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Thursday 22 December 2016

Thursday 22: Neutral climate indicators point to variable rainfall

Most international models are pointing to continued neutral conditions for the main climatic drivers of our weather over the next few months. The BoM's fortnightly ENSO Wrap-Up shows the Pacific Ocean remains balanced between El Niño and La Niña conditions, the Southern Oscillation Index continues neutral resulting in more high pressure systems than usual for this time of year over southern Australia, and the Indian Ocean Dipole, which typically has little impact on Australia during summer or early autumn anyway, is also in neutral territory.

Much interest therefore turns to the two other significant drivers of our weather at this time of year, the MJO and sea surface temperatures around the continent. The Madden-Julian Oscillation is often the main driver of the monsoon for us, though the BoM's Weekly Tropical Climate Note points out that MJO is weak and is expected to remain so. The BoM says that it "is unlikely to have had a significant influence on the development of the [current] monsoon trough. Instead, the recent development of the monsoon was mostly driven by a surge of northeasterly winds from the northwest Pacific and South China Sea. These winds crossed the equator and fed into the developing monsoon trough in the southern hemisphere. At the same time, enhanced westerly winds across the eastern tropical Indian Ocean provided further energy for the deepening of the monsoon trough."

Also of interest are two areas of well above normal sea surface temperature: a broad area around the whole NW of the continent and a smaller area in western Tasman Sea off NSW and southern QLD. They show up well in this weekly animation. The northwestern area, in combination with the monsoon, has the potential to enhance rainfall and the chances of tropical cyclone development, while the Tasman Sea warm SST can enhance rainfall along the nearby coast and eastern ranges in the right circumstances.

The Rural Bank provides a monthly Weather Update presented by meteorologist Jane Bunn and tailored for farmers. These explain the impact of these climatic drivers on our likely weather in the next few months. The presentations provide very useful 5-minute summaries, one for each state, and the December ones have just been released.

Thursday 22: Heavy rain in WA Kimberley as TC Yvette approaches

With Tropical Cyclone Yvette still well out to sea but approaching, torrential downpours have been reported from across the WA Kimberley and nearby areas in the northwestern NT as the monsoon makes its presence felt. In the 24 hours to 9am today, there were many reports of 100 to 150mm of rain with the highest in the Kimberley to the NE of Fitzroy Crossing where Yulmbu registered 175 and Siddins Creek 218mm. In the NT Labelle, SW of Darwin, was highest on 169mm. For some of the rainfall stations in the Kimberley, it was their highest December 24-hour total on record.

As of 2pm WST this afternoon, Yvette continued to meander about 800km NW of Port Hedland with a central pressure of 990hPa, but is expected to begin a clear movement to the SE tomorrow and cross the coast between Port Hedland and Broome around Christmas Day as a category 1 cyclone, the lowest category. The BoM is currently issuing information bulletins, which will turn to warnings as Yvette comes closer to shore and its movements are better known, as they are still far from certain. A forecast track map is also available and, for those that like more technical detail, there is the High Seas Warning, and even more detail is available from the US Navy Joint Typhoon Warning Center.

Wednesday 21: Prolonged rain brings flooding in Fiji, Vietnam and Indonesia

A tropical depression that has hung around Fiji during the past week has brought heavy rain, strong winds, flooding and landslides across many areas, and the whole country is subject to heavy rain and flood warnings. Rakiraki, on the northern tip of Viti Levu, recorded 268mm over a 24-hour period during 18-19 December and 182mm the previous 24 hours. There are traffic disruptions and power cuts, and the National Disaster Management Office of Fiji says 1,545 people have been evacuated across most Divisions in the country as of 21 December. The Fiji Meteorological Service indicates that heavy rains have now eased.

Heavy rain over the past week in Vietnam, following similar rain earlier in the month, has caused widespread flooding in central and southern parts of the country. As of 21 December, the UN reported that 26 people had been killed and over 11,850 houses damaged in the provinces of Thua Thien Hue, Quang Nam, Quang Ngai and Binh Dinh, while there were unconfirmed reports that nearly 200 square kilometres of paddy fields had been submerged. Hue, 75km up the coast from Da Nang in central Vietnam, recorded 405mm over three days to 16 December after recording 346mm over four days in the previous week. Further rain was expected for at least the next 24 hours.

There has also been heavy rain in Indonesia, especially in the provinces of Lampung and in North Sulawesi, where five people died in the city of Manado at the far northern tip of Sulawesi, formerly known as Celebes.

Thursday 22: Newsbits

What is the monsoon?: With the monsoon, and a Tropical Cyclone that often accompanies one, very much in the news at present, here's an article in The Conversation that acts as a good introduction as well as going more deeply into the science behind the Australian monsoon.

Know your clouds and wind: I've come across these two very useful posters from New Zealand's MetService that will be handy for all weather-watchers. The first shows common cloud types, with useful information on each type and the weather that it presages. The second shows the full World Meteorological Organisation wind scale, from calm right up to the speeds you'll only find in the worst tornadoes and the strongest known cyclones, and well beyond the standard Beaufort Scale which runs out at a mere force 12. It describes the effect of the wind on the sea, on trees, and on objects on land with illustrations for each level. The posters are in Acrobat pdf format, and of course apply equally in Australia as in New Zealand.