For the second time this May, Australia's normally mobile weather systems have ground to a halt.
The lack of movement on the charts during the coming week will lead to another prolonged spell of invariable day-to-day weather, including a record run of unseasonable warmth across Western Australia, another spell of showery days for parts of the eastern seaboard, and an extension of the extreme-dry finish to autumn for South Australia and northern Victoria.
The source of the abnormally stable conditions is again a "blocking high" — the name given to strong high-pressure systems which stall for an extended period and block the usual eastward movement of weather.
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High-pressure system replicating previous two-week pattern
A glance at the current weather map shows a near identical set-up to the first fortnight of the month – a strong high south of the mainland and the absence of other notable features near Australia, a configuration which is likely to remain unchanged until at least next Thursday.
Analysing the position of the jet streams also shows a similarity – the subtropical and polar jets are splitting west of WA, allowing the high to remain cradled over Australian longitudes with little wind to keep the system moving.
The high should finally become mobile again late next week, however almost immediately a new blocking high may form in the Tasman Sea.
This potential third near-stationary high would bring another period of uniform weather through the last week of May, although it's likely position further east would at least bring a change to the prevailing weather during the current block.
Can NSW expect further heavy rain and flooding?
With another inactive high to the south, it's no surprise showers are on the forecast for the NSW coast into the foreseeable future, but will the drenching match the previous event?
The answer is thankfully no, due to the system's location west of Adelaide, as opposed to the earlier blocking pattern when the high was west of Tasmania.
This change in position may seem inconsequential considering the system's impact stretches more than 5,000km, however because winds spin anticlockwise around a high in the Southern Hemisphere, the small shift north will lead to a slight deviation of winds across eastern Australia from a south-easterly to a southerly (as depicted on the map below).
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Since southerlies blow near parallel to the NSW coast, compared to a south-easterly which is directly onshore, they bring firstly, less rain, and secondly, its mostly confined to the coastal fringe.
Take Sydney for example — the first two weeks of May dumped 213mm of rain to Observatory Hill and 61mm at Penrith, while the next week should deliver well below 50mm to the city and just a few millimetres to the outer western suburbs.
Despite lighter falls during the coming week, the ongoing wet weather for the state is breaking May records.
Jervis Bay has already collected 532mm this month, already the highest May total since observations commenced in the region back in 1900.
Hillston in south-west NSW has received 124mm this month, also a record, with data for the town dating back to 1882.
WA warmth rolls on as Perth annihilates records
The ongoing location of the blocking highs to WA's east are leading to a predominant warm north-easterly on the west coast, setting the platform for Perth's warmest May on record.
The city has averaged 26C for maximums so far this month, well above the long-term May mean of 21C and on track to easily surpassing the old record of 24.4C from 2009.
The run of warm May days is so unusual for Perth that since records began in 1897 the most consecutive 25C plus days in May before this year was eight in 1985 and 2009, a spell annihilated by a current run of 11 days which should extend to 16 days by Wednesday.
The unseasonable warm month also follows above-average temperatures through March and April and should ensure Perth achieves an average maximum this autumn around 28C, breaking the old record of 27.5C from 2011.
Parts of SA, VIC suffer through dry May
When the weather blocks up, there are always regions which experience inordinate spells of dry weather.
Since the stationary highs are preventing rain-producing cold fronts from reaching the mainland, it's currently the southern states missing out.
Adelaide is a prime example with the official Bureau of Meteorology rain gauge yet to measure any precipitation this month, well below the city's May average of 67mm and possibly on track to remain below the current record of 2.6mm from May 1934.
Across the border, northern Victoria is also suffering from a lack of fronts – much of the Wimmera has received less than 1mm this month, which will become the lowest May total since 1934 if the last week of May fails to deliver significant rain.
