MetMalaysia Forecasts Storms and Flash-Flood Risk During October’s Inter-Monsoon Phase

Malaysia is expected to enter its inter-monsoon phase on Oct. 9, bringing a higher risk of afternoon thunderstorms, flash floods and sudden strong winds as the southwest monsoon ends, the Malaysian Meteorological Department said.

The transition is forecast to continue until the end of October. MetMalaysia Director-General Dr. Mohd Hisham Mohd Anip said weak winds blowing from different directions would encourage thunderstorm clouds to develop, according to an Oct. 6 report by Bernama.

Heavy rain, lightning and brief bursts of strong wind are most likely in the afternoon and early evening across much of the western and interior parts of Peninsular Malaysia, as well as western Sabah and Sarawak. The department identified flash floods, sudden surges of water, fallen trees and damage to unstable structures among the possible consequences.

The announcement marks the end of the southwest monsoon that began May 14. MetMalaysia said changing wind directions were also expected to gradually reduce the influence of transboundary haze from neighbouring countries. It urged the public to follow forecasts and warnings through its official website, the myCuaca mobile application and its official social media accounts.

Local Storms and the Regional Outlook

The immediate thunderstorm risk sits alongside a broader outlook for relatively dry conditions in parts of Southeast Asia. The ASEAN Specialised Meteorological Centre's forecast, updated Oct. 5, predicted below-normal October rainfall across much of the Maritime Continent, with the strongest likelihood in its southern half.

For October through December, the centre expected an increased probability of below-normal rainfall across much of the same region, while identifying exceptions in the northwest and parts of western Borneo. It also forecast above-normal temperatures over most of Southeast Asia and said El Nino conditions were present and expected to strengthen in October and November.

The centre warned that the regional hotspot and smoke-haze situation could remain elevated under dry conditions. More showers as the region moves into the inter-monsoon period toward the end of October could begin to ease it.

These forecasts operate at different scales. A seasonal assessment describes the likely average conditions over a broad area and extended period. It does not establish whether a particular district will receive a dangerous downpour on a particular afternoon. ASMC specifically directs readers to national meteorological services for country-level updates.

Why Short Warnings Matter

MetMalaysia's thunderstorm warning criteria explain how the department handles the more immediate threat. Warnings are issued when observations and forecasting tools show thunderstorms with rainfall intensity exceeding 20 millimetres an hour occurring or expected to occur.

Each warning is valid for no more than six hours. If conditions are expected to deteriorate and last beyond that period, the warning can be upgraded to the continuous-rain category. That distinction gives residents a reason to check the latest issuance time and affected areas instead of relying only on a monthly outlook.

In its weather education guidance, the department explains that warm, moist air rising from the surface can build into thunderstorm clouds. It says thunderstorms are common in Malaysia throughout the year, often in the afternoon and around dusk, and become more frequent during inter-monsoon periods.

The department advises people to check forecasts before spending extended periods outdoors and postpone outdoor activities when thunderstorms are expected. During a storm, it recommends sheltering in a substantial building or a car, avoiding isolated trees and small shelters, and leaving boats and staying away from water.

Its flood precautions include avoiding flooded roads and following instructions from local disaster-management authorities. For people arranging outdoor work, travel or events this month, the practical information is therefore the latest local forecast and short-term warning, alongside the broader seasonal outlook.

The Oct. 9 transition date is a forecast for the start of a weather phase, rather than a prediction that every location will experience a storm that day. The department's warning areas and validity periods will provide the more specific information as conditions develop.

File photo: Vyacheslav Argenberg via Wikimedia Commons, licensed under CC BY 4.0.