Forecasting the next few hours from live radar, satellite and lightning data rather than from a model run. It is how a Kalbaisakhi squall or a Mumbai cloudburst gets a warning at all.
ForecastingWhat is Nowcasting?
Nowcasting is very short-range forecasting — conventionally the next zero to six hours, and most valuable in the first one or two. It differs from ordinary forecasting in what it is built from. A conventional forecast comes out of a numerical model solving equations forward from an analysis of the whole atmosphere. A nowcast comes from watching weather that already exists and projecting where it goes next.
That distinction matters most for the weather that kills people in South Asia. A monsoon trough can be forecast days ahead. The individual thunderstorm cell that floods one Mumbai suburb while the next one stays dry cannot be — it is smaller than the model grid and shorter-lived than the model’s update cycle. So the region runs two systems in parallel: models for the days ahead, nowcasts for the hours that matter.
How a nowcast is made
Four observation streams feed a modern nowcast, and each answers a different question.
Weather radar is the backbone. It shows where rain is falling right now, how hard, and — with Doppler radar — which way the air inside the storm is moving. The simplest nowcast tracks each echo over successive scans, works out its speed and direction, and advects it forward. More sophisticated versions allow for cells growing, merging and decaying rather than treating them as rigid blobs.
Satellite imagery covers what radar cannot reach: the open Arabian Sea and Bay of Bengal, and the mountain valleys where radar beams are blocked. Geostationary satellites such as India’s INSAT series watch cloud tops continuously. Rapidly cooling tops mean a cumulus tower is building fast, which is often the earliest sign of a storm that radar will pick up ten or twenty minutes later.
Lightning detection networks sense radio pulses from strikes and triangulate them within seconds. A sudden jump in strike rate is a reliable signal that a cell is intensifying, and lightning itself is the hazard most in need of a short warning.
Surface observations — automatic weather stations, rain gauges, airport reports — anchor everything else to ground level, including the wind shift and pressure jump that mark a gust front arriving ahead of the rain.
Increasingly, machine-learning models trained on archived radar imagery run alongside straightforward extrapolation. They are better at the hard part — anticipating that a cell will grow or die rather than simply drift — but they still depend on the same live observations.
Nowcasting in South Asia
The India Meteorological Department operates a national Doppler weather radar network and issues nowcast bulletins for cities and districts, updated through the day and escalated to warnings when a severe cell is tracked. The Pakistan Meteorological Department and the Bangladesh Meteorological Department run their own radar and warning services on the same principle.
Three regional weather types make nowcasting unusually important here.
Pre-monsoon squalls. In March, April and May, violent thunderstorm lines sweep across West Bengal, Bihar, Jharkhand, Odisha, Assam and Bangladesh — the storms known as Nor’westers or Kalbaisakhi. They form fast, move fast, and bring destructive straight-line winds, hail and dense lightning. The useful warning window is measured in tens of minutes, which is nowcast territory and nothing else.
Monsoon cloudbursts and urban flooding. Intense convective cells embedded in monsoon flow can drop extreme rain on a few square kilometres. Mumbai, Chennai, Bangalore and Dhaka all flood on this timescale, and so do Himalayan catchments where a cloudburst becomes a debris flow within the hour.
Dust storms and heat-driven convection in the northwest. Across Rajasthan, Haryana, Punjab and Sindh, pre-monsoon storms often arrive as a wall of dust ahead of the rain, collapsing visibility and knocking down power lines. Radar sees the gust front; satellite sees the dust plume.
Aviation, ports and the power grid all consume nowcasts directly, because a thirty-minute warning is enough to hold aircraft, secure cranes and reroute load.
Where a nowcast saves lives
The value of a nowcast is almost entirely in what people can do with a short warning.
- Lightning. Lightning is among the deadliest weather hazards in India by annual death toll, and most victims are outdoors and unaware. Getting people indoors half an hour early is the entire intervention.
- Fishing fleets. Small boats off the Konkan, Odisha, Sindh and Bengal coasts can make for shore if a squall line is called early.
- Outdoor work and commuting. Cranes, scaffolding, rooftop work and school runs all need lead time measured in minutes, not days.
- Crops at harvest. Grain lying on a threshing floor can be covered if the hail warning arrives in time.
The limits of a nowcast
A nowcast is confident and short-lived, which is the opposite failure mode from a model forecast.
Skill decays quickly. Extrapolation works well for the first thirty to sixty minutes, then degrades as cells grow, split and die in ways the last few radar scans did not reveal. Beyond a few hours the advantage over a numerical model disappears entirely — which is why operational systems blend the two, weighting the nowcast heavily at the start and handing over to the model as the lead time grows.
Coverage is the other constraint. Radar beams are blocked by terrain, so mountain valleys in Himachal Pradesh, Uttarakhand, Gilgit-Baltistan and the northeast are exactly where cloudbursts do most damage and exactly where radar sees least. Offshore, coverage ends not far from the coast. Satellite fills part of the gap but sees cloud tops rather than rain.
And a nowcast cannot tell you about weather that has not formed yet. If no cell exists on the radar, the nowcast is silent — which is correct, and is not the same as saying the afternoon will stay dry.
Frequently asked questions
How far ahead does a nowcast go? Conventionally up to about six hours, with the strongest skill in the first one to two. Past that, a numerical weather model does better and forecasting systems switch over to it.
Why can’t a weather model predict a single thunderstorm? Individual convective cells are often only a few kilometres across and live for under an hour — smaller than a typical model grid box and shorter than the interval between model runs. Models predict the conditions that favour storms; radar sees the storm that actually formed.
Is nowcasting the same as a rain radar app? A radar app shows you the raw echoes. A nowcast is the step after that: tracking those echoes, estimating growth and decay, and projecting where the rain will be at a stated time. Many apps present a simple extrapolation nowcast as a moving radar loop.
Why do nowcasts sometimes miss storms in the hills? Radar needs line of sight. Mountains block the beam, so cells forming inside a valley can be invisible to the nearest radar until they are already producing rain. Satellite and surface reports partly compensate, but hill coverage remains the weakest part of the network.
Where can I see short-range rain for my city? Mausam Online city pages show hour-by-hour rain probability and intensity for the hours immediately ahead, which is the practical form of a nowcast. Check Mumbai, Kolkata, Delhi, Patna, Dhaka and Lahore.