How do we work out the weather forecast?

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 its 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.

Image courtesy of ECMWF, one of the international weather models

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 are missing information in between. 

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 then 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 can be quite a 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:

ECMWF model output from Metservice NZ

This is a sample of the output from a European model (the ECMWF). It shows:

  • MSL - the dark lines - each line joins observations of constant pressure at sea level. You can see a line of 1028hPa pressure south and east of Tasmania. This is an area of high pressure. Well southwest of Perth are two deep lows, shown by lines with numbers less than 970hPa. hPa is a unit of measurement for pressure, the hectopascal.
  • 1000-500 thickness - the grey dashed lines, and the highlighted levels of 5760 in red, and 5400 in blue (for more see Thickness). This is how tall or short a column of air is, between the ground and the 500mb pressure level up in the atmosphere. Higher numbers are taller = warmer air, lower numbers are shorter = colder air. 
  • Rainfall - the coloured areas, using the legend in millimetres on the right hand side. The time step shows that this is rainfall over the last six 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 six hours, but there are some in the short term (next few days) that have a resolution of one hour in a 12 by 12 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 one km 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.


The main models have 'the answer' by about 6am and 6pm. On my radio crosses to 3aw each morning, and in the 6pm news on Channel 7, I will have analysed the latest run of these models. The BoM need to have their forecast issued ahead of these times, so they are ready for all the rest of the media to use. This is why my forecasts can sometimes differ. 

Coming soon...
Stylised, easy to use maps, of current and future weather conditions, based on the best of the weather models.
More details to follow.

What is the difference between wintry hail and snow?

When we have one of those really icy-feeling days, we could end up with wintry hail. But what exactly does that mean?

Its not the like the warm, summer thunderstorms that bring large hailstones. Those stones are produced by raindrops going up and down in the storm cloud, growing in size every time they ride the updraft.

In fact, they don't necessarily come from thunderstorms.

All winter precipitation starts out as snow but how it looks when we see it at the ground all depends on what temperatures it falls through.

Generally, we will have raindrops reach the ground.

Rare for Australia is freezing rain. Snowflakes turn to raindrops that partially refreeze in cold air, coating the ground with ice. The temperature at the surface is close to zero.

Ice pellets are the wintry hail that I mention. BoM use the term "possible hail" or "local hail" in the forecast, and separate it from thunderstorms.

  • They are small (pea-sized), translucent balls of ice, that bounce when they hit the ground and make a high-pitched tapping noise.
  • They fall within a mass of raindrops, but my theory is that they have fallen faster, so each pellet is still frozen by the time it reaches us.
  • The temperature at the surface is still well above freezing, so its not snow. 

Snow is precipitation that remains frozen as snowflakes all the way to the ground. The temperature at the surface is in the negatives, 0C, or up to 2C - and very rarely as high as 4C (it would need to be a very low dew point to still remain as snow).

We can also add sleet to that list. This one is cold air for much of the trip down, warming up just enough near the ground. That is a wet snow mix, or snow that is melting by the time it reaches us.


When the weather map shows a long southwesterly fetch - ie the lines on the map show a 'road' coming straight up from Antarctica - with a cold pool (thickness dropping below 5400, the blue line), then we have the chance of wintry showers (ice pellets within raindrops, that stops and starts).

This is what a cold pool looks like as it approaches Victoria: 

And this is what it delivers - little balls of ice, not snow (its still +10C!):

Photo: alezamo44 (instagram)

Photo: alezamo44 (instagram)

This may help explain further:

How the wind blows

Its generally a given, in Australia, that a southerly wind is cool and a northerly wind is warm. 

But this is not always the case... and here's why.

First of all, lets clear up what a northerly wind means. The terms northerly or southerly indicate where the wind is blowing from. A northerly blows air down from the north, a southerly blows it up from the south. 

Simplified maps use an arrow to indicate this, but you may also see a wind barb

Image: noaa.gov

Image: noaa.gov

Image: eoas.ubc.ca

Image: eoas.ubc.ca

Each half barb (line) equals 5 knots, each full barb 10 knots, and each flag (triangle) 50 knots. A rough guide is to double the knots to get km/h.

You can see this in action here (with Obs Rose turned on): http://www.weatherzone.com.au/radar/vic 

To determine how hot or cold the wind will actually be, its helpful to look at the bigger picture, and where the wind has originated.

For example - in Victoria:

A northerly could have started over the Tasman Sea, so it will be warm and a bit humid.

A northerly could have started over the Tasman Sea, so it will be warm and a bit humid.

But a northerly that starts over the outback is generally hot and dry.

But a northerly that starts over the outback is generally hot and dry.

A southerly that loops around a high over the Bight is cool.

A southerly that loops around a high over the Bight is cool.

