Backgrounders on various weather services: chapter 3

Monthly and Seasonal Weather Outlooks

Canadians love their weather, which is why Environment Canada (EC) is dedicated to providing Canadians with daily weather forecasts and weather alerts that are often issued alongside them.  However, there is also a benefit in knowing what the weather may be like in the future, which is why EC produces both Monthly and Seasonal Weather Outlooks.

  • Monthly Weather Outlooks are issued on the 1st and 15th day of each month, for temperature only.  
  • Seasonal Weather Outlooks are issued on the 1st day of each month, for temperature and precipitation. A season is three months long.  

These monthly and seasonal outlooks are not meant to describe a sequence of specific weather events, nor to give details about upcoming weather, but are useful because they provide advance notice of possible long range weather conditions.  Monthly and seasonal weather outlooks are of particular interest to weather-sensitive sectors such as agriculture, construction, transportation, forestry, tourism, hydro-electricity generation and to various provincial and federal agencies. 

How are monthly and seasonal outlooks produced?

In December 2011, the Canadian Meteorological Centre (CMC) began running a newly developed Canadian global coupled (one-tier) multi-seasonal prediction system, called the Canadian Seasonal and Interannual Prediction System (CanSIPS), for forecasting monthly to multi-seasonal climate conditions.

CanSIPS is a multi-model ensemble system based on two climate models developed by the Canadian Centre for Climate Modeling and Analysis in Victoria, B.C.  It is a fully coupled atmosphere-ocean-ice-land prediction system, integrated into the CMC operational prediction suite and relies on CMC data assimilation infrastructure.

Deterministic and Probabilistic Forecasts

Before 2013, EC produced only deterministic monthly and seasonal forecasts.  A deterministic forecast is one that gives only one description (example: tomorrow at 1:15 p.m. the temperature will be 3.2 degrees Celsius, and it will begin to rain five minutes later at 1:20 p.m.).

To produce a deterministic forecast, the results from CanSIPS models are averaged and compared to past climatology (currently covering the years 1981 to 2010) to create the temperature and precipitation outlooks.

The monthly deterministic outlook shows three forecast categories, color coded to indicate if the temperature will be:

  • below normal* values in blue;
  • above normal values in red;
  • near normal in white.

For examples of temperature and precipitation probabilistic forecasts, visit the website.

Since 2013, EC also produces probabilistic forecasts (with probability expressed in percent), which is considered a better approach when it comes to monthly and seasonal forecasts.  A probabilistic forecast would say “tomorrow, there is an 85% probability that the temperature will be 3 degrees Celsius or higher at 1:15 p.m., and a 65% probability that rain will have begun by 1:20 p.m.”.

To produce a probabilistic forecast, ten simulations from each model are produced, creating an ensemble of 20 predictions used to calculate probability distributions.  If all of the predictions have similar results, the probability and the confidence level of the outlook is higher.  The more the results differ from each other, the lower the probability and confidence level.

*Note: Presently the monthly weather outlook is a deterministic forecast and is available for temperature only on the website.  EC is currently working at designing a probabilistic monthly weather outlook which will use the Global Ensemble Prediction System, making it more precise and reliable.

For examples of temperature and precipitation deterministic forecasts, visit the website.

At a glance, you can see by region which category- above, near or below normal- is the most probable and the probability that this forecast will come true. 

For more information on the science behind weather forecasting, visit the following links:

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