Reviews of weather events and climate news are normally written within a few days to give time for accurate information to become available, and as I have the time to research and write it. For weather news as it breaks, I've listed reliable, organised sources on AWN's Weather and Climate Media Reports page.
Major front gives NZ snow, severe gales and high seas
Saturday 20 May saw a major cold front sweep across New Zealand bringing snow to low levels, and severe gales and high seas that peaked at 11.4m in Cook Strait.
Snow settled as low as 100m above sea level in Dunedin, and NZ Metservice reported it was deep enough to close parts of the main State Highways 1, 85 and 87 to the north, and gave 12cm snow cover at Kingston, south of Queenstown. Much deeper snow fell in the high country in the South Island Alps. On the North Island, there was a dusting of snow on the hills from Wellington to the Bay of Plenty and motorists were trapped by snow and ice on the Desert Road which gets to 1,000m SW of Taupo [Radio NZ]. Icy roads caused problems over much of the country for a few days following the change as temperatures fell well below -5° [Radio NZ]. MetService put together this montage of the country's first major snow event of 2017.
A large swell developed behind the change, driven by severe gales across the huge fetch of the Southern Ocean. A pulse of swells averaging over 6.5m pushed up the east coast of both islands on 21 May. MetService, in its Blog, said "A wave buoy in the Cook Strait has been recording average wave heights of five to six metres, with a peak wave early this morning of 11.4 metres." Not surprisingly, inter-island ferry sailings were cancelled [Radio NZ].
Monster wave one of largest recorded in southern hemisphere
The system that brought gales and snow to NZ also produced a monster 19.4m wave at the southern-most wave-measuring buoy in the Southern Ocean, moored in 150m of water 11km S of Campbell Island or 700km S of Invercargill.
The buoy, which was put in service in February by MetOcean Solutions, is designed to collect observations from an area lacking in hard data. MetOcean's Senior Oceanographer Dr Tom Durrant said "During the depths of winter, Southern Ocean waves are enormous, with significant wave heights averaging over 5m, and regularly exceeding 10m. Individual waves can double that size. Accurate measurements of these conditions will help us understand waves and air-sea interactions in these extreme conditions. This, in turn, will lead to improvements in the models used to simulate the waves, providing better forecasts, both for the Southern Ocean and for the wider region."
It's important to note the difference between "highest wave" and "highest significant wave height". Oceanographers describe highest significant wave height as the highest third of waves over a period of time. This is sometimes described as the highest averaged over 15 or 20 passing waves. A highest wave is simply the highest wave of all waves, which may be the highest within the significant wave period or a rogue wave that is far larger than would normally be expected in a normal distribution of waves.
While rogue or freak waves of phenomenal size have been reported for centuries (or not reported, as the crew didn't live to tell the tale), it has only been since two events, in 1995 and 2000, that science has begun to take them seriously. This Wikipedia article on Rogue waves has riveting reading on the history, current knowledge and research on one of the least known, most dangerous yet not uncommon aspects of the ocean.
Here's the relevant MetOcean Blog and, to appreciate the size and power of the size of waves we're talking about, look at this Royal New Zealand Navy video on Radio New Zealand. The World Meteorological Organisation maintains a list of highest significant wave heights on record globally - go to the bottom and click the "Phenomena characteristics" for more information on each one. And this BBC article has some information on high individual waves, and that other breeding ground of mighty seas, the North Atlantic.
Outlook calls for a warm, dry winter
The Bureau of Meteorology seasonal outlook for winter, released today, is forecasting a high probability of above average temperatures and below average rainfall across most of southern Australia grading to close to normal conditions across the northern half where little rain falls anyway during the dry season. Exceptions are the southeast coast and TAS, where rainfall is likely to be closer to average, and the northern half of Cape York Peninsula which can expect above average temperatures.
Driving factors are a warm Pacific Ocean and cool eastern Indian Ocean. The Bureau's climate model is suggesting further Pacific warming is unlikely, which would also reduce the possibility of an El Niño developing. However, the BoM points out that "The majority of international climate models surveyed by the Bureau still suggest El Niño will develop later this year."
