What are Long range forecasts? |
How far into the future does a forecast have to go to be considered long range? The World Meteorological Organisation (WMO) defines various forecast ranges as short (up to 3 days ahead), medium (3 to 10 days), extended (10 to 30 days) and long (30 days to 2 years). It notes that extended and long range forecasts are "usually averaged and expressed as a departure from climate values for that period".
Different countries vary in both the forecasts they issue and what they call them. In Australia, the Bureau of Meteorology issues state and weather district forecasts for the next 4 days and forecasts for individual towns and cities for the next 7 days. It also issues what it calls Seasonal outlooks which are in fact three monthly outlooks. It issues nothing in between.
Other organisations, both within and outside Australia, issue extended and long range forecasts for Australia, or for all or some part of the globe that includes Australia, and I've chosen what I think are the most useful of these in the links below. These mostly look 3 to 4 months into the future. A list of centres producing long range forecasts is maintained by the WMO.
Finally, here's a warning. If you want to know what the weather will be like on the afternoon of your wedding day in 6 months time you'll be disappointed. Weather science just isn't that advanced, and the chances are it never will be.
Here's why. Meteorologists try to predict future weather by looking at the current weather situation, then using complex models that apply the laws of physics to advance that situation, one small step at a time, into the future. Unfortunately, chaos theory, also known as the butterfly effect, gradually takes over. We never know the exact "current weather situation" because we don't have weather information for every square millimetre of the globe. Our knowledge of the physics that affect the atmosphere isn't perfect. Gradually these imperfections create errors that then build on errors that then build on errors... You get the picture.
Forecasters can produce reasonably certain forecasts, in most situations, up to 4 days ahead. Between there and 10 to 14 days the level of certainty rapidly drops off until the forecast becomes so inaccurate as to be meaningless.
So how do seasonal forecasts work? Instead of trying to build a forecast from an initial state of the atmosphere, they look at what is happening with the giant teleconnections (e.g. El Niño) that drive global climate as well as other factors. This area of science is still in its infancy, and accuracy is nowhere near as great as it is for forecasts over the next four days or so. Because of the economic importance of long-range forecasting, many countries are devoting considerable resources to improve the science, and the results of some of these efforts appear below.
There are also a number of people and companies that say they provide accurate forecasts for months or years ahead using cyclic patterns of the sun, moon and planets, or through the use of superior forecasting models. Some provide day-by-day forecasts for many days, up to a year, into the future. Unfortunately, hard independently verified evidence of their accuracy is usually impossible to find and, while there may be something in the methods used, the motives to make money or gain a competitive edge also have to be taken into account. |
North American Ensemble Forecast System (NAEFS)
From Canada |
Temperature anomaly forecast between 8 and 14 days ahead
---on one chart
---on three charts |
NAEFS, a joint project of the Canadian and US weather services, produce these North American and global temperature anomaly forecasts for the period 8 to 14 days ahead twice daily. Go to the bottom for the global maps. They are produced from an ensemble model, the first link at left showing increasingly deep reds for greater chance of heat, blues for greater chance of cold compared to normal. The second link shows separate charts for likelihood of above normal, average, and below normal temperatures. If you can't see or if you lose the charts, go to the top and reduce the date by one day. Help is here. |
North American Ensemble Forecast System (NAEFS)
From USA |
Forecast anomalies between 8 and 14 days ahead
---temperature
---rainfall |
These experimental NAEFS charts, from the US NWS, show expected temperature anomalies 8 to 14 days ahead in a similar way to those from Canada, above. The rainfall forecasts for the periods 6 to 10 and 8 to 14 days ahead show the percent probability of receiving over 25mm and over 75mm. Click the maps for larger ones. Go here for help. |
National Centers for Environmental Prediction |
15-day global rain forecast
15-day global 500hPa level forecast |
From the NCEP Global Ensemble. If you get a broken image symbol, reduce by 1 the date above the image (second from right). Using the controls at the bottom and top, you can now step or loop through the daily probabilities of rainfall exceeding 1, 5, 10 or 25mm over the next 15 days.
Select Global 15 days forecast in the second drop-down menu above the chart. The 15-day global 500hPa level forecast is used in the same way as the rain maps to see forecast upper troughs around Australia and in the southern hemisphere. |
Bureau of Meteorology |
Seasonal outlook main access page
ENSO tracker
Climate and Water Outlook video |
The Bureau's three-month outlooks are issued each month about one week before the period to which they refer. This page neatly wraps up rainfall, temperature, stream flow and tropical outlooks with information on the main teleconnections, including El Niño/La Niña, MJO and IOD, as well as a summary of what the main long-term models are saying. It's informative, easy to understand, and well illustrated with maps and diagrams. 16/8/14
The Bureau's ENSO tracker, updated fortnightly, keeps a watch on the El Niño-Southern Oscillation (ENSO). This global teleconnection has been responsible for droughts/heat and flooding rain across much of eastern Australia, so its movements are keenly observed.
For those that want to cut to the chase, the Bureau provides a short video each month summarising the long term outlooks. |
University of Southern Queensland - International Centre for Applied Climate Sciences (ICACS) |
Climate outlook and short review |
Professor Roger Stone provides this monthly outlook for the next three months around the beginning of each month. It is aimed at those in Queensland, but much of its content and some maps are useful for all users in Australia. The outlook is in pdf format and changes its title each month - use the ICACS link at left and click on Climate outlook and review. |
