Abstract The synoptic-scale weather systems significantly influenced the advance of the summer monsoon over eastern India in June 2025. In this paper, a case of heavy-to-extremely heavy rainfall spells that occurred from June 18 to 21, 2025, over eastern India is studied using the India Meteorological Department's (IMD) weather bulletins and ERA5 reanalysis fields. Analysis showed that a pre-existing upper air cyclonic circulation was present over southern Bangladesh and the adjoining north Bay of Bengal (BoB) on 14 June, which deepened vertically, leading to the formation of a Low Pressure System (LPS) on 17 June. It intensified further into a Well-Marked LPS (WML) over Gangetic West Bengal by June 18. It migrated slowly west-northwestward across Jharkhand and Bihar, reaching southeast Uttar Pradesh by June 21, enabling prolonged moisture incursion from the BoB and enhancing the positive lower-level convergence over the region, which ultimately aided in the development of deep convection, coinciding with the monsoon advance phase over eastern India. Beyond synoptic-scale features, dynamic fields showed strengthened low-level convergence, upper-level divergence, significant cyclonic vorticity, and pronounced negative omega, indicating strong vertical motion. Thermodynamic indices also revealed high instability and abundant moisture, further favoring deep convection. A comparison with recent June events (2020–2025) reveals that weak-to-moderate LPSs with slow movement can produce extreme rainfall even without strong dynamical forcing, thus emphasizing the role of system persistence. This case study demonstrates that the vertical structure, intensity, and movement of monsoon LPSs strongly influence early-season rainfall distribution and intensity. The results are relevant at the process level and for forecasting, and they emphasize the role of LPS kinematics in the early stages of the summer monsoon. Research highlights <UnorderedList Mark="Bullet"> <ItemContent> A slow-moving monsoon LPS leads to widespread heavy to extremely heavy rainfall during the June 2025 monsoon advance over eastern India. </ItemContent> <ItemContent> The event was controlled primarily by system persistence (7-day life cycle and ~6.4 km h−1 translation speed), demonstrating that the rainfall effect is more sensitive to residence time than peak intensity. </ItemContent> <ItemContent> Vertical moisture flux convergence and lower-tropospheric vorticity showed the strongest relationship with rainfall variability, validating dynamical-moisture coupling as the dominant rainfall driver. </ItemContent> <ItemContent> Comparative analysis (2020–2025) identifies three pathways to June extreme rainfall, with the 2025 event falling under the category of slow weak-to-moderate LPS + thermodynamic support, emphasizing the importance of translation speed as a key forecasting predictor. </ItemContent> </UnorderedList>

Influence of synoptic-scale systems on heavy rainfall during the 2025 Southwest monsoon advance over Eastern India

期刊论文
发表日期：

2026年7月8日

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Rachna Sharma,
Shashi Kant,
Vivek Kumar

摘要

Abstract

The synoptic-scale weather systems significantly influenced the advance of the summer monsoon over eastern India in June 2025. In this paper, a case of heavy-to-extremely heavy rainfall spells that occurred from June 18 to 21, 2025, over eastern India is studied using the India Meteorological Department's (IMD) weather bulletins and ERA5 reanalysis fields. Analysis showed that a pre-existing upper air cyclonic circulation was present over southern Bangladesh and the adjoining north Bay of Bengal (BoB) on 14 June, which deepened vertically, leading to the formation of a Low Pressure System (LPS) on 17 June. It intensified further into a Well-Marked LPS (WML) over Gangetic West Bengal by June 18. It migrated slowly west-northwestward across Jharkhand and Bihar, reaching southeast Uttar Pradesh by June 21, enabling prolonged moisture incursion from the BoB and enhancing the positive lower-level convergence over the region, which ultimately aided in the development of deep convection, coinciding with the monsoon advance phase over eastern India. Beyond synoptic-scale features, dynamic fields showed strengthened low-level convergence, upper-level divergence, significant cyclonic vorticity, and pronounced negative omega, indicating strong vertical motion. Thermodynamic indices also revealed high instability and abundant moisture, further favoring deep convection. A comparison with recent June events (2020–2025) reveals that weak-to-moderate LPSs with slow movement can produce extreme rainfall even without strong dynamical forcing, thus emphasizing the role of system persistence. This case study demonstrates that the vertical structure, intensity, and movement of monsoon LPSs strongly influence early-season rainfall distribution and intensity. The results are relevant at the process level and for forecasting, and they emphasize the role of LPS kinematics in the early stages of the summer monsoon.

Research highlights

A slow-moving monsoon LPS leads to widespread heavy to extremely heavy rainfall during the June 2025 monsoon advance over eastern India.

The event was controlled primarily by system persistence (7-day life cycle and ~6.4 km h⁻¹ translation speed), demonstrating that the rainfall effect is more sensitive to residence time than peak intensity.

Vertical moisture flux convergence and lower-tropospheric vorticity showed the strongest relationship with rainfall variability, validating dynamical-moisture coupling as the dominant rainfall driver.

Comparative analysis (2020–2025) identifies three pathways to June extreme rainfall, with the 2025 event falling under the category of slow weak-to-moderate LPS + thermodynamic support, emphasizing the importance of translation speed as a key forecasting predictor.