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Showers and storms in WA's west, SA's east, NSW's west & QLD's southwest in unstable air are most intense in WA's southwest. Showers in the NT's eastern Top End, QLD's east & NSW's east with help from easterly winds. A high is keeping much of the remainder of the country dry.
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Today, 7:05AM UTC
Will another positive Indian Ocean Dipole brew in 2024?
Signs are pointing to the second consecutive positive Indian Ocean Dipole (IOD) developing in the next few months. However, May is shaping up to be a time to make-or-break this event. What is a positive IOD? The IOD is a coupled ocean-atmosphere climate driver that changes the circulation patterns over the Indian Ocean. A positive IOD occurs when colder waters are upwelled (brought from below to the surface) in the eastern Indian Ocean near Jakarta, forcing warmer waters to travel west, where they build up near Africa. The cold water in the east reduces atmospheric moisture, leading to clearer skies and stronger southeasterly winds in the eastern Indian Ocean and the Australian continent. These winds then help to continue the upwelling of more cold water. Meanwhile, near Africa, warm waters boost convection, increasing rain and thunderstorms, helping draw more air in from the east. This positive feedback loop allows an established positive IOD to become stronger and self-sustaining. Positive IOD events typically reduce the amount of moisture that flows from northwest Australia to the southeast of the country. This leads to clearer skies, reduced rainfall, and increased daytime temperatures during winter and spring. Minimum temperatures typically increase in spring throughout Australia with a positive IOD, but they decrease in the southeast during winter under clearer skies, increasing the occurrence of frosts. Recent positive IOD events were in 2019 and last year in 2023, both of which featured very dry periods in late winter to mid-spring. Note that if a positive IOD forms this year, it will be the first time since records began in 1960 that we have seen two consecutive positive IOD events. Image: Standard rainfall anomaly over Australia from August to October 2023 as the positive Indian Ocean Dipole developed. Source: Climate Change Institute, University of Maine How close are we to a positive IOD? Like the Niño3.4 index used to monitor the El Niño Southern Oscillation in the Pacific Ocean, the Indian Ocean has an equivalent index, simply named the IOD Index. This index compares the relative ocean temperatures in the western (near the Horn of Africa) and the eastern (near Jakarta, Indonesia) Indian Ocean. When this index is above +0.4°C for at least 3 consecutive months, it is a sign that a positive IOD is underway. Over the past 7 weeks, the IOD Index has been above +0.4°C and is currently sitting at a value of +0.68°C. If the index keeps this up over the next 5 or so weeks, it is likely that a positive IOD event will be declared. How certain is it that a positive IOD will form? A critical time in the development of this IOD event is approaching in mid-to-late May. This event is a strong pulse of the Madden-Julian Oscillation, a pulse of tropical activity that circumnavigates the globe near the equator. During an active pulse, tropical activity increases, and the result can alter both ocean and atmospheric patterns. It is well documented and researched that this kind of tropical activity can act for or against particular phases of the IOD. Over an active area of tropical activity, especially if a tropical cyclone forms, air is constantly rising, and cold water is helped to upwell underneath. If this converging air and cold-water upwelling occurs in certain locations in the Indian Ocean, it can amplify or dampen the establishing IOD circulation. For example, if a tropical cyclone develops in the Bay of Bengal during the months of April or May, there is a very high likelihood that a positive IOD event will occur in that year. This is because it draws moisture away from the 'upwelling zone' labelled below, setting up the circulation necessary for a positive IOD. Around mid-May, a pulse of the MJO will pass over the eastern Indian Ocean. If significant tropical activity occurs near India or near Australia, this positive IOD event will become far more likely to occur. Conversely, if significant activity occurs over Indonesia, and specifically in the 'upwelling zone' near Jakarta, it may act to hinder the formation of the positive IOD. The occurrence of this tropical interference leads to the 'autumn predictability barrier', a term to reflect how climate driver forecasts at this time of year have reduced accuracy. Any tropical activity forming in particular locations in the Indian Ocean, as well as other ocean basins, may interrupt the establishing circulation, and lead to a sudden shift in climate driver expected for the remainder of the year. This barrier applies in the Pacific Ocean with the El Niño Southern Oscillation for the same reasons. So far in 2024, there are strong signs that a positive IOD is forming, with southeasterly winds in the east, and warm waters built up in the west. However, it is too soon to say with certainty that one will develop this year, but we should have a good idea if it will by the end of the month.
