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Tuesday 31 January 2017

All quiet on the El Niño/La Niña front

The Bureau's ENSO Wrap-Up, issued today, shows that the major teleconnections* that affect Australia continue to be neutral and are likely to stay in neutral territory at least to the end of autumn. "Most models surveyed expect the tropical Pacific Ocean to warm during this period," says the BoM, "meaning La Niña [with above average rainfall] is the least likely scenario for winter/spring 2017."

There is a caution. "It should be noted that model outlooks that span the southern autumn period tend to have lower accuracy than outlooks issued at other times of the year. This means outlooks beyond May should be used with some caution." Note that the Indian Ocean Dipole gets little mention as the movement of the monsoon trough into the Australian area, typically between December and April, prevents the IOD forming.

An additional rider is that a "neutral ENSO state does not necessarily mean average rainfall or temperature for Australia. Rather it means that ENSO patterns are not driving Australia's weather toward generally wetter [La Niña] or drier [El Niño] conditions. Other shorter-term or smaller-scale climate drivers may dominate and hence influence Australia's climate." The most important "shorter-term or smaller-scale climatic driver" that has been making its presence felt in the past month has been the monsoon trough which has been bringing heavy rain in northern NT and WA, and affecting pressure distribution, heat and weather conditions in the south.

*If you're struggling with what all this teleconnection stuff is about, the BoM has some very helpful explanations on the drop-down menu you get by clicking the button to the right of the tabs.

Severe weather affects all continents in the past few weeks

AWN's international news coverage has fallen behind recently, so here is a review of major events around the globe in the past fortnight.

Europe: Winter returned with a vengeance

Wintry weather returned to Mediterranean Europe between 18 and 20 January, only 10 days after the massive snow event that embraced most of the continent (see AWN 9 January and 11 January). Unlike that event, which moved gradually away to the east, this one was slow-moving as it was caused by a near-stationary deep low pressure system that remained west of Italy for around a week.

Snow fell to sea level on the Costa Blanca in SE Spain at Torrevieja for the first time in 118 years, while a little inland at Murcia it snowed for three hours leaving several centimetres on the ground, the first snow since 1939. Snow spread across the water to Algeria where it fell lightly at Bejaia, east of Algiers (and see the Africa report below). Heavy snow fell in Sicily, and was a metre or more deep where this photo was taken just below 1800m on the slopes of Mt Etna. Al Jazeera produced this photo selection of snow in Mediterranean countries during the event, while this photo, taken on an ice-choked Danube River in Belgrade, Serbia, show that farther inland a thaw is still far away.

With warmer weather moving into the Mediterranean around 20 January, violent thunderstorms wracked the area around the low. Sardinia, Sicily and Calabria (the toe of Italy) were badly hit by strong winds, waterspouts, hail, torrential rain, flash flooding, main river flooding, and landslides with one death on Sicily according to the Italian News Agency ANSA (summary in English on Floodlist). Licata, on the SW coast of Sicily, recorded 162mm in three hours in one downpour.

Over the United Kingdom, high pressure brought clear skies but heavy freezing fog and frost in London and the south. The Guardian reported widespread problems on the roads from 22 to 25 January with many flight cancellations at all airports in the area.

AWN has also learnt that during the earlier storm, on 13 and 14 January, over 550,000 homes were protected by east coast flood defences in the UK. An alignment of strong winds driving waves of over 7m, high tide and a storm surge (caused when very low barometric pressure causes sea levels to rise) had authorities on full alert. However, the danger was not as bad as expected because the alignment did not coincide in all areas, and the major flood defences that exist along the UK east coast all did their job.

North America: Drought eased in California while major winter tornado outbreak hit southeast USA

Record mountain snowfalls and widespread river flooding in northern California have helped ease drought conditions there though, state-wide, the drought is still still far from over. California's drought, which has been getting steadily more severe, has been in progress for nearly 10 years, apart from a two-year respite between 2009 and 2011. During the second week of January, a succession of storms dumped rain or snow across the state, causing flash flooding, river flooding in 15 streams, mudslides, avalanches and power blackouts. Mammoth Mountain, 300km E of San Francisco, has had a record 6.25m of snow to 25 January according to WeatherNation.

