Abstract

Typhoon Lekima (2019) with its heavy rains and floods is an excellent example of the need to provide the earliest possible warnings of the formation, intensification, and subsequent track before a typhoon makes landfall along a densely populated coast. To demonstrate an opportunity to provide early (10 days in advance) warnings of the threat of Typhoon Lekima, the ensemble models from the European Centre for Medium-Range Weather Forecasts and the National Centers for Environmental Predictions have been used to provide time-to-formation timing and positions along the weighted-mean vector motion track forecasts. In addition, the seven-day intensity forecasts after the formation using a weighted analog intensity prediction technique are provided. A detailed description of one European Center ensemble forecast is provided to describe the methodology for estimating the formation time and generating the intensity forecasts. Validation summary tables of the formation timing and position errors, and the intensity errors versus the Joint Typhoon Warning Center intensities, are presented. The availability of these ensemble forecasts would have been an opportunity to issue alerts/watches/warnings of Lekima even seven days in advance of when Lekima became a Tropical Storm. These ensemble forecasts also represent an opportunity to extend support on the 5–15 day timescale for the decision-making processes of water resource management and hydrological operations.

Highlights

  • The formation of Typhoon (TY) Lekima in the far western North Pacific, its rapid intensification into a Super-Typhoon as it approached Ishigaki-jima and Miyako-jima of the Ryuku Islands, and its landfall on the densely populated East China coast make Lekima an excellent example of the great threat from landfalling tropical cyclones

  • It is essential to know the likely storm category at peak intensity. Will this tropical disturbance only reach tropical depression (TD, 25 kt according to the Joint Typhoon Warning Center) stage and not attain Tropical Storm (TS, 35 kt) stage so that no wind damage or high storm surge is to be expected? Or with the large areas of the western North Pacific with sea-surface temperatures much higher than the minimum 26 ◦ C for tropical cyclone (TC) formation, will a pre-TC circulation in a numerical weather prediction model become a typhoon, and perhaps a Super-Typhoon like Lekima? If so, the most critical question becomes the future track of that pre-TC circulation, but the track uncertainty is important and the global ensemble models provide that track spread information

  • One of the most important results in this study is that the ECEPS consistently predicted the pre-Lekima circulation from as early as seven days in advance of the verifying T2F (35) as a Tropical Storm, and the ECEPS weighted-mean vector motion (WMVM) track forecasts consistently made landfall near Zhejiang and continued poleward along the East China coast

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Summary

Introduction

The formation of Typhoon (TY) Lekima in the far western North Pacific, its rapid intensification into a Super-Typhoon as it approached Ishigaki-jima and Miyako-jima of the Ryuku Islands, and its landfall on the densely populated East China coast make Lekima an excellent example of the great threat from landfalling tropical cyclones. Atmosphere 2020, 11, 1162 deaths in China occurred in the city of Wenzhou when the heavy rainfall during landfall caused a landslide to form a natural dam that led to a rapid pooling of water before the dam collapsed and people could not escape the downstream flood. In such cases as TY Lekima that form relatively close to densely populated islands or coastal areas, it is essential that the formation (either 25 kt or 35 kt depending on the warning center definition) timing and position be known as early as possible. Will this tropical disturbance only reach tropical depression (TD, 25 kt according to the Joint Typhoon Warning Center) stage and not attain Tropical Storm (TS, 35 kt) stage so that no wind damage or high storm surge is to be expected? Or with the large areas of the western North Pacific with sea-surface temperatures much higher than the minimum 26 ◦ C for tropical cyclone (TC) formation, will a pre-TC circulation in a numerical weather prediction model become a typhoon, and perhaps a Super-Typhoon like Lekima? If so, the most critical question becomes the future track of that pre-TC circulation, but the track uncertainty is important and the global ensemble models provide that track spread information

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