The first major westerly change of the season swept across south east Australia at Easter, with the Southern Annular Mode swinging to the negative. However, this may be only temporary. The transition into a full El Nino is expected to be fairly slow over the coming months so its effects will be quite varied initially. In general, rainfall over winter and spring in much of eastern Australia is likely to be a little below normal - quite a change to the conditions of last year. However, often in these situations there can still be the occasional brief but significant rain event which will locally boost totals towards and even above average. This is partially because at least in the next month or two, such a pattern can give rise to increased upper troughs and given the warm temperatures still present in the Coral and Tasman seas, this means a slightly increased chance of East Coast Lows developing. Short of a random, extreme event occurring, average rainfall is favoured for the rest of autumn before progressively tending drier from later in May onwards with a slightly higher risk of early frosts emerging as the lower atmosphere becomes drier too. As always, the Indian Ocean will also have an effect. Most medium-term models suggest that a positive Indian Ocean Dipole event may develop in the coming months. Long-range forecasts of IOD conditions made in autumn have lower accuracy than those made at other times of the year, but a positive IOD decreases rainfall potential in the south east states in late autumn and winter due to it reducing the number of so-called north west cloud bands. The effect of these developing patterns on temperatures will be more interesting. Initially, such developments will likely bring slightly below average temperatures in south east states in late autumn and the early half of winter. It can result in the above mentioned sharp upper troughs bringing brief bursts of colder temperatures as well. Often these types of patterns can result in a reasonable start to the snow season in the Snowy Mountains given the right elements coming together in the region. In addition, and later in the year, such a pattern can also result in northern Australia warming up more quickly than normal towards the end of winter and into spring. The implications here (other than a preliminary indication of an earlier start to summer) is the concerning bushfire potential that may emerge next spring and early summer. After such a long period of high growth from the high amounts of rainfall, hotter and drier conditions will potentially be concerning.
