In India, the primary authority for weather and rain data is the India Meteorological Department (IMD). IMD feeds every trusted app and Google Weather with the raw observations from its national surface stations, doppler-radar network and radiosondes, and re-issues district-level nowcasts every 15 minutes during the monsoon. Understanding the difference between a nowcast (0–3 hours) and a short-range forecast (3–48 hours) helps you trust the right number.

This guide walks through how a modern rain forecast is built, how to read the block-by-block outlook, which apps to trust for your city and the exact safety steps to follow when a heavy rain band is on the way. Keep the page open on your phone during monsoon — it’s the fastest daily reference.

A rain nowcast blends the last three doppler-radar sweeps with the current cloud-motion vector to project rainfall for the next 30 to 180 minutes. Because radar returns light rain from a distance, it can ‘see’ a rain band before it arrives at your locality — usually 40 to 90 minutes ahead.

For horizons longer than three hours, apps switch from nowcast to short-range weather-prediction models such as WRF, IMD-GFS or ECMWF-HRES, and the confidence drops from 90% to about 65%. Always trust the near-term hours first and re-check every couple of hours as the rain window closes in.

In India, IMD has doppler-radar coverage in 30+ cities including Mumbai, Delhi, Chennai, Kolkata, Bengaluru, Hyderabad, Nagpur, Bhopal, Lucknow, Patna and Guwahati. Cities without radar coverage rely on satellite-inferred nowcasts, which are less precise for locality-level rain but still useful for the coming 1–2 hours.

Mumbai, Chennai, Kochi and Kolkata check the rain nowcast round the clock during monsoon — a 50mm/hour band can flood streets and cause commute chaos in under an hour. Delhi, Jaipur and Ahmedabad watch pre-monsoon and post-monsoon showers.

Hill towns Shimla, Manali, Darjeeling and Gangtok watch it for landslide and hail risk. Bengaluru, Hyderabad and Pune watch for flash storms and traffic snarls. Coastal Vizag, Bhubaneswar and Panaji watch for cyclone-linked rain bursts. Farmers in every district watch it for spraying and harvest windows.

IMD Mausam app is the primary official source — free on Android and iOS, updated every 15 minutes for radar cities. Windy shows a beautiful radar animation you can scrub across the next 12 hours. Google Weather and Apple Weather give a simple hour-by-hour rain bar with ‘MinuteCast’-style detail on many devices.

AccuWeather’s minute-by-minute forecast is very good in radar-covered cities. Windy.com’s ECMWF layer is the model of choice for many meteorologists. Cross-check any single source against IMD before making a serious decision like postponing a wedding, a match, a delivery run or a farm operation.

How often is the rain nowcast updated? Doppler-radar based rain nowcasts update every 10–15 minutes in most cities. Longer-horizon forecasts refresh every 3–6 hours.

Why do two rain apps disagree on the exact hour? Because they use slightly different models (WRF, GFS, IMD-GFS) and different post-processing. Trust the direction — rain coming or clearing — more than the exact minute.

What is a cloudburst? A cloudburst is a sudden, very intense rain event — more than 100 mm in one hour over a small area — usually in the hills, often triggered by orographic uplift.

Can I still trust a rain forecast at midnight for the next morning? Yes — the 0–12 hour horizon has 85–90% accuracy for rain occurrence in radar-covered cities. Refresh the forecast when you wake up.

What is the difference between rain and drizzle? Drizzle is precipitation with drops smaller than 0.5 mm falling from stratus clouds; rain has larger drops from taller clouds. Drizzle rarely accumulates more than 1 mm/hour; rain can hit 30–100 mm/hour.

A rain nowcast tells you what will happen in the next 3 hours; a 7-day rain outlook tells you which days of the week will carry the wet spell. Use both together — the nowcast for today, the 7-day for the week’s planning.

During active monsoon, IMD also publishes a ‘medium-range’ 10-day rainfall probability at district level. This is the go-to source for scheduling weddings, cricket matches, film shoots and outdoor events — and for cancelling them politely if the numbers move against you.