The Monsoon
The southwest monsoon, 1 June to 30 September, measured against its own 1991–2020 climatology.
- Actual
- — mm
- Normal for these dates
- — mm
- Rain arrived
- —
- Season measured to
- —
- Next 10 days, ECMWF
- —
Rainfall through the season
Cumulative from 1 June. The normal is the mean of the same dates over 1991–2020.
This season in the record
Every region, state and union territory
Departure from the 1991–2020 normal for 1 June to the date above, in IMD's classes. Sorted wettest first.
| Area | Actual | Normal | Departure | Wetter ← → drier | Class | Next 10 d |
|---|
What the model adds next
The rainfall ECMWF's operational run puts down over the next ten days, against the 1991–2020 normal for those same calendar days. It is summed from the decoded 0.1° field rather than from the published rasters, which are banded to a palette; and it stops at ten days rather than the model's fifteen, because a deterministic run's day-thirteen rainfall over one state is not a number worth setting beside an observed one.
Nineteen subdivisions carry no rainfall figure
Rainfall here is averaged over polygons, and the only Indian boundaries this site has are states and union territories. Seventeen of IMD's thirty-six meteorological subdivisions are unions of whole states and are computed exactly. The other nineteen are part of a state — West and East Rajasthan, the four Maharashtras, the three Karnatakas and so on — and their boundaries follow district lines this site does not have.
Cutting a state at a round meridian would produce a departure published to a tenth of a per cent across a boundary that was invented, so it is not done. Each of these links to its state and its homogeneous region, both of which are computed.
What this is, and what it is not
The normal is CHIRPS 1991–2020, not IMD's long-period average. IMD's LPA is computed over 1971–2020 from its own gauge network. CHIRPS begins in 1981, so that period is unavailable at any resolution; 1991–2020 is the current WMO standard normal. Departures here will differ from IMD's by a few per cent. As a check on the method rather than a claim of agreement, the all-India full-season normal computed here is 883 mm against IMD's published 868 mm.
Both halves come from the same product. Measuring a satellite season against a gauge-network normal produces a departure that is partly an artefact of changing instrument. The season and the normal here are both CHIRPS v2.0 daily at 0.25°.
The last few weeks are preliminary. CHIRPS final lags about a month; the tail is filled from the CHIRPS preliminary product, which is the same pipeline without the final gauge merge. The chart marks where that begins.
“Rain arrived” is not IMD's monsoon onset. IMD declares onset over Kerala on a three-part test — rainfall at fourteen named stations, the depth and strength of the westerlies, and outgoing longwave radiation — which no rainfall dataset alone can reproduce. What is shown here is the day an area has normally received a tenth of its season's rain, which is a different and stated quantity.
Tamil Nadu's rainfall year is not this season. The southwest monsoon is not its main one: most of its rain comes from the northeast monsoon of October to December, which is a separate season and is not folded in here.