Monsoon in Nepal: Normal Onset and Withdrawal Dates (Calendar)

The Department of Hydrology and Meteorology (DHM), Nepal's national weather and climate agency, puts the normal monsoon onset over eastern Nepal at 13 June and the normal withdrawal at 2 October, giving a normal season length of 112 days. In the Bikram Sambat (BS) calendar these dates fall around 30 Jestha and 16 Ashwin respectively, which is why the months of Asar, Saun and Bhadau are Nepal's archetypal rainy months. DHM's Climate Analysis Section has kept onset and withdrawal records continuously since 1968 AD (2025 BS). The dates on this page are climatological normals — long-term averages — not forecasts for any particular year.

For rainfall statistics, DHM treats the four calendar months from June to September — often abbreviated JJAS — as the monsoon season, even though the actual onset and withdrawal drift from year to year. The summer monsoon is a large-scale seasonal reversal of winds that carries warm, moisture-laden air from the Bay of Bengal across South Asia; Nepal receives this moist easterly flow after the monsoon has advanced across northeastern India. Once the flow establishes, scattered pre-monsoon thundershowers give way to sustained, widespread rain.

In practice, the start date varies by up to about two weeks either side of 13 June, and the season also pauses during within-season 'break' spells when rainfall weakens for days at a time. DHM declares onset only when persistent monsoon circulation and widespread continuous rainfall are established over eastern Nepal — an early shower in May or the first week of June does not by itself mean the monsoon has begun. Note also that some older reports still cite 23 September as the normal withdrawal date; that figure comes from an earlier reference period, and DHM's current bulletins use 2 October.

The monsoon always enters Nepal from the east, crossing into Koshi Province from the Bay of Bengal branch of the South Asian monsoon, and then spreads westward along the Himalayan foothills. Covering the whole country typically takes about a week, and in some years ten days or more, so far-western Nepal (Sudurpashchim Province) normally sees its monsoon rains begin in the third week of June even when the east gets them by mid-June.

DHM's onset and withdrawal map for 2023 illustrates the normal progression well. That year the monsoon entered eastern Nepal on 14 June and needed nine days to reach the far-western border, while in autumn the withdrawal unwound in the opposite direction over ten days.

Withdrawal, in other words, is a mirror image of onset: it begins from the dry far west and clears the moist east last. Eastern Nepal is therefore both the first region to receive monsoon rain and the last to lose it, giving the eastern hills and Tarai an effectively longer rainy season than Sudurpashchim — one reason the east is generally the wetter half of the country.

Averaged across DHM's 20 major monitoring stations, normal monsoon-season (June–September) precipitation is 1,472 mm against a normal annual total of about 1,833 mm — meaning the monsoon delivers roughly 80 percent of Nepal's yearly rainfall in just four months. These normals are calculated for the 1991–2020 reference period. The remaining fifth of the year's precipitation is split among the pre-monsoon, post-monsoon and winter seasons, with July and August typically the wettest individual months.

The national average hides enormous local contrast. The wettest belt lies on the southern slopes of the Annapurna massif in Kaski and Lamjung districts, where seasonal totals exceed 3,000 mm; Lumle station in Kaski, Nepal's wettest recording station, measured 4,030.3 mm in the 2023 monsoon alone. At the other extreme, trans-Himalayan rain-shadow valleys north of the main range receive less than 500 mm in the whole season — in 2023, Humde in Manang recorded just 246 mm and Jomsom in Mustang about 338 mm — because the high Himalaya wrings the moisture out of the northward-moving air.

DHM classifies a season's performance against these normals: below 90 percent of normal precipitation counts as a below-normal monsoon, 90–110 percent as near normal, and above 110 percent as above normal. Seasons that deliver only 50–75 percent of normal rain are categorised as moderate drought, 30–50 percent as severe drought and under 30 percent as extreme drought. The 2023 monsoon, for example, produced 89.9 percent of normal precipitation nationwide and was classed as slightly below normal.

DHM's published record table, which begins in 1968, long listed 31 May 1996 as the earliest monsoon onset. That mark fell on 29 May 2025 (15 Jestha 2082), when DHM's Meteorological Forecasting Division announced that monsoon clouds had crossed into eastern Nepal a full 15 days ahead of the normal date — the earliest arrival in the department's records. The latest onset ever recorded remains 27 June 1982, exactly two weeks behind schedule.

