Quick answer
The southwest monsoon usually starts in Sri Lanka in the second half of May, often in the third or fourth week, bringing rain to the west and south coasts and the western hills. The northeast monsoon usually becomes established in November or early December, bringing rain to the east and north, after the second inter-monsoon storms of October. Both dates can vary by a week or two from year to year.
Key takeaways
- The southwest monsoon usually arrives in the second half of May, often in the third or fourth week.
- The northeast monsoon builds gradually in November and is usually established by early December.
- Onset dates vary by roughly one to two weeks from year to year.
- The southwest onset is usually sudden, with wind, swell and heavy rain; the northeast onset is more gradual.
- Trips at the edge of a season should keep the beach at the dry end of the route.
The southwest monsoon onset: May
The southwest monsoon usually reaches Sri Lanka in the second half of May, a little before it reaches the Kerala coast of India, where its arrival is officially tracked and usually falls around 1 June. In most years, the onset over the island falls between about 15 and 31 May, with the third and fourth weeks most typical.
The arrival is usually unmistakable. Over two or three days, the wind swings to the southwest and strengthens, cloud thickens over the west coast, heavy rain sets in, and the sea on the south and west coasts rises into big, messy swell. Beaches put up red flags and whale-watching boats from Mirissa stop.
In some years the monsoon is early, arriving in mid-May, and in others late, into the first days of June. A late or weak onset can leave the southwest with April-like weather for longer.
- →Usual onset: second half of May.
- →Earliest typical: around mid-May.
- →Latest typical: first days of June.
The northeast monsoon onset: November to December
The northeast monsoon has a less clear start. Through October and November, the island is in the second inter-monsoon, with storms everywhere. The northeast wind starts to blow in November, usually strengthening through the month, and meteorologists generally treat the monsoon as established by late November or early December.
For the east coast, the practical onset is when the rain becomes persistent and the sea turns rough, typically in November. For the southwest coast, the practical change is the opposite: as the northeast monsoon settles, the southwest dries out, usually by the second half of November or early December.
- →Northeast wind begins: November.
- →Monsoon established: late November to early December.
- →Southwest dries out: second half of November to early December.
Signs that a monsoon is arriving
The signs are worth knowing for travellers already on the island. Before the southwest onset in May, the afternoon storms of the first inter-monsoon become longer, the wind starts to blow steadily from the west, and the swell on the south coast grows. The Department of Meteorology usually announces the onset once the pattern is established.
Before the northeast monsoon, the wind on the east coast swings to the northeast, the sea at Trincomalee and Pasikudah becomes choppy, and the rain there changes from evening storms to longer periods of cloud and drizzle.
- →Steady wind from a new direction.
- →Rising swell on the coast facing the wind.
- →Longer rain spells rather than short storms.
How much it varies
The variation is a matter of a week or two, not months. The monsoon is one of the most reliable weather systems in the world, but its exact arrival depends on sea temperatures, the strength of the Asian heat low and larger patterns such as El Nino and the Indian Ocean Dipole.
That variation matters most for trips booked at the very edge of a season. A trip to Mirissa in the last week of May has a fair chance of catching the onset; a trip in the first week of May usually does not. Likewise, a south coast stay in mid-November is less certain than one in mid-December.
Because the onset is forecast only a week or so ahead with confidence, flexible bookings at the edges of seasons are worth the extra cost.
Planning around the onset
The simplest rule is to put the beach at the dry end of the route. In May, finish on the east coast rather than the south. In November, finish on the south coast rather than starting there. Keep at least one flexible night near the change of season.
Lankan Stays & Trails plans trips at the edges of the monsoons with the beach at the dry end of the route and checks the onset forecast in the week before travel.
Frequently asked questions
When does the monsoon start in Sri Lanka?
The southwest monsoon usually starts in the second half of May, often in the third or fourth week, on the west and south coasts. The northeast monsoon builds through November and is usually established by early December on the east and north coasts. Both dates vary by a week or two.
When does the rainy season start in Colombo?
Colombo's main rainy season starts with the southwest monsoon, usually in the second half of May. Colombo also gets heavy inter-monsoon storms in April, October and November. Its driest months are usually January and February.
When does the rainy season start on the east coast of Sri Lanka?
The east coast's rainy season usually starts in November, as the northeast monsoon begins, though storms can arrive in late October. The heaviest rain is usually in November and December, easing through January. The sea there often stays rough until March.
Can the monsoon arrive early in Sri Lanka?
Yes. In some years the southwest monsoon reaches Sri Lanka in mid-May, and in others it is delayed into the first days of June. The variation is usually a week or two. Watch the Department of Meteorology forecasts in the week before a trip that falls near the edge of the season.
How do you know the monsoon has arrived?
The wind swings to a steady direction, from the southwest in May or the northeast in November, the swell rises on the coast facing the wind, and rain changes from short storms to longer, more widespread spells. The Department of Meteorology usually announces the onset once it is established.
Is late May a good time to visit the south coast?
It is uncertain. The southwest monsoon often arrives in the third or fourth week of May, bringing heavy rain and rough seas to the south coast. Early May is usually more settled. For beach time in late May, the east coast is a safer choice.
How does Lankan Stays & Trails plan around the monsoon start?
Trips that fall near an onset date are planned with the beach at the dry end of the route: the east coast in late May, the south coast in late November. Flexible nights are kept near the change of season, and forecasts are checked before travel.