The West Pacific Ocean became quite active last weekend, with four concurrent storms. This tropical activity was triggered by an active Madden-Julian Oscillation (MJO) phase coupled with the seasonal positioning of the monsoon trough. There have been seven named storms since July 1, the most recent being Typhoon Nesat.
Earlier this month, Tropical Storm Nanmadol formed northeast of the Philippines and within 48 hours made landfall along the west coast of the Japanese island of Kyushu. Wilkens Weather meteorologists closely monitored this storm for customers operating in the East China Sea and commenced storm advisories 42 hours in advance of the Joint Typhoon Warning Center (JTWC).
Marine Meteorologist Bryce Link explained his decision to initiate advisories at 09UTC on June 30th, “Deep thunderstorms were centered around a developing surface low and the system was in a fantastic environment”. Bryce pulled the trigger on this storm when operational models such as the GFS and ECMWF generally showed no signs of tropical development. ECMWF ensembles showed a stronger signal for tropical development, but the operational signaled nothing at the time the first WWT advisory was issued.
Sea surface temperatures (SSTs) have been on the rise since Nanmadol tracked across the East China Sea in early July. Early in the month SSTs were in the mid-80s across a small portion of the East China Sea, where Nanmadol tracked, and generally anywhere south and east of Taiwan. The graphic above (from July 24th) shows that SSTs are now in the upper 80s to 90°F (31-32°C) across much of the same region. These above average SSTs and favorable ocean heat content have created a conducive environment for tropical development. In fact, the last two tropical cyclones to form, Roke and Nesat, developed in this very warm area near the Philippines.
Currently, Typhoon Nesat is flaring convection northeast of Luzon. Further intensification is expected prior to landfall over Central Taiwan. Nesat formed along the monsoon trough, which is active from the Gulf of Thailand through the central Philippines as depicted in the 850mb wind graphic below. Tropical cyclones will not form in the area of enhanced westerly flow due to the presence of moderate to high vertical wind shear. Nesat was the second typhoon to develop in 2017.
Despite convective and tropical cyclone activity in this region, the first typhoon of 2017 (Noru) developed at a higher latitude (near 28°N). Noru became a tropical depression over 1,000nm east-southeast of Yokosuka, Japan on July 20th. Early on, Noru tracked westward in response to two competing ridges to the south and northeast. Then, the cyclone’s movement slowed and the steering flow pulled Tropical Storm Noru and Tropical Storm Kulap around a central point. Noru became a typhoon on July 24th and the two cyclones “danced” cyclonically around each other the following day, a phenomenon known as the Fujiwhara Effect. The end result was Tropical Storm Kulap tracking across cooler waters and dissipating within 24 hours and Noru maintaining typhoon strength southeast of Japan. Wilkens Weather meteorologists forecast Typhoon Noru to persist over the Northwest Pacific Ocean into early August.
Author’s note: This blog was updated July 28th to reflect the latest tropical information.