On the withdrawal side, the earliest retreat came on 2 September 1995 — a full month before normal — while the latest, 19 October 2013, stretched the season 17 days past the normal date. Late withdrawals have become the rule rather than the exception: in every monsoon from 2016 onward the season has ended after the 2 October normal, usually in the second week of October, and DHM's trend analysis shows withdrawal dates drifting steadily later since 1968.

Season length varies accordingly. The shortest monsoon on record lasted just 73 days in 1979 (late onset on 24 June, abrupt withdrawal on 4 September), and DHM's bulletins cite the 130-day season of 2008 among the longest. The 2022 season — onset on 5 June, withdrawal on 16 October — ran even longer at about 134 days, and 2025 (29 May to 10 October) was of similar length, so recent years have produced the longest monsoons in the record.

The list below shows the onset and withdrawal dates for eastern Nepal declared by DHM for the last eleven seasons. It illustrates both the spread around the normals (13 June and 2 October) and the recent tendency for the monsoon to arrive on time or early but leave one to two weeks late.

Across this period the average onset falls close to the 13 June normal, while the average withdrawal is around the second week of October — well after the 2 October normal. That asymmetry is why recent monsoon seasons have averaged closer to 120–130 days than the textbook 112.

DHM divides Nepal's year into four seasons: pre-monsoon (March–May), monsoon (June–September), post-monsoon (October–November) and winter (December–February). 'Pre-monsoon' and 'post-monsoon' are simply the transition seasons on either side of the rains, and each has a distinct character worth knowing.

The pre-monsoon, roughly Chaitra to Jestha in the BS calendar, is Nepal's hot, unstable season. Daytime heating builds towering afternoon and evening thunderstorms that bring short, violent bursts of rain, hail and frequent lightning — one of Nepal's deadliest weather hazards, which peaks in these months. The Tarai often sees heat waves above 40°C, and haze and forest fires are common before the first big storms. These scattered thundershowers are not the monsoon: they lack the persistent moist easterly flow, and DHM does not count them toward onset.

The post-monsoon, roughly Ashwin–Kartik, is the reverse: once the monsoon trough retreats, dry, stable air settles over the country, giving the clear skies and long views that make October–November the classic season for trekking and for the Dashain and Tihar festivals. The season is normally dry, but remnants of Bay of Bengal cyclones can occasionally push in hazardous late rain and high-mountain snow — most infamously the October 2014 blizzard from Cyclone Hudhud's remnants, which killed more than 40 people on the Annapurna Circuit. Winter, by contrast, brings only a small fraction of annual precipitation, mostly from westerly disturbances that favour the far-western hills.

Nepal's agricultural year is built around monsoon onset. Rice — the staple crop — is transplanted into flooded paddies once the June rains soak the fields, an event celebrated on Asar 15 (around 29 June), observed as National Paddy Day or Dhan Diwas with mass rice-planting festivities. Because the majority of Nepali farmland is rain-fed rather than irrigated, a two-week delay in onset can compress the transplanting window and cut yields, while a timely monsoon underpins a good harvest and, with it, agricultural GDP.

The same rains drive Nepal's water and energy systems: rivers rise several-fold, run-of-river hydropower plants reach peak generation, and groundwater and springs recharge for the dry months ahead. The monsoon is also Nepal's principal disaster season, when saturated slopes fail and rivers flood — the 2024 monsoon (10 June–12 October) was the deadliest in years, with official end-of-season tallies reporting 494 people killed and record-breaking downpours, including the late-September 2024 floods in the Kathmandu Valley. Government agencies publish monsoon preparedness and response plans keyed to the onset date each year.

For visitors, June–September is the tourism low season: trails are muddy and leech-prone, mountain views are often clouded, and domestic flights to hubs such as Lukla face frequent weather delays. The pattern is not uniform, though — much of the monsoon rain in the hills falls overnight and in the early morning, and the trans-Himalayan rain-shadow districts of Upper Mustang, Dolpo, Manang and Nar-Phu stay relatively dry, making them Nepal's established monsoon-season trekking destinations.