The long range weather forecast is a computer-generated product. Computer models of the atmospheric behavior have been developed and are constantly being improved through intensive research.
However, the longer the range of the "forecast", the less accurate or reliable it becomes.
This is one of the many reasons why Environment Canada issues its Seasonal Forecast in very general terms.
The Seasonal Forecast gives temperatures and precipitation as - above normal - below normal - normal. The product is available in graphic as well as text format.
Note: To understand how long-term weather forecasts are produced, I recommend Meteorology: An Introduction to the Wonders of the Weather!
Why A Long Range Forecast?
Why attempt to issue a long range forecast in the first place? Because there is tremendous demand from commercial and industrial sectors. They have to plan ahead, often months and years ahead.
Any effort to improve the accuracy of long term weather forecasts above that of "coin flipping" is worth billions to the world economy.
How To Interpret A Long Term Weather Forecast
Environment Canada states that the "skill" of its Deterministic Forecast System - for its 1-3 month temperature anomaly forecast - averages around 46.3%. Its best average score is 50.7% for the April-May-June period.
Please note that if you were to flip a perfectly balanced coin thousands of times, you would obtain "heads" 50% of the time, and "tails" 50% of the time!
Therefore, a score of 50.7% is about what pure chance would yield.
I certainly do not want to belittle the efforts of meteorological researchers around the world but, for now, a score of 50.7% is not far from "crystal ball gazing"!
In areas of the world where the temperature variations and weather patterns do not change much from one month to the next (i.e. the Caribbean), long range forecasting models could potentially score better.
Shorter Term Forecasts Reliable
In contrast, Environment Canada's short term 48 hour forecasts consistently scores slightly better than 85%!
Beyond the first 48 hours, the reliability of the "forecast" falls rapidly. In fact, beyond 5 days, the "forecast" becomes more like a general outlook. But it is reliable enough to be useful!
The secret is in knowing what its limitations are ... and in getting updates every day!
After a while, with practice, you eventually get better at "reading" a forecast. Just keep in mind that it is not cast in stone.
Every forecast, short term and longer term, is regularly updated to reflect the constant flow of weather observations received since the last issue.
Just keep in mind that, by the time you get the forecast, the atmosphere has "morphed" itself in a state that, hopefully, the forecast has taken into account!
Countries Collaborate To Improve Models
The atmosphere is in constant motion. It's dynamic, never static. Predicting its behavior beyond 48 hours, using present state of the art computer models of the atmosphere, is a challenge ... to say the least.
The member countries of the World Meteorological Organization (WMO) collaborate closely by constantly exchanging research results aimed at improving the long range weather forecast models in use around the world.