But a southerly that comes up from near Antarctica brings much colder air with it.

But a southerly that comes up from near Antarctica brings much colder air with it.

If we want to get more technical we have a look at the thickness. Thickness tells us how short or tall a column of air is. Warm air expands so its tall, cold air is short:

Image: psu.edu

Image: psu.edu

Thickness (from 1000 to 500, two different heights in the atmosphere, labelled THK) is represented on these maps using grey lines. mb stands for millibars, a measure of pressure, and it decreases as you go up, because the air is less dense with height (gravity pushes air towards the ground).

There are three important ones, marked red, blue and purple.

Red = 5760 = if that line is down over Victoria its likely to get to 40C in Melbourne. This line indicates mighty hot air.

Blue = 5400 = "the infamous 540 line" talked about in snow communities. This indicates that it should snow down to about 1500 metres.

Purple = 5200 = this is the really exciting one. If this line is over Tasmania or Victoria, it represents snow to sea level to the south of that line, and some bitterly cold air. 

So, we look at thickness and where the wind originated from to gauge how hot or cold the wind will actually be.

And this last map really puts it into perspective - a warm southerly(!): 

warm southerly

What cloud is that?

We've got some pretty interesting names to describe what we see in the sky: lenticular, mammatus, asperitus, castellanus.

What Cloud is That, and does it mean it will rain?

Lets dissect, starting with the ten basic types.

Image from Geo for CXC

Image from Geo for CXC

Clouds are classified by how high up the base of the cloud is (low, mid and high) and what sort of shape they form (stratus or cumulus). This can tell us what kind of weather to expect.

Stratus are clouds in a layer, usually a grey, widespread sheet. Cumulus are the puffy, cauliflower-like, clumpy cloud, often with vertical development.

CUMULUS

Can range from 'fair-weather' cumulus (or if you want to be really weather nerdy: 'cu', like 'Q'), to great towering beasts of a cloud, extending well up into the atmosphere. 

They form with clear air around them, and move with the wind, so it goes from sunny to cloudy to sunny. If they have enough vertical development, the particles inside become too heavy, and rain falls out. So, we go from sunny, to downpour, back to sunny. 

If they grow enough, they become cumulonimbus, the thunderstorm cloud. 

A perfect example of cumulus clouds, growing enough to produce wet weather. One part is in a downpour, the rest are dry. Image from Australian Sky and Weather

A perfect example of cumulus clouds, growing enough to produce wet weather. One part is in a downpour, the rest are dry. Image from Australian Sky and Weather

STRATUS

Limited vertical development means less rain particles inside, so its usually associated with drizzle. 

But altostratus are beefier, and can be a sign of rain on the way, particularly associated with the huge cloudbands that come down from the tropics. When they develop into nimbostratus, it can be the favourite type of weather in agricultural areas: days of light to moderate, steady rain. 

Stratus or altostratus (depending on the elevation of this land), forming nimbostratus, with a hint of stratocumulus too. Image from skypix photography

Stratus or altostratus (depending on the elevation of this land), forming nimbostratus, with a hint of stratocumulus too. Image from skypix photography

COMBO CLOUDS

Clouds do like to get together, forming combinations of stratus and cumulus, keeping the properties of both genes. 

Stratocumulus (weather nerds pronounce as 'strato-cu') is often seen in Melbourne - our mostly cloudy days, with the odd drizzly shower underneath. Altocumulus altostratus ('ack as') is the mid level version of this, and let us take the rain parts of altostratus, and embed heavier showers inside to make even more rain. 

Stratocumulus as a grey sheet with lumpy bits - both genes coming together to create a fairly grey day. Photo by cloud-maven.com

Stratocumulus as a grey sheet with lumpy bits - both genes coming together to create a fairly grey day. Photo by cloud-maven.com

HIGH CLOUDS

Cirrus are the ones right at the top, not attached to any cloud below them. Made of ice crystals, they don't produce any wet weather. 

Cirrus are the wispy clouds, cirrocumulus are small and rippled (and look amazing at sunrise/sunset), and cirrostratus are sheets of the wispy cloud, forming halos around the sun and moon.

Cirrus won't rain on us, but they can let us know that rain is on the way. Winds are strongest at that cloud level, so they are pushed here first, before the rest of the cloud associated with the weather system. 

Cirrus cloud in Swifts Creek, Victoria. Photo by Fir0002/Flagstaffotos

Cirrus cloud in Swifts Creek, Victoria. Photo by Fir0002/Flagstaffotos

These are the ten basic types, but that's just skimming the surface. The variations on these are endless, ranging from cumulus humilis and cumulus mediocris (that don't sound very impressive) to altocumulus stratiformis translucidus undulatus!

Want more? See my Weather Resources page.