Intro Guide Part Four: How to Read a BoM Forecast

You've seen the official forecasts from BoM - detailing rain percentage, rainfall amount, temperatures, icons, wind and more.

This summary should help you decipher what it all actually means.

An example:

This forecast has a lot of information on it - lets break it down to explain the different parts.

SUMMARY BOX

The summary box covers all the essentials:

Top Temperature

The highest point the temperature will reach during the day. This could be at any part of the day (ie before a cool wind change), and may only be for a short period of time.

Low Temperature

The lowest point the temperature will reach, before 9am. This is generally around dawn but it could be at 10pm the night before, or just before that reset time of 9am. 

Ever seen a forecast in the alps of Min 3 Max 3? This lets you know a cold change is coming, and could refer to: midnight 8C, 6am 8C, 9am 3C, midday 0C, 3pm -2C. So yes, it can snow with a max of 3. It helps to look at what the next days low temperature is, to see how much it will fall after the wind change.

Precis

"Windy. Mostly sunny." Less than 5 words, to describe the main parts of the expected weather. Wet weather is mentioned only if its a 40 percent chance or higher. Otherwise it tells you about the cloud or sun. It usually includes a mention of wind if the wind is strong.

Chance of Rain

This is the likelihood of any rain in that location, from midnight to midnight.  

Rainfall Amount

If it does rain, the likely amount.

The range is also a 'percentage' chance. In the second example (possible rainfall: 0 to 1 mm), out of every ten days with this weather pattern:

• half of these days would produce 0mm,
• a quarter of these days would produce 1mm. 

If the range was 2 to 5 mm, then out of every ten days with this weather pattern:

• half of these days would produce 2mm,
• a quarter of these days would produce 5mm,
• the remaining quarter of these days would produce less than 2mm.

If the forecast said:

Chance of any rain = 60 %      Possible rainfall = 2 to 5 mm

You can read it as:

"There's a 60 percent chance of rain, and if it does rain we could see up to 5 mm".

If the forecast said:

Chance of any rain = 20 %      Possible rainfall = 0 mm

You can read it as:

"There's a 20 percent chance of rain, so it most likely won't rain, and if it actually did rain, it wouldn't produce anything measurable".

MAIN FORECAST

This is the "full text" part - the full description. It includes information about the sky cover (cloud/sun), the wet weather, and the wind.

Sky Cover

Sunny, mostly sunny, partly cloudy, mostly cloudy or cloudy - and a description on how that could change over the day (ie sunny morning, cloudy afternoon). The first part of this series can help you see the contrast between these terms - the difference between mostly sunny and partly cloudy. 

The wet weather expected in the area (if any), and its likelihood. The second part of this series describes the difference between showers and rain.

Wind

Wind direction and speed, and changes to this across the day.

These speeds are averages, not the highest gusts. Light winds are generally less than 25 km/h, strong winds are 40 km/h and over - but here is the official guide:

Image: BoM

SEVERE WEATHER

If the weather is expected to be severe, a separate Severe Weather Warning is issued, detailing:

• Sustained winds of gale force (63 km/h) or more
• Wind gusts of 90 km/h or more
• Very heavy rain that may lead to flash flooding
• Abnormally high tides (or storm tides) expected to exceed highest astronomical tide
• Unusually large surf waves expected to cause dangerous conditions on the coast
• Widespread blizzards in Alpine areas

If thunderstorms are forecast, and they occur and become severe, a separate Severe Thunderstorm Warning is issued, detailing:

• Large hail (2cm in diameter or larger)
• Damaging or destructive wind gusts (generally wind gusts exceeding 90 km/h)
• Heavy rainfall which may cause flash flooding
• Tornadoes

That's the end of the series Understanding the Weather. Hopefully this has clarified a few weather concepts, but if you want more please move on to the advanced concepts. Happy reading!