Weather

Medium Term Climate Outlooks

Mixed rainfall odds for the March quarter

Mixed rainfall odds for the March quarter.

Issued 17th December 2008

The national outlook for total March quarter rainfall (January to March), shows mixed odds for exceeding the seasonal median. There is a moderate to strong shift in the odds favouring higher than normal rainfall over southwestern WA as well as for an area covering northeast NSW and southeast Queensland. Conversely, there is a moderate shift in the odds favouring lower than normal rainfall over parts of central and southwestern Queensland.

The pattern of seasonal rainfall odds across Australia is mainly a result of continued warmth in the central Indian Ocean.

The chance of exceeding median rainfall over Australia during the March quarter is between 60 and 75% over southwest WA and between 60 and 65% over northeast NSW and the far southeast of Queensland. This means that for every ten years with ocean patterns like the current, about six or seven years are expected to be wetter than average in these regions, while about three or four years are expected to be drier.

In contrast, the odds of exceeding median rainfall over parts of central and southwestern Queensland are between 35 and 40%, which means that these areas have a 60 to 65% chance of being drier than normal.

Across the rest of the country, the chance of exceeding the median rainfall during the March quarter is between 40 and 60%, meaning that above average falls are about as equally likely as below average falls in these regions.

Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian rainfall. During the March quarter, history shows the effect to be moderately consistent through eastern parts of NSW and Queensland, large parts of the NT and over much of southern and western WA. Elsewhere the effect is only weakly or very weakly consistent.

The tropical Pacific Ocean remains ENSO-neutral. Sea surface temperatures across most of the equatorial Pacific are near normal, however, cool sub-surface temperatures persist from the dateline to the South American coast and the trade winds are now stronger than normal across most of the basin. The SOI remains positive at approximately +13 for the 30 days ending 14 December. Most international computer models indicate that neutral conditions are likely to continue through summer and autumn, however, two models predict cooling. For routine updates and comprehensive discussion on any developments please see the ENSO Wrap-Up.

Click here or further detail.

Short Term Observations

Current Satellite (Bureau of Meteorology)

Synoptic Chart (Bureau of Meteorology)

24 Hour Rainfall observations (Bureau of Meteorology)

Bureau Of Meteorology: ENSO Wrap-Up

A regular commentary on the El Niño-Southern Oscillation
CURRENT STATUS as at 10th December 2008

Some La Niña characteristics developing across the Pacific

The tropical Pacific remains neutral with respect to ENSO, although it is beginning to develop some characteristics of a La Niña. Sea surface temperatures are close to the long-term average across most of the equatorial Pacific, although some parts of the central Pacific have recently cooled to around 1³C below normal. Subsurface waters have also been cooling over the past few months; a large volume of cooler than average water now stretches from the central to eastern Pacific. In addition, the Trade Winds have been persistently stronger than normal for some months across the western half of the basin; a fact reflected in the continuing strongly positive SOI which had an approximate 30-day value of +15 as of 8th of December.

Given current conditions and recent trends, the development of a La Niña during the southern summer cannot be ruled out. Some computer models do, in fact, forecast this to occur, although the majority of climate model forecasts show neutral conditions, with a cooler than normal Pacific, until the end of the southern summer. Historically, it is unusual for La Niña thresholds to be reached during our summer, but 1999/2000 is one recent example where this did occur.

The Indian Ocean Dipole is now neutral and is expected to remain neutral through the rest of the southern summer.

In Brief 

  • Weak cool anomalies have recently developed in the central Pacific.
  • Sub-surface cool anomalies persist in the central and eastern equatorial Pacific.
  • The latest 30 day SOI for the 8 December is +15. The monthly value for November was +17.
  • Trade winds are stronger than average across the western half of the tropical Pacific.
  • Cloudiness near the date-line has been below average during the past few months.
  • The majority of dynamic computer models predict neutral conditions to continue through the southern summer, however, some models are predicting a return to La Niña conditions.

     Source: BOM (chart updated 3 Dec 2008)

    Click here for further detail.

 

WA Dept of Agriculture: Growing Season Outlook

December 2008 (next update by 16 January 2009)
Contributors: David Bowran, Meredith Fairbanks, Ian Foster, Phil Goulding, David Stephens

Summary

  • Neutral conditions have continued, though DAFWA’s ESS is indicating border-line La Niña conditions to develop through summer with neutral to borderline La Niña persisting through 2009.
  • Warmer than normal sea surface temperatures (SSTs) to the north of Australia, falling pressure over Australia and a positive SOI suggests good summer rainfall for much of Australia and at least a normal cyclone season.
  • The median rainfall from the ESS analogue years (1947, 1949, 1967, 1995, and 1999) is mainly indicative of average to above average rainfall across much of Australia. Confidence in this outlook will increase if Australian barometric pressure trends to lower values in coming months.

Click here for complete Growing Season Outlook.