The graphics and statistical information on this page fill gradually as they become available, with some not available until the next day. The page is updated every 30 minutes at about 20 and 50 minutes past the hour.

For weather news as it breaks that is tagged and organised, use the links on the Weather and Climate Media Reports page.

Thursday 25 May 2017

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.