Hot and dry to the end of 2019, but hope is on the horizon
Thu 26 Sep 2019
The BoM's Climate Outlook was issued this afternoon with heat and little prospect of rain in most parts of the country, but a ray of hope towards the end.
For October to December it's expected to be drier than average for much of Australia apart from the northern half of WA, where decent falls from wet season buildup thunderstorms are likely from November. Daytime temperatures are very likely to be warmer than average across the whole country with only the western half of TAS seeing cooler conditions. The situation is more complex for overnight temperatures, with warmer than average minima expected over WA and in a band from the QLD Gulf Country to the NSW coast, but cooler than average temps in the southeast, along the QLD coast and in the NE Top End.
The main climatic drivers behind the BoM's reasoning are that a positive Indian Ocean Dipole (IOD) will persist for a few months, pushing rain-producing influences well west of the country. Meanwhile a negative Southern Annular Mode (SAM) will continue to bring Southern Ocean low pressure systems closer to southern Australia, with approaching fronts that drag hot, dry inland air southeastwards ahead of them. So it's a double-whammy of no moisture from the Indian Ocean and frontal changes across the south of the continent that produce heat ahead and little rain behind.
The interesting feature of the Outlook is the new forward view into January 2020 which shows up in 3-month expectations for November to January. If you compare the Oct-Dec to Nov-Jan outlooks, you can see the breakdown of the IOD beginning. In Nov-Jan, most of the continent north of a line from Perth to Townsville is likely to have median rainfall, and a little more in northern WA. That suggests January is likely to see good rains across the tropical north after a late-breaking wet season. Below that line, the likelihoods of below average rain are still below median, but not as dire as in the Oct-Dec quarter suggesting improved rainfall generally during January.
Comparing Oct-Dec to Nov-Jan temperatures to gauge the impact of January on quarterly figures, daytime temps continue with a high probability of being above average across the continent, but become close to median in the Kimberley, NT and far NW QLD. That is because of the wetter conditions in this area. By the Nov-Jan quarter, minimum temps have warmed compared to Oct - Dec, indicating business as usual in January with warmer nights in the southeast and on the QLD coast, and significantly warmer nights in the NT Top End. The moister air accompanying rain can be thanked for these.
The Outlook is here and the video Outlook is here.
For those in the tropics, the Weekly Tropical Note issued on Tuesday says "A marked increase in humidity and the first thunderstorms for several months across the Top End of the Northern Territory and northern Western Australia hint at the start of the transition from the dry season to the northern wet season, which officially commences 1 October." Given the forecast late start to the true wet season, it remains to be seen how long the stormy buildup lasts, but on average the onset of the wet in Darwin is in the last week of December.
A notable tropical event has been a mid-level trough crossing central Australia early in the week which gave 10 to 30mm of rain to large parts of the southern NT and eastern Kimberley. Parts of these areas have not seen any rain in three months.
BoM issues Special Climate Statement on NSW/QLD fires. Thu 26 Sep 2019. The BoM has issued one of its detailed analyses on the weather surrounding the disastrous fires in SE QLD and NE NSW in the first half of September. All the ingredients of the worst fire weather were present: high temperatures, very low humidity, gusty winds and, for some locations, the driest January to August period on record resulting in a buildup of tinder-dry fuel. The SCS is here (pdf), and a list of all the BoM's interesting SCS reports is here.
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