An El Niño tends to produce below average winter and spring rainfall in the east of the country and warmer than average daytime temperatures in the south. The BoM says "Some El Niño-like effects may still be felt even if an event doesn't fully develop, but Pacific Ocean temperatures remain warmer than average." The other factor, the cooler than average eastern Indian Ocean, will also reduce the likelihood of much moisture streaming into Australia from the northwest.
The full Outlook is here (archive version here), and a video version is here. The latest fortnightly Bureau ENSO Wrap-Up, issued last Tuesday, is here giving more detail on the current state of the Pacific and Indian Oceans, El Niño and the Indian Ocean Dipole. The Bureau's summary of a number of international climate models is here.
Fog a sign the season's turning
Fog is a sure sign of autumn and that winter is not far away. Over the past week, fog has been widespread across the country following good rain in most areas at the weekend.
Abundant ground moisture, cold, clear and long nights and light winds are perfect conditions for the formation of fog and the country has had no shortage of them since the rain finished and high pressure systems have dominated. Around sunset, the ground begins to reradiate its heat to space, cooling a narrow layer of air immediately above it as well. If the air and ground are moist, the air in this layer soon reaches 100% humidity and a very small inversion layer forms where there is cold air below warm. Air is a poor conductor of heat, so the air above the inversion will remain warm while that below will continue to cool slowly and, if there is no wind, a heavy dew will form instead of fog. But a slight wind - 6 to 10km/h is ideal - will set up turbulence that slowly extends the inversion level upwards and fog will begin to form in a deepening layer of saturated air. This is known as radiation fog.
As the night progresses, the cold, dense air will begin to drain downhill like water, setting up its own light wind and often gradually filling valleys with fog that can be clearly seen on the first satellite images after the sun comes up. Even stronger wind, up to about 20km/h, will continue to mix the fog deeper and deeper, but over this speed the fog usually rises to become low cloud. Soon after sunrise, if the heat of the sun can penetrate, the warming ground also warms the lowest air beginning a process of drying and evaporating the fog from below which appears to make the fog "lift".
Gradually, the ground warmth and developing turbulence will break apart the inversion and the fog will dissipate. Sometimes, though, if the fog has developed to a considerable depth overnight, the sun can't penetrate to begin the warming process until late in the morning when the higher sun will find tiny gaps in the cloud and will also attack the fog around the edges of the main fog area. That happened this morning around and north of Canberra, where a large shield of fog could be seen on the satellite images until early afternoon.
Perth, Adelaide, Launceston, Melbourne, Canberra, Sydney and Brisbane have all experienced fog during the past week with visibility as low as 50m in places. Large areas from the coastal valleys to the inland slopes in the east and the coast to the Great Southern in WA have also had fog. Brisbane had fog on Sunday, Monday and Wednesday, while Perth enjoyed its fog on Tuesday after receiving 48.6mm over the five days to Wednesday 24 May, the heaviest rain by far since an unseasonal dump of 114.4mm on 10 February.
Unusual May rain event covers eastern Australia from Gulf to Tasmania
A sweeping band of rain that began on Wednesday 18 May in tropical north QLD and the Gulf Country and travelled south to cross four states and the ACT finally exited the East Coast of TAS on Sunday morning after taking over four days to make the journey. What makes it notable is the expanse of country it traversed, almost the whole nation from north to south, and this in a month not normally associated with large-scale rain bands or events.
Many people have commented that the warnings and forecasts ahead of the event were overstated, and it is true that in some areas the expected rainfall amounts didn't happen. Yet widespread May one-day rainfall records were broken in the 24 hours to 09.00 on each of 18 May, 19 May, 20 May and 21 May, and many locations had more than their monthly average rain in 24 hours.