The California Department of Water Resources said in a press statement, "Although October through December rains in Northern California provided an encouraging start to the 2016-2017 water year (Oct. 1, 2016 – Sept. 30, 2017), much of the state has not recovered from the severe drought conditions that have persisted for the past four years. Moreover, measurements by the Department of Water Resources indicate that the statewide snowpack is about 70 percent of average for early January." It will be interesting to see how the situation is changed when full data for January - particularly snow pack information along the Cascade Range - is in, but it seems likely the drought at least in northern California will be significantly eased.

An ice storm lasting for three days across Midwest USA and the Great Plains from 14 to 16 January killed six people, all in car crashes where drivers lost control on ice-covered roads. Ice storms, a rarity in Australia but not uncommon in North America, happen when rain and frozen precipitation mix, or rain falls onto frozen ground, creating a surface glaze on everything from road surfaces to power lines and trees. Ice storm or winter storm warnings were in force from the Texas panhandle to Iowa and Indiana, as well as Missouri, Oklahoma and Kansas. Thousands were left without electricity as poles collapsed under the weight of ice on wires.

On 21 to 24 January, a complex storm system brought three rounds of deadly tornadoes across the SE USA states of Georgia, Mississippi and Alabama, killing at least 21 people and injuring many more, before moving NE to give snow, rain and gales to northeastern states. This contrasts with 17 killed by tornadoes during the whole of 2017.

Bob Henson, who co-writes the leading blog on Weather Underground, writes "Although midwinter outbreaks don’t happen every year in the United States, they’re most likely to be across the South when they do occur. Many of the deadliest tornadoes in these outbreaks happen overnight, when residents may be caught asleep or otherwise unaware and when getting to shelter can be difficult. The high proportion of manufactured/mobile homes across the South adds to the vulnerability of residents." And so it was in this case.

The following is summarised from media reports and Bob Henson's blog on the event, which has unfortunately lost its illustrations. First, a supercell thunderstorm in the hours of darkness before dawn, 21 January, charged across southern Mississippi and Alabama, spawing five tornadoes. The most damaging had a track 50km in length and up to 800m wide, passing through Hattiesburg, Mississippi, around 04.00 local time. Rated an EF3 on the enhanced Fujitsa intensity scale, it caused at least four deaths, 56 injuries and at least $US200m in damage.

Later the same day, a more widespread outbreak of tornadoes occurred from noon until the early hours of 22 January. The most lethal of these tore across southern Georgia after midnight on 21/22 January taking at least 10 lives around 03.45 local time, 8 of them in a mobile home park near Adel, a little north of the Georgia-Florida border. By late on the afternoon of 22 January, NOAA Storm Prediction Center (SPC) issued its first "high risk" warning since 2014, and tornadoes were sweeping from the Gulf of Mexico into Georgia, one killing at least four people in mobile homes in Albany, southern Georgia. Altogether, over 40 tornadoes were logged for the event by SPC, an unusually high number for January.

Having done with the south, the storm system roared up the eastern US as a powerful nor'easter on 23 and into 24 January. A strong temperature difference across this system's cold front brought dramatic weather, including heavy rainfall and wet snow, flooding and strong to gale force winds. About 750 flights were cancelled from New York metropolitan airports, there were power blackouts and delayed commuter trains due to fallen power lines, and some schools across the region closed because glazed or snowy roads prevented students and staff from reaching them.

South America: Historic bushfires create "Dante's inferno" in southern Chile

The worst bushfires in decades are ravaging central Chile with up to ten days of hot, dry, windy weather to continue. In a situation dubbed "Dante's Inferno" by the mayor of Constitución, Carlos Valenzuela, the entire town of Santa Olga, 460km south of the capital Santiago, was razed - over 1,000 buildings - as were a number of smaller towns in the area according to The Guardian.

