Rain & the Pai River
Where the monsoon stands — the last 30 days against the long-term average and the week ahead — and what heavy rain means for the Pai River.
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Where the monsoon stands — the last 30 days against the long-term average and the week ahead — and what heavy rain means for the Pai River.
Pai sits in a river valley with two faces. From November to April it is dry — the clear cool months, then the hot, smoky burning season. From around May to October the southwest monsoon takes over, heaviest in July and August. Pai's floods come from heavy overnight rain swelling the Pai River. This page shows where the season stands: the last 30 days against the 30-year average for the same dates, and the week ahead.
When the river rises, it rises fast
It is the monsoon — the southwest monsoon. Rain builds from May and is heaviest in July and August. Heavy overnight rain is what floods the Pai River.
| Season | When | |
|---|---|---|
| Cool, dry season | 1 November – 31 January | |
| Hot, smoky season | 1 February – 30 April | |
| Monsoon | 1 May – 31 October | now |
Dates and monthly amounts come from 1991–2020 weather-model averages (ERA5) for this valley; the monsoon's start and end drift by a few weeks each year, and the first real rains can arrive any time from late April.
That is close to typical for these dates.
7 of the next 7 days are forecast to bring 1 mm of rain or more, with about 39 mm in total (typical for this week: 52 mm).
| Day | Rain | Chance of rain |
|---|---|---|
| Sun 21 Jun (today) | 10 mm | 100% |
| Mon 22 Jun | 2 mm | 67% |
| Tue 23 Jun | 7 mm | 100% |
| Wed 24 Jun | 5 mm | 98% |
| Thu 25 Jun | 3 mm | 92% |
| Fri 26 Jun | 7 mm | 90% |
| Sat 27 Jun | 5 mm | 93% |
Route 1095 to Chiang Mai is Pai's one lifeline, and its 762 curves climb through steep, slip-prone hillsides. Heavy monsoon rain brings landslides that can slow or close the road, and the wider Mae Hong Son area saw landslips damage hundreds of homes in 2024. In a wet spell, ask your guesthouse or the minivan operators about the road before you travel, and allow extra time.
There is one upside to the monsoon: the first heavy rains wash months of burning-season haze out of the valley. This page is about rain and the river — if it is the air you are tracking, the air-quality page is the one to follow.
These figures compare the area's rainfall to its own long-term average — useful for seeing whether the monsoon is early, late, or about on time. They do not measure the river itself, and one storm cell can soak one valley while the next stays dry. For how high the Pai River actually is right now, neighbours upstream and the town's Facebook groups will often know before any model does.
Updated 21 June at 23:55. Rain figures are weather-model estimates for the area around central Pai — useful for comparing periods, not exact bucketfuls; one valley can catch a storm the next one misses. The “typical” figures are 1991–2020 averages from the same modelling family (ERA5), so the comparison is like-for-like. Weather data by Open-Meteo (CC BY 4.0).