall. This lowers the peak storm intensity while broadening the wind field.

The actively-scanning DPR allows for a 3D depiction of Sinlaku’s precipitation field. Areas shaded in blue show frozen precipitation aloft, mainly in the form of snow but also graupel (rimed snow particles) and frozen drops present in the cores of active thunderstorms. The DPR shows a complete ring of higher tops associated with the intense inner eyewall as well as bands of higher tops associated with the outer rainbands separated by areas of lower tops. This structure again suggests Sinlaku is undergoing or about to undergo an eyewall replacement cycle. At the time of the overpass, Sinlaku’s maximum sustained winds were reported at 170 mph, making it both a super typhoon and Category 5 storm.

Sinlaku reached its peak intensity of 175 mph about 6 hours after the GPM overpass as it continued tracking towards the Northern Marianas. The storm maintained this intensity for about 12 hours, after which, the peak intensity began to drop off due to the effects of an eyewall replacement cycle. Sinlaku then continued on to strike the islands of Tinian and Saipan with the center making landfall over northern Tinian around 14:30 UTC April 14th (00:30 am April 15th LST) with maximum sustained winds estimated at 145 mph by JTWC, equivalent to a Category 4 storm on the US Saffir-Simpson scale. Both islands were impacted by Sinlaku’s inner eyewall with a peak wind gust of 130 mph reported at Saipan International Airport. Guam International Airport, located about 100 nm south-southeast of where the center made landfall, reported a peak wind gust of 84 mph.

After passing through the Northern Marianas, Sinlaku remained a strong typhoon but did slowly weaken due in part to another eyewall replacement cycle before starting to recurve to the north then north-northeast into the mid-latitude westerlies.