Long-Range Forecast Users Guide
Description of the maps
The forecasts charts are composed of 2 panels: the forecast (upper panel) and the skill (lower panel) of the forecast system for this particular forecast. The forecast is presented in three categories: below normal, near normal and above normal. The method used to define these categories is explained in the next section. The above normal and below normal categories are indicated respectively by the hatched red and blue areas. The white areas are predicted as being near normal. The months for which the forecast is valid are indicated in the lower left corner of the upper panel while the issue date is shown in the lower right corner of the same panel. The lead times and the season for which the skill map is valid are indicated in the upper right corner of the lower panel. The climatology was calculated using the data of the period indicate on the upper panel.
The lower panel shows the percent correct (PC) associate with the forecast. The values equal to or greater than 45% are colored. The percent correct corresponding to each color are shown in the legend at the bottom of the lower panel. The grey areas indicate areas where the percent correct are below 45% which is not better than a pure chance forecast. The percent correct were calculated using the Historical Forecasting Project data (Derome et al., 2000 ; Plante and Gagnon, 2000) that cover the 26-year period of 1969 to 1994. The PC is an evaluation of the past performance of the models. Although there is no warranty that this skill map will be an accurate estimation of future skill, this is the best estimate we can use so far. For more information on the skill maps please click here.
Definition of the categories
The forecasts are categorized as below normal, near normal and above normal. The threshold used to define the category is 0.43 times the interannual seasonal standard deviation of the variable (i.e. temperature or precipitation). This choice makes the category equiprobable (same probability) on average. The observed climatology and threshold maps are available here.
More details on the categories:
For temperature forecast:
- ABOVE NORMAL
The seasonal forecast is warm compared to the 31 seasons in the 1963 to 1993 period. One third of the seasons in that period was as warm as the forecast.
- BELOW NORMAL
The seasonal forecast is cold compared to the 31 seasons in the 1963 to 1993 period. One third of the seasons in that period was as cold as the forecast.
- NEAR NORMAL
This is forecast in all other cases. This means that the forecast anomaly is weak and the forecast temperature is close to its 31 year average.
For precipitation forecast:
- ABOVE NORMAL
The seasonal forecast is wet compared to the 30 seasons in the 1961 to 1990 period. One third of the seasons in that period was as wet as the forecast.
- BELOW NORMAL
The seasonal forecast is dry compared to the 30 seasons in the 1961 to 1990 period. One third of the seasons in that period was as dry as the forecast.
- NEAR NORMAL
This is forecast in all other cases. This means that the forecast anomaly is weak and the forecast precipitation is close to its 30 year average.
How to use the maps ?
Locate the area of interest on the skill map and make sure that the percent correct is higher than 45% (colored areas). If it is the case, you should go to step 2. However, if it is not the case, the confidence in the forecast must be considered as very low. Therefore, it is not recommended to use the forecast for this particular area of interest!
Locate the area of interest on the forecast map and note the forecast category (ABOVE, BELOW or NEAR NORMAL).
On the temperature climatology map (in degree Celsius), note the average seasonal temperature for the area of interest.
On the threshold map at the area of interest:
- for temperature: note the threshold value (in tenths of degree Celsius) that defines the ABOVE, BELOW and NEAR NORMAL categories.
- for precipitation: note the threshold value (in millimetres) that defines the ABOVE, BELOW and NEAR NORMAL categories.
For each category, the forecast value is:
ABOVE NORMAL : the temperature (precipitation) is forecast to be warmer (greater) than the climatological value plus the threshold value
BELOW NORMAL : the temperature (precipitation) is forecast to be colder (less) than the climatological value minus the threshold value
NEAR NORMAL : the temperature (precipitation) is forecast to be near the climatological value plus or minus the threshold value
It has to be noted that the surface air temperature forecast is a prediction of the anomaly of the mean daily temperature at 2 metres (i.e. at standard observation Stevenson screen height). It is not a forecast of the maximum nor of the minimum daily temperature. For more information on what is predicted by Environment Canada seasonal forecasts please read this frequently asked questions page.
Examples
If the climatological temperature for the area of interest is -18 Celsius and the threshold value is 12 (which means 1.2 Celsius), the ABOVE, BELOW and NEAR NORMAL categories are defined by the following values :
ABOVE NORMAL : temperature forecast to be equal to or warmer than -16.8C (which is equal to -18.0C + 1.2C)
BELOW NORMAL : temperature forecast to be equal to or colder than -19.2C (which is equal to -18.0C - 1.2C)
NEAR NORMAL : temperature forecast to be between -16.8C and -19.2C
If the climatological temperature for the area of interest is +6 Celsius and the threshold value is 3 (which means 0.3 Celsius), the ABOVE, BELOW and NEAR NORMAL categories are defined by the following values :
ABOVE NORMAL : temperature forecast to be equal to or warmer than 6.3C (which is equal to 6.0C + 0.3C)
BELOW NORMAL : temperature forecast to be equal to or colder than 5.7C (which is equal to 6.0C - 0.3C)
NEAR NORMAL : temperature forecast to be between 5.7C and 6.3C
If the climatological precipitation for the area of interest is 300 millimetres and the threshold value is 30 millimetres, the ABOVE, BELOW and NEAR NORMAL categories are defined by the following values :
ABOVE NORMAL : precipitation forecast to be equal to or greater than 330 millimetres in water equivalent (which is equal to 300 mm + 30 mm)
BELOW NORMAL : precipitation forecast to be equal to or less than 270 millimetres in water equivalent (which is equal to 300 mm - 30 mm)
NEAR NORMAL : precipitation forecast to be between 270 and 330 millimeters
References
Derome J., G. Brunet, A. Plante, N. Gagnon, G. J. Boer, F. W. Zwiers, S. J. Lambert, J. Sheng, et H. Ritchie, 2001: Seasonal Predictions Based on Two Dynamical Models.Atmos. Ocean., 39, 485-501. [paper]
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