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Using a seasonal weather forecast

If you have Internet access, you have useful, free long-run weather predictions at your fingertips

weather map

Every farmer knows how important the weather is to success: too many hot, dry days and the crop will be stunted; too many cold, wet days and fungi can run rampant, cutting down yields. Knowing what the weather is going to be a few days from now is vital, and having a general picture of the weather a few months from now can help you plan for all contingencies, protecting your harvest and your farm.

Unfortunately, the weather is a notoriously tricky thing to predict. While you can be reasonably confident about the weather tomorrow, whether it will be sunny or rainy a week from now can be less certain than a coin toss. How can you expect to predict the weather months from now? By learning to read and interpret a seasonal forecast. We asked Dan Kulak, a meteorologist at Environment Canada, for some help.

Finding the information

Environment Canada issues many types of weather predictions through its “Analyses and Modelling” website. There is a lot of data here, but don’t feel overwhelmed: we’re only interested in the “probabilistic forecast map” and the “climatology of temperature and precipitation chart,” both of which can be found under the heading “Seasonal Forecast.” These are big picture looks at the weather to come.

First, click on “Probabilistic Forecast.” This will take you to a map of Canada with the provinces outlined, showing large blobs of colour: blue, purple, yellow and red. This is a map of air temperature predictions for a three-month period. The box in the upper right hand corner will tell you the date the forecast was issued.

The colours correspond to climate variations: either below, above or at normal. An area shaded blue is more likely than not to have below seasonal temperatures for the forecast period, purple shows average predicted temperatures, and yellow through red shows above average. The exact shade shows the forecasters’ confidence in the prediction, based on the past performance of their climate models. For example, a band of yellow across southern Saskatchewan indicates that the meteorologists at Environment Canada have a 40 per cent confidence that temperatures in that region will be above normal for the area. The areas that are not coloured, that have been left white, are areas where the forecasters’ confidence is below 33 per cent, meaning that they consider it equally likely the temperatures could be below, at, or above the normal.

On the same page as the probabilistic forecast of temperature, you can also get precipitation predictions. Immediately below the map there is a table listing temperature and precipitation, with columns for one to three months, two to four months, four to six months and so on. You’ll want to take a look at the precipitation map for one to three months in advance, as the reliability of the forecast is greatly reduced the farther into the future it projects. The precipitation map is similar to the temperature map — colour coded to show how confident Environment Canada is in its rain and snowfall predictions for an area matching or differing from the seasonal norm.

All this information isn’t much help if you don’t have some idea about what “normal” means for your region. To find it, go back to the main “Analyses and Modelling” page and look again under Seasonal Forecasts. Click on “Climatology of Temperature andPrecipitation.” This will take you to a new page where you can, using pull down menus, look up maps and tables of normal temperature and precipitation. The most useful for our purposes is the climatology table for the current three-month period.

The table lists average temperatures for different regions. Scroll down until you find the nearest city to you, then read across to see the climatology data. Under precipitation the data is given in millimetres, while under temperature it is given in degrees Celsius. This is the average of the precipitation or temperatures for the whole three-month period. This is the number that’s important, the one against which the probabilistic forecast is weighted, so a 40 per cent chance of above normal precipitation or temperature means a 40 per cent chance of warmer days or millimetres of rain above this base line.

As in so many things in life, there is a lot of chance involved in using a seasonal forecast. The trick is to know how to read and measure the chances, while understanding what you’re doing. Remember that it’s not a forecast of whether there will be rain or shine on any particular day, but how the overall weather will be: whether there will be more hot days than cooler days, more sunny days than wet days, and vice versa, vital info for a farmer wondering what to grow and when to expect the harvest.

About the author


Michael Flood is a business writer and columnist. You can reach him at [email protected]



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