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The truth behind a weather forecast


Meteorologists at The Weather Network look at things like computer models, radar, satellite and surface observations
Meteorologists at The Weather Network look at things like computer models, radar, satellite and surface observations

Staff writers

September 10, 2012 — The weather is always changing and so is the forecast. How do meteorologists prepare a forecast that Canadians rely on?

Different models can predict different tracks for tropical systems as well
Different models can predict different tracks for tropical systems as well

Weather is the number one topic of conversation across the country and one that seems to strike a nerve among most Canadians.

Whether celebrating the beautiful conditions together or commiserating when the weather spells trouble, all eyes are on the forecast daily.

But what goes into creating that forecast and how are certain changes reflected?

Computer models are the main tools that meteorologists use.

"There are several computer models that we look at to come up with the forecast," says Gina Ressler, a meteorologist at The Weather Network. "Two Canadian models (run by the Canadian Meteorological Centre), two American models (run by the National Centers for Environmental Prediction), and two European models (run by the UK Met Office and the European Centre for Medium-Range Weather Forecasts)."

When new model data comes in, the meteorologists will compare the model fields against actual observations like radar, satellite and surface observations. That helps to decide which models are doing the best and identify any possible errors as well.

"We will also compare the models to each other. If all the models have the same general solution, confidence in the forecast is high. If there are differences between the models, then the meteorologist must use their expertise to decide which solution is best," explains Ressler.

Forecast uncertainty increases the further into the future you go
Forecast uncertainty increases the further into the future you go

Once the meteorologist has decided which combination of models represent the most likely situation, the forecast can be prepared.

"The meteorologist will make adjustments to the model output if necessary based on current weather observations, known model biases and experience with similar weather systems," says Ressler.

Even with intuitive forecasting skills and the latest computer guidance however, there are still limitations and uncertainties, especially the further into the future you go.

"Most models are updated twice a day, some four times a day and often, small changes to the track or timing of a weather system will lead to a large change in the forecast for some locations," notes Ressler.

She says summer time precipitation is generally considered to be the most challenging weather forecast to nail down.

"Precipitation in the summer usually comes in the form of thunderstorms, and these storms are often too small for the models to accurately resolve," says Ressler. "That's why you often hear the words 'risk' and 'chance' when it comes to thunderstorms."

Be sure to check the forecast in your city before heading out. You can also tune into The Weather Network on TV for your local forecast and up-to-date coverage on any severe storms.

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