Today, 5:06AM UTC
Rare rainless Agfest thanks to blocking high
Tasmania’s Agfest is enjoying an unusual run of dry, sunny and mild weather this week thanks to a big blocking high pressure system sitting over the Great Australian Bight. Agfest is a 3-day agriculture field day that attracts 60,000 people to the Carrick region in Tasmania each year. Those attending Agfest usually only need two things to enjoy the show: a ticket and an umbrella. Agfest is well-known for typically having cold and wet weather at some point during the event. This is because cold fronts usually start to become a more prominent feature over southern Australia in May, increasing the likelihood of showers and cold air outbreaks. This year’s show, however, is enjoying a spell of dry and mild weather thanks to a large high pressure system centred over the Bight, which is blocking any cold fronts from reaching Tasmania. Image: Agfest on a sunny morning in 2010. Source: Peripitus via Wikimedia Commons Launceston, which is located about 19 km northwest of the Agfest showgrounds, is forecast to reach highs of 17 to 18°C under mostly sunny skies on each of the three days during this year’s event. This is a couple of degrees warmer than average for this time of year. The dry and sunny 2024 Agfest weather is a contrast to last year’s event, which had days only reaching 12 to 15ºC while also picking up 5 mm of rain. This week’s dry weather continues a run of lower-than-average rainfall that has affected much of Tasmania over the last three months. Between February and April, large areas of the state registered rainfall totals that were in the lowest 10 percent of historical records. Looking ahead, the Launceston region may not see any decent rain until at least the middle of May as blocking high pressure remains a dominant feature to the south of Australia.
Today, 12:42AM UTC
400km line of severe thunderstorms lashing southwest WA
A line of severe thunderstorms is moving onshore towards the southwest of WA, which could produce heavy rainfall and large hail on Thursday morning. These thunderstorms are forming on a trough offshore ahead of an approaching cold front and sweeping across the region. The image below shows a shelf cloud over Bunbury on Thursday morning. Credit:@the_greenhouse_goddess Cape Naturaliste has recorded 12.2mm in the half an hour leading up to 8am WST on Thursday. The image below shows a 400km long line of thunderstorms moving into the state’s southwest on Thursday morning, in a ‘train-like’ fashion as the storm cells follow each other's path. Image: Himawari satellite image showing cloud, rain and lightning in the three hours leading up to 7:40am AWST on Thursday, May 2. This movement thunderstorms is commonly known as the ‘train effect’, which is when thunderstorms and the associated heavy rainfall repetitively impact the same location. This train effect could cause flash flooding in the region over the next few hours. These thunderstorms are also moving over a very dry landscape, which could increase this risk of flash flooding further. The map below shows the forecast rainfall on Thursday, with widespread falls of 5-15mm forecast in the Central West, Goldfields, Central Wheat Belt, South West and Southern Coastal districts. Pockets of heavier rain are also possible. Image: Accumulated rainfall forecast for the day leading up to 8pm AWST on Thursday, May 2, according to Access-G Looking ahead, rainfall should ease on Friday and Saturday across WA, however a low pressure system near South and South East Coastal districts on Sunday and early next week will bring another burst of rainfall to those districts.