BoM Senior Forecaster Scott Williams commented, as the event was beginning, that "it's unusual to get a band that extends right across the country this late, outside the traditional wet season. The average rainfalls in western QLD, western NSW and northwest VIC for May are only about 25 to 40mm, so this event in many places should deliver more than the average monthly rainfall in May." He added that further east, as well as in VIC and TAS which are moving into winter rain in May, amounts are normally heavier anyway. "So the real unseasonal rain with this is the northern parts over inland NSW and QLD in particular."
The BoM rainfall map for the week to 09.00 on 21 May shows the distribution of the rainfall well. Very little rainfall, other than from this system, fell in the week. The map clearly shows the patchy and irregular nature of the rain, which is why some people were commenting that the forecasts had overstated the expected rain. Many of the areas that received heavy falls were under thunderstorms or one of the long, rather thin but long-lasting rainbands that were the main mechanisms for delivering the rain. As a result, while some areas received no real rain to talk about, many from the north to south of the country had their heaviest May rain in decades.
The heaviest rain was on the QLD tropical coast and inland to the Gulf. On and near the coast between Townsville and Mackay, many locations received between 150 and 250mm in the 48 hours to 09.00, Friday 19 May, with the heaviest of the official or hydro recordings 244.6mm at Alva Beach. Bowen Pump Station gauge was not far behind on 220mm and minor flooding was reported in the Don River inland form the city. Over the two days, Townsville recorded 154.8mm (May average 39mm).
Some notable long-term records were broken. Mingela Post Office, in the hills 70km SSW of Townsville, which has continuous rainfall records going back 1899, set a new record on 18 May with 108.0mm, then surpassed that on 19 May with 110.6mm. Other records on 19 May at stations with long histories were Ravenswood, near Mingela, 127.4mm with records back to 1887, Townsville Aero 115.2 (1941), Collinsville 120.4 (1939) and Croydon 118.0 (1889). Croydon's previous record was only 63.5, set in 1956, and its average May rainfall is 7.8mm. For the statisticians, Croydon's median rainfall is a mere 0.4mm.
While the wet weather was winding down in northern QLD on Friday 19 May, rain and thunderstorms were pushing into western NSW and VIC and across to the SE QLD coast. During the day, these spread across much of NSW and VIC with the heaviest rain in a band that spread from SW NSW and NW VIC to the NSW southeast. Even far southeastern SA saw some action with falls over 25mm E and SE of Murray Bridge; Parrakie's 38.0mm was the highest.
While VIC had no rain in record territory, NSW broke many one-day rainfall records over Friday and Saturday with the most significant 56.0mm to 09.00 on Friday 19 May at Gunbar in the far western Riverina (records back to 1949) and 51.2mm (1971) at Berriedale west of Cooma to 9am Saturday 20 May. Canberra had a drenching with 33mm in the city centre to 09.00 Saturday, which triggered emergency crews to prepare sandbags [The Age]. Tuggeranong recorded 52.2mm, its highest May total in a 22-year history, and Mt Ginini, on the Divide west of the Capital, recorded 88mm in the same period. The heaviest falls in SE NSW were on the South Coast where Central Tilba recorded 151.0mm, the highest since it began recording in 2003.
Rain began falling in NW TAS overnight into Saturday 20 May, and by 09.00 had deposited a record 50.2mm in the gauge at Marrawah (records began 1971) and 61.6mm at Smithton, the highest May fall since records began in the town in 1911. During Saturday and into Sunday, the rain spread across NE TAS and Flinders Island and down the East Coast with most locations picking up 50 to over 120mm.
Flinders Island Airport recorded 73.8mm to 09.00 Saturday, its heaviest May fall since 1969, and a further 36.6mm during the day. Notable new records were set in the NE to 09.00 on Sunday 21 May where Fingal recorded 102.0mm (records began in 1888) and Gladstone 83.0mm (1914). The heavy rain in the northeast produced minor flooding in the South Esk River. Only ten emergency callouts were required in the north, but blustery winds helped produce widespread though small power outages [The Advocate].