According to the Emergency Response Co-ordination Centre (ERCC), as at 27 January there were 142 fires of which 77 were active, 51 under control and 14 extinguished. 289,974 hectares (2,899 square kilometres) of forest had been burnt as well as large tracts of farmland. One thousand houses were damaged or destroyed, 5,000 people evacuated and nine had been killed. Hot and dry weather, with locally intense gusts of wind, were forecast to continue for the next ten days. Other sources indicate that 5,000 local firefighters were at work with additional equipment, aircraft and personnel flown in from the US, France, Peru and Mexico. This additional Guardian story gives further details.

The awesome MODIS satellite image for 26 January shown in Jeff Masters' WunderBlog gives some idea of the scale and intensity of the fires. On the same day, at least 12 weather stations broke the previous national all-time temperature record, some by up to 3.3C. In addition to extreme heat, central Chile has been in an extreme drought, which began in the late 1970s and is unprecedented in Chilean history. As with Australia's SW WA, rainfall has been gradually declining, at the rate in Chile of about 7% a decade. Citing a 2016 study, Jeff Masters writes "While at least half of the change in precipitation can be blamed on natural causes, primarily due to atmospheric circulation changes from the Pacific Decadal Oscillation, the authors estimated that a quarter of the rainfall deficit affecting this region since 2010 was due to human-caused climate change."

While this has been happening at the southern end of the populated part of Chile, the northern end has been experiencing flooding. Torrential downpours in the Andes around the junction of Peru, Bolivia and Chile have sent raging streams of mud down the rivers in the Atacama Desert that are normally dust. There have been mudslides, and around 350 people were isolated earlier this week according to Al Jazeera. In Bolivia and Peru there has also been heavy rain, falling in bursts in the mountains and leading to flash flooding, Al Jazeera says.

Asia: Smog chokes Beijing as Japan is buried under snow

After choking smog enveloped large areas of eastern China at the beginning of January, reducing visibility to 100m in places and closing both airports and highways, residents of Beijing ignored official pleas to refrain from setting off fireworks to celebrate Lunar New Year on Friday night 27 January. As a result they awoke on Saturday morning to dense smog that gave the second-highest particulate level in five years, according to Reuters.

Japan is seeing immense snowfalls this winter, not just on the northern island of Hokkaido which is legendary for its snow depths, but in the next island down the chain, Honshu. This, the largest island, is where the bulk of the population live and snow has fallen so heavily it is being being measured in metres rather than centimetres according to Al Jazeera. In Ishikawa Prefecture, on the side of Honshu opposite Tokyo, 60cm fell in the mountains in one 24-hour period bringing resort snow depths to nearly 4m. Great for skiing, but unfortunately also great for creating avalanches.

Africa: Heaviest snow in Sahara Desert in memory; floods in the east

The wintry blast that pushed across the Mediterranean on 19 January (see Europe entry above) gave mountainous areas of the northern Sahara Desert in Algeria their first heavy snow since 1979. Metro News describes it as "the biggest snowfall in living memory", and says "The red sand dunes of Aïn Séfra were blaketed [sic] in one metre deep snow". Aïn Séfra lies in the Atlas Mountains at an elevation of 1100m. The Guardian featured a spectacular set of pictures of deep red dunes against a deep blue sky, covered in snow. It is very reminiscent of Australia's Simpson Desert - apart from the snow.

Very heavy rain has fallen in central and southern Mozambique and in Zimbabwe, with falls of up to 650mm in southern Mozambique in the week from 12 to 18 January. About 6,500 people are isolated and about 1,200,000 people across the two countries are in the area that has had over 300mm and would be experiencing flooding according to FloodList.

Pacific nations: French Polynesia and the Philippines awash

Flooding has affected French Polynesia since 22 January, with the islands of Tahiti and Moorea the worst affected. A state of "natural disaster" has been declared for Tahiti. Over 200mm was recorded at Thaiti-Faa’a between 22 and 23 January, according to FloodList. Thousands have been left without power and the airport temporarily closed. As of 25 January, about 800 homes had been destroyed or damaged and 4,000 people affected according to the Pacific Disaster Center.

As of 17 January, several days of heavy rain had caused flooding, six deaths and the displacement of 6,800 people in the Philippines. 311mm had fallen over three days in Borongan, on the eastern side of Visayas. At Dipolog City, Mindanao, 186mm fell in six hours.