Weather in Business
23 Apr 2024, 12:07AM UTC
Wind returning to southeastern Australia
After a prolonged period of light winds, a surge of powerful winds and cool air will push through southeastern Australia during the next 48 hours. As we wrote about last week the National Electricity Market (NEM) saw very low wind power generation during the seven days leading up to Thursday, April 18, with the daily wind generation having been under 41 GWh/day for this period. Wind power has improved slightly during the past week, with wind power increasing for a few days over last weekend. The chart below shows that the winds across the NEM have already increased in response to a cold front approaching SA and Vic on Tuesday. The increase in wind power is associated with a cold front sweeping across southeastern Australia, after a stubborn high-pressure system forced many cold fronts south for over a week. The map below shows the cold front marching across the Bight, approaching SA on Tuesday morning. You can also see a broad area of speckled cloud in the Bight, which is a tell-tale sign that a large mass of cold polar air has broken away from the Antarctic region and is venturing into the relatively warm mid-latitudes. Image: Himawari-9 satellite image at 9:30am AEST on Tuesday, April 23. Northwesterly winds will continue to strengthen ahead of the cold front on Tuesday in SA and Vic before a strong southwesterly change hits Adelaide on Tuesday afternoon and Melbourne in the evening. The images below show the wind gust forecast for Tuesday and Wednesday afternoon. Image: Instantaneous wind gust forecast at 1pm AEST on Tuesday (top) (bottom)and Wednesday, April 23 and 24, according to ECMWF While most of this wind should hit the wind power ‘sweet spot’, cut outs could occur at southeastern Vic and southern NSW wind farms late Tuesday into Wednesday. Generally, cut outs occur when wind speeds exceed 90km/h (25 m/s), which is when the wind turbines are turned off to prevent damage. In the image above you can see the front and associated southerly change moving up the NSW coast on Wednesday, hitting Sydney on Wednesday afternoon. Winds will then become light later this week as another high-pressure system moves into the Bight. Cool change coming Ahead of this change, much of southeastern Australia has experienced pleasant autumnal weather, with Sydney’s temperature for the next two days reaching 27°C. However, there will be a large temperature contrast ahead and behind the cold front in the next few days. The image below shows forecast air temperatures around 1.5km above sea level on Wednesday morning, showing the cool airmass moving over parts of SA, VIC, Tas. Behind the cold front strong southwesterly winds will cause temperatures to plummet to around 2-3°C below average across southern Australia; Melbourne and Adelaide's temperature dropping to around 16-18°C for several days following the front. Hobart’s temperature will drop from 22°C on Tuesday to 15°C on Wednesday. Sydney’s temperature will go from 5°C above average on Wednesday to average on Thursday. The coolest air will move over Tas, bringing snow to the alpine region there. Looking ahead, temperatures will recover later this week ahead of another cold front and burst of wind power early next week.
18 Apr 2024, 3:20AM UTC
Generation gone with the wind
A weeklong stretch of low wind power was broken on Wednesday evening, as a weak cold front marched across southern Australia. The chart below shows that the National Electricity Market (NEM) has endured a weeklong stretch of low wind generation, with wind power providing around 6% of the NEM’s electricity, down from last year’s average of 13.1% (Mon, April 15 to Wed, April 17). The daily wind generation has been under 41 GWh/day for the last seven days leading up to Thursday, April 18. The calmest winds occurred on Sunday, April 14 with only 24 GWh/day produced or 4.6% of grid demand. To put this in perspective, the average daily wind generation during the last year up to Wednesday, April 17, 2024, was 71 GWh/day, according to data from the open NEM. This prolonged period of low wind was caused by blocking high pressure systems sitting over southern Australia, forcing cold fronts or strong wind further south. The image below shows the Mean Sea Level Pressure chart on the calmest day, Sunday, April 14, with high pressure stubbornly sitting over the southern half of the country. Image: Mean Sea Level Pressure Analysis at 4am AEST on Sunday, April 14. Source: Bureau of Meteorology. As we mentioned earlier today, a positive Southern Annular Mode (SAM) has led to consistent high pressure systems positioned over latitudes well south of Australia during the first three months of the year and during this week. This has kept cold fronts south of the mainland, and even south of Tasmania. READ MORE: WHAT IS THE SAM? The period of light winds ended abruptly on Wednesday evening, with WattClarity reporting that wind production peaking above 3,000 MW at 9:10pm AEST. The increased wind power on Wednesday night, was caused by a couple of factors; A weak cold front swept across the southeast on Wednesday, bringing stronger winds. A very shallow temperature inversion formed in the early evening protecting the surface from the stronger winds, but the wind turbine hub-height (150 metres above the ground) winds remained strong at around 28-38 km/h in SA. Temperature inversions form at night when the ground cools quicker than the air above it, meaning that the temperature increases with height for a thin layer of the atmosphere. During autumn, the temperature inversions are commonly shallow meaning that while the wind could be weak at the surface, the hub heights could continue to experience strong winds. A similar phenomenon could happen again tonight across southern Australia, with another cold front passing across the south. Looking ahead, light winds are expected to continue from Friday as a stubborn high-pressure system builds over the Bight until early to mid-next week.
