The Northeast and Southwest Monsoons: Why the East Coast Floods Every Year

Every November, before checking the news, many residents of Kota Bharu, Kuala Terengganu and Kuantan start checking the water levels of rivers near their homes. This is not a habit born of unexpected disaster — it is a predictable schedule that repeats every year, and its cause is the same seasonal wind system that also determines when the west coast of the Peninsula turns dry.

Malaysia sits in the path of two monsoons that switch direction with the seasons: the Northeast Monsoon and the Southwest Monsoon. Both are studied and officially reported by the Malaysian Meteorological Department (MetMalaysia). What’s interesting isn’t just when the rain falls — it’s why the country’s geography causes one coast to be drenched while the other stays dry at the very same time.

Summary of the two monsoons

	Northeast Monsoon	Southwest Monsoon
Duration	November to March	May to September
Dominant wind direction	Northeast, across the South China Sea	Southwest
Rainfall character	Heavy and sustained in exposed areas	Relatively dry across the country except Sabah
Most affected areas	Peninsula east coast, western Sarawak, northeastern Sabah	Northwestern Sabah and Sarawak (winds can reach 20 knots or more)
Peak rain months (east coast)	November–January	No major peak
Related phenomenon	Cold air flow from Siberia can accompany the northeast winds	”Sumatra” — a squall line that crosses the Melaka Strait to the west coast

Source: MetMalaysia, “Iklim Malaysia” and “Malaysia’s Climate”.

How the Northeast Monsoon forms

The Northeast Monsoon occurs when high air pressure builds up over the cold Asian landmass, driving winds to blow consistently from the northeast across the warmer South China Sea. According to MetMalaysia, these winds sometimes also carry a cold air flow from Siberia that can lower temperatures on the east coast.

Winds passing over this warm sea absorb large quantities of water vapour. When they meet the landmass of Peninsular Malaysia, that water vapour turns into rain — and the areas most exposed to the path of these winds receive the greatest amount of rainfall. MetMalaysia records that both the Peninsula’s east coast and western Sarawak experience maximum rainfall in the same month, January.

Why the east coast, not the west coast

The answer lies in the map, not the wind alone. The Peninsula’s east coast faces directly onto the South China Sea — nothing blocks the moist winds of the Northeast Monsoon before they reach the shores of Kelantan, Terengganu, Pahang and Johor.

The west coast, meanwhile, is shielded by the Titiwangsa Range, which splits the Peninsula from north to south. This range acts as a barrier: moist air from the northeast is forced to rise, dropping most of its rain on the eastern slopes of the range before reaching the western side. That’s why areas like Kelantan, Terengganu and Pahang — as well as the eastern part of Johor — are consistently named in official announcements as high-risk areas every monsoon season, while Perak, Selangor or Melaka are rarely mentioned in the same context.

The scale of flooding: what the official figures say

This risk is not just a general impression. According to the Department of Irrigation and Drainage (JPS), flood-prone areas across Malaysia are estimated to cover approximately 29,000 square kilometres — equivalent to 9% of the country’s land area — and involve more than 2.7 million people, or 18% of the country’s total population.

For the 2024/2025 season, the National Security Council (MKN) announced that the Northeast Monsoon began on 5 November 2024 and was expected to continue until March 2025, with 5 to 7 episodes of heavy rain forecast over that period. What is less widely known is that the affected areas also change according to the phase of the season itself:

• Early phase (November–December): Kelantan, Terengganu, Pahang, Johor, Sarawak and Sabah
• Late phase (January–February): Pahang, Johor, Sarawak and Sabah — Kelantan and Terengganu are no longer listed as highest-risk in this phase

MKN also warned that this heavy rain “could cause flooding in low-lying areas and river basins,” alongside the risk of rough seas, coastal erosion and high waves in the South China Sea due to strong winds and rough seas.

The Southwest Monsoon: an uneven dry season

If the Northeast Monsoon is the flood season, the Southwest Monsoon is its opposite — but not evenly so across the country. MetMalaysia describes this period as relatively drier for the whole country except Sabah, because lower air humidity reduces the formation of rain clouds in most areas.

The major exception is northwestern Sabah and Sarawak, where the southwest winds can become stronger and reach 20 knots or more. The Peninsula’s west coast, meanwhile, has its own distinctive phenomenon known as the “Sumatra” — a line of thunderstorms that forms when Southwest Monsoon winds cross the mountain ranges of Sumatra, then gather additional moisture from the warm Melaka Strait before moving toward the Peninsula’s west coast, usually in the early morning.

Transition phases: afternoon storms and floods with a different cause

Between the two main monsoons are two shorter transition phases — April–May and October–November — characterised by weak, variable winds and clear morning skies. These conditions encourage the formation of convective thunderstorms in the afternoon, and according to MetMalaysia, it is these transition phases — not the Northeast Monsoon — that contribute the highest monthly rainfall in the Peninsula’s west coast areas.

This also explains why flooding on the west coast cannot always be linked to the Northeast Monsoon. In November 2025, the Director-General of MetMalaysia, Dr. Mohd Hisham Mohd Anip, explained via RTM that flooding in west coast states such as Perak, Selangor and Perlis at that time stemmed from a separate low-pressure system — not the Northeast Monsoon that was hitting the east coast at the same time. That low-pressure system was detected near southern Thailand, moving into the South China Sea, then toward Sumatra, before potentially turning back toward Malaysian waters — a movement pattern entirely different from the consistent northeasterly wind flow of the Northeast Monsoon.

Common mistakes

“Monsoon season” means heavy rain evenly across the whole country. In reality the effect is very uneven — the Peninsula’s east coast, western Sarawak and northeastern Sabah bear the majority of the impact, while inland areas and those shielded by mountain ranges are far less affected.

The west coast never floods during monsoon season. The west coast does flood less often as a direct result of the Northeast Monsoon, but it can still be hit by flooding from a separate low-pressure system or transition-phase thunderstorms — a different mechanism from the one that strikes the east coast.

The Northeast Monsoon is always equally severe throughout Nov–Mar. Official MKN data shows the highest-risk areas change between the early and late phases of the same season; Kelantan and Terengganu are most affected in Nov–Dec, while the Jan–Feb phase is more concentrated in Pahang, Johor, Sarawak and Sabah.

All floods in Malaysia are “monsoon floods.” Flash floods caused by inadequate urban drainage systems are a separate phenomenon and can occur at any time regardless of the monsoon.

What’s next

For current weather forecasts and heavy rain warnings, refer to the official MetMalaysia website or the myCuaca app. For the latest flood status and river water levels, JPS operates the Public InfoBanjir portal.

For context on the states most frequently mentioned in Northeast Monsoon announcements, see the profiles for Kelantan, Terengganu, Pahang and Johor.

This article remains a draft until a human reviewer confirms it.