The fact that Australia is enduring an unprecedented warm spell is hardly breaking news to anyone these days. Rather than feeling like we are two weeks from the beginning of winter it feels like we are still at the tail-end of summer. Further east, the Pacific Ocean has been steadily warming up over the past months. The heating effect has not only affected the ocean surface (which you would expect with a warming atmosphere over it) but also in the sub-surface. In fact, the oceanic temperature of the equatorial Pacific at depth (top one kilometre of the ocean) has reached unprecedented levels. Last March saw the warmest subsurface waters for that month since 1979 while April was the second warmest, only second to April 1997. International forecasting models are all in agreement that the oceanic temperatures will continue to warm up over the coming months reaching El Nino thresholds by early summer. Some models even have these temperatures reaching the threshold as early as winter. Now, the 3 most important thing to remember is that: 1) El Nino - Southern Oscillation (ENSO) is a coupled phenomena and has an oceanic component as well as an atmospheric component (the latter being more difficult to forecast so far in advance). The main atmospheric response being the weakening of the trade winds across the Pacific. 2) Although El Nino events are associated with below average rainfall (weaker trade winds bringing less moisture from the Pacific Ocean) in eastern and southern Australia and above average temperatures, not all El Nino events have the same effects on Australian weather. 3) There is no direct correlation between the intensity of an event and its effects in Australia, i.e. a strong El Nino might not necessarily bring a significant drought or extreme heat while a weaker one might. Currently, there is a lot of research going on to differentiate between different types of El Ninos depending on where the warming is occurring in the tropical Pacific Ocean. In the short term, this warm weather will continue over southern and eastern Australia for much of the week with a high pressure system over the Tasman keeping most cold fronts at bay. During the weekend a front will sweep across southern parts of the nation bringing some cooling. This system is likely to bring daytime temperatures down to about average, with a drop in night-time temperatures being the most significant.