# Montréal Forecast Reliability

2026-08-20 · https://ibraverse.ca/tech/montreal-forecast-reliability/

> Thirty days of Montréal forecasts graded against the station next door. Temperature is 1.3 °C off a day ahead; seven in ten rain calls were false alarms.


[Montréal Forecast Reliability Report](https://ibraverse.ca/reports/montreal-forecast-reliability.html)


## Key findings

- Temperature is the reliable part: 1.2 °C off six hours ahead and 1.3 °C a day ahead, on average. Humidity holds as well.
- Rain is not: six hours ahead, 419 of 583 rain calls passed without a drop (72% false alarms), and the precipitation error does not improve as the hour approaches.
- No station here reports cloud cover, so sunshine stands in for it and that measure reads "not measurable".

## Data and methods

From 14 August to 13 September 2026, I recorded the hourly forecasts of ECMWF, GFS and HRDPS (through Open-Meteo) for Parc Henri-Julien and Montréal–Trudeau, saving a new record only when a service changed its forecast. Each forecast is graded against Environment Canada’s McTavish and Trudeau stations at one, three, six, twelve or twenty-four hours ahead, or three days ahead.

A forecast counts only if it was published before the hour it describes. Instants are matched within ±5 minutes; rain is compared against the hour it fell in. One month of late summer in one city is not a verdict on forecasting.