One of the favourite parts of my job (besides the hair/makeup/wardrobe for TV ;) ) is analysing the next run of the weather models.
The main 'runs' are available twice a day, and I actually don't like talking to people in the hour or so before they are ready, because I feel the information I have is 'old'. If anyone asks me a question about the weather at that point, I will usually back away and mutter something about "I'll tell you in an hour" - even if it's the news director!
This analysis leads to the forecast that you read or hear, how we come up with the words to describe what is most likely to happen.
Weather models have three steps to them: input, crunch, and the answer.
INPUT
The current state of the atmosphere.
All the observations from satellites, radars, weather stations, buoys, aeroplanes, ships, weather balloons, soil probes - all of it - are mapped into a current state of the atmosphere.
It would be wonderful if we could grab every bit of information about every inch of the earth and its atmosphere, but we can't.
For example, weather stations measure what is happening at that exact location, but they may be a long distance from the next weather station. Melbourne Olympic Park is 16 km from Essendon Airport, the next closest station, while a station called Carnegie in outback Western Australia is more than 300 km from the nearest station.
This means that we have a very good set of data for certain locations, but the information in between is missing.
This is supplemented by satellites, but its not every minute. In fact, satellites now record once every ten minutes, and radar's once every six minutes. Good but not absolutely every piece of information.
CRUNCH
The model takes all this information and produces an approximate current state of the atmosphere.
This is used to feed a set of mathematical equations that describe how the atmosphere works - its dynamics and physical processes.
The earth is huge, and that's a lot of data (even with the missing parts), so the numbers are crunched by supercomputers that can take anywhere from two to eight hours to come up with the solution.
THE ANSWER
When its all processed, 'the answer' is produced.
What went in at the start was an approximation, so what comes out at the end won't be the exact answer of what is going to happen.
And this gets worse with time.
Say they missed a part of a cold front, or thought it was 200 km away from where it was actually located. That front's location could then become out of sync with the rest of what has been modelled, and this gap will grow with each day that we look ahead. This is why I limit my forecasts to 8 days, anything after that is a very poor representation of day to day conditions.
But, the technology gets better every year, and the next model run may have a better idea of exactly where that front is, because it affected a passing ship, or went through a denser part of the observation network.
The answer arrives looking like this:
This image shows:
- MSL - the dark grey lines - each line joins observations of constant pressure at sea level. You can see a line of 1020hPa pressure over the Bight. This encloses an area of high pressure. hPa is a unit of measurement for pressure, the hectopascal (it is the same as millibar [mb]).
- Rainfall - the coloured areas. This example shows us the potential rain over 24 hours.
We analyse lots of different maps like these, from all the different models (which could be saying rather different things, based on a different crunch equation, and a different approximation of current conditions) and come up with the forecast.
The example above shows a time step of 24 hours, but there are some in the short term (next few days) that have a resolution of one hour in a 9 by 9 km square.
But have a think about how a shower moves through (for more see Showers vs Rain). They can last for less than 30 minutes, and be less than 1km wide. So, we can't see every shower in these weather models. This is why we describe it as a chance of precipitation - and can't guarantee that you will see wet weather exactly on your spot.
-> For more on the weather models we feature here at Jane's Weather please read more here.
The main models have 'the answer' by about 6am and 6pm AEST. That is why our weather detail emails start arriving in your inbox shortly after.
