Weather forecasting has come a long way in recent years, but there are some forecasts that you just can't trust.

Finding a weather forecast is easier than ever these days. Open up a mobile app, go online, turn on the TV, read the newspaper or listen to the radio and you'll find a weather forecast. If you're enthusiastic enough, you can even access weather models online and have a go at producing a forecast yourself.

However, despite the exceptional advancements in forecasting during recent years, there are still some firm limitations on how far ahead we can accurately predict the weather.

Day-to-day weather forecasts come from powerful supercomputers that run complex mathematical models to predict how the atmosphere will change during the next few days. These models, which are called Numerical Weather Prediction (NWP) models, sift through millions of real-life observations and solve complex mathematical equations to predict how the atmosphere will change in the near-future.

Today's NWP models give us a pretty good idea of what the weather will be like, at any location on Earth, during the next few days. Their accuracy starts to wane beyond four days into the future and looking out more than a week becomes more of a rough guide than a discreet forecast.

When you start to look at what the weather might be like beyond the next fortnight, NWP models are unreliable and forecasters start to look at something else: climatology and climate models.

Climatology refers to what type of weather a location typically experiences at a given time of year based on its historical observations. For example, how much rain or how many rainy days Adelaide sees during September, or how hot days typically are in Sydney during February.

Climate models then look at how these long-term averages might be swayed in the coming months by broad-scale climate drivers like El Nino or the Indian Ocean Dipole.

While climatology and climate models are useful to glean information about what type of weather you might get on any given day, it's only a broad guide. Using climate information to determine how likely you are to see rain in two month's time is not the same as an NWP producing a weather forecast of rain in three days time.

Unfortunately, the most important forecasts in our lives are often sought weeks or months ahead of time, to help plan for things like weddings, outdoor functions and holidays. For these cases, you should only ever use climate-based outlooks as a rough guide. Wait until you get within a few days of the event to find out what the weather is actually expected to be like.