Forecast

Issue Notes - Issued Oct 1

ENSO status: La Niña Watch. IOD status: Neutral. SAM status: Negative. Trending slightly negative. The El Niño Southern Oscillation (ENSO) is in a La Niña Watch, meaning there is approximately a 50% chance of a La Niña developing this year. Overall, models are now starting to look more like La Niña, with more widespread ocean cooling occurring in the South Pacific, helped by the consistently strong trade winds. While these are good signs that La Niña is starting to develop, a declaration would likely happen no earlier than November, meaning this is more likely to be a late and weaker event if it does eventuate. A La Niña typically increases rainfall over northern and southeastern Australia during spring, but has little effect for southwestern WA. The Indian Ocean Dipole (IOD) is in a neutral phase and is now unlikely to develop into either phase this spring. Conditions are closer to a negative IOD at the moment, so there may be some minor increases in the frequency of northwest cloudbands impactingAustralia. A neutral IOD tends to bring a small increase in spring rainfall for southwest WA, but has little effect elsewhere. The Southern Annular Mode (SAM) is in a negative phase, continuing on from the initial sudden Stratospheric Warming (SW) events over Antarctica since mid-July. These events increase the chances of negative SAM events, but are having a lesser effect moving forward in October. A negative SAM increases the frequency of cold fronts. During spring, this increases rainfall over western Tas and southern Vic, but decreases rainfall over eastern Australia. Rainfall outlooks are showing close to average for most of the country during October, with a higher chance of above average rainfall for northern and eastern Australia, increasing further into November and December. Models are also hinting that rainfall is more likely to arrive in the form of showers and thunderstorms, rather than broad rainbands, meaning rainfall in practice will likely be hit-and-miss for most of Australia thisspring.