SEASONAL FORECASTS

Long term forecasts can tell us if it will be wetter or drier (compared to the average) in the months ahead.

We look at the Pacific and Indian Ocean's. If they are 'neutral' it is not clear cut. But if they cross a positive or negative threshold, then that can indicate whether rain will be pushed to Australia, or away from Australia.

We also look at instability. A pulse of tropical energy, or a strong cold front, working to turn the moisture from the ocean into rain.

Here is a selection of indicators and forecasts to tell us if its more likely to be wet or dry in the months ahead.


CURRENT FORECAST

BoM forecast for the next month
BoM forecast for the next three months

The latest forecast from the Weather Bureau for rain in the next month and the next three months

Green and blue indicates higher odds for above average rain.  Orange and brown indicates higher odds for below average rain.

BoM forecast for the northern wet season

As we approach the northern Australian wet season, this forecast shows the chance of reaching 50mm earlier or later than average. Green is earlier, brown is later.


RECENT RAINFALL AND THE STATE OF THE OCEANS

Last month's rain, as a percentage of the average:

Last 3 month's rain, as a percentage of the average:

Last Month's Rain
Last 3 Month's Rain

The current month's rain so far, as a percentage of the average:

Month to Date Rain

Sea Surface Temperature Anomaly (how much warmer or cooler than average):

Sea Surface Temperature Anomaly

PACIFIC OCEAN (EL NINO OR LA NINA)

INDICATORS

SOI stands for the Southern Oscillation Index.

This index shows the difference in pressure from the central Pacific (Tahiti) to Australia (Darwin).

Above +7 indicates there is more rain in Darwin, below -7 indicates there is more rain in Tahiti. Sustained low values can indicate an El Nino:

SOI (Southern Oscillation Index)

Looking under the Pacific Ocean surface (Australia on the left, South America on the right). What is feeding the anomaly at the top - cool blues or warm reds. Can indicate upcoming changes at the surface:

Under the Pacific Ocean

PACIFIC OCEAN INDEX

Looking at the centre of the Pacific Ocean, on the equator. The sea surface temperature, how above average or below average. 

Above +0.8C (the brown area) can indicate El Nino. The Pacific Ocean won't help it rain here.

Below -0.8C (the green area) can indicate La Nina. The Pacific Ocean will encourage rain here.

The black line is how the index has tracked so far, and the dotted line is the average of all the international model forecasts.

The Pacific Ocean Index. Observations so far, and forecasts from the international weather models.

The Pacific Ocean Index. Observations so far, and forecasts from the international weather models.


INDIAN OCEAN (INDIAN OCEAN DIPOLE)

INDIAN OCEAN INDEX

Looking at the Indian Ocean, on the equator. This is the sea surface temperature, how above average or below average. 

Above +0.4C (the brown area) can indicate a Positive IOD.

This can lead to dry conditions in much of Australia in winter and spring - the Indian Ocean won't help it rain here.

Below -0.4C (the green area) can indicate a Negative IOD .

This can lead to wet conditions in much of Australia in winter and spring - the Indian Ocean will encourage rain here.

The black line is how the index has tracked so far, and the dotted line is the average of all the international model forecasts.

The Indian Ocean Index. Observations so far, and forecasts from the international weather models.

The Indian Ocean Index. Observations so far, and forecasts from the international weather models.

(IOD is the Indian Ocean Dipole - the Indian Ocean equivalent of La Nina or El Nino).


The Southern Annular Mode (SAM) index.

Southern Annular Mode (SAM)observation and forecast

Black is observation, red is forecast.

This index tells us if the cold fronts are stronger or weaker as they move around the Southern Ocean. This can have a big impact on Australia’s weather, as these examples show:

WINTER

Positive (left/first diagram) means the rain producing fronts and lows are too far south to affect southern Australia in winter.

Negative (right/second diagram) encourages rainfall over southern Australia in winter, letting fronts and lows pass through.

Positive Southern Annular Mode (SAM) effects on rain in WINTER
NEGATIVE Southern Annular Mode (SAM) effects on rain in WINTER

SUMMER

Positive (left/first diagram) means a high can sit off Tasmania in summer, letting humid air interact with an inland low or trough, potentially bringing rain to eastern Australia.

Negative (right/second diagram) can lead to a dry stretch of weather in the southeast and east coast in summer, as high pressure sits overhead and acts to block any weather systems. Or, it produces westerly winds in the south without any ‘energy from a low’, so the rainfall totals are only light.

POSITIVE Southern Annular Mode (SAM) effects on rain in SUMMER
NEGATIVE Southern Annular Mode (SAM) effects on rain in SUMMER

MADDEN-JULIAN OSCILLATION (MJO) INDEX.

A ‘pulse of energy’ in the tropics that moves around the globe.

When the pulse is in Australian longitudes (“Maritime Continent” - the right hand side of the diagram), and its not weak (extends outwards from the inner circle) then it can encourage feeds of tropical moisture to push rain into southern Australia. Especially good if it meets up with the SAM.

Different model forecasts:

Madden-Julian Oscillation (MJO) observation and forecast
Madden-Julian Oscillation (MJO) observation and forecast
Madden-Julian Oscillation (MJO) observation and forecast
Madden-Julian Oscillation (MJO) observation and forecast