Clarity Helping you understand the facts about weather forecasting Forecasting the weather is essential to help you prepare for the best and the worst of our climate. Met Office forecasters work 24/7, 365 days of the year, using one of the world’s largest supercomputers to predict the weather for hours, days, weeks, seasons and even years ahead. Operating as part of an international network to collect weather data, we also partner with research institutes worldwide to develop the very latest techniques. We strive to ensure that you always have the very best advice in print, on air, and via the web. Our forecasts for tomorrow are right six times out of seven. Here you will find some facts about weather forecasting – an insight into our foresight.

Weather forecasting — the big picture 01 Observations are essential

FACT 1 — Before we forecast the weather, we collect observations.

Observations Assimilation Observations of the weather are made Next we turn observational data into a 24 hours a day, all over the world. numerical representation of the current The main observations are from weather atmospheric conditions. This process is satellites, balloons, land­based known as assimilation. instruments, ships, buoys and aircraft. Small changes in atmospheric conditions Each day, the Met Office uses around lead to very different weather patterns, half a million observations of so it’s vital that the current state of the temperature, pressure, wind speed and atmosphere is represented as accurately direction, humidity and many other as possible. properties to provide the starting conditions of our weather forecast model. We continually update our Copyright European Space Agency (ESA), D. Ducros knowledge of the current state of the atmosphere.

� Satellites measure radiation from Earth’s surface and the atmosphere.

� Balloons and aircraft measure the bit of the atmosphere that they are passing through.

� Buoys and land stations measure the lower part of the atmosphere.

� Radar systems measure the reflection of radiation from rain drops and snowflakes.

Weather forecasting — the big picture 02 We use numerical models and supercomputers

FACT 2 — We use numerical computer models and supercomputers to forecast the weather. Numerical Weather Prediction (NWP) Met Office research scientists work to We predict how atmospheric conditions improve understanding of atmospheric will change with our knowledge of the processes, so they can be better simulated atmosphere, oceans and land surface. in our forecast model, and ultimately, lead to more accurate forecasts. Our numerical model starts with the current atmospheric conditions in the This is done by a combination of making area of interest from the surface to the and studying new types of observations upper atmosphere at points on a three­ and advanced numerical simulations. dimensional grid. The Met Office model is called the The atmosphere is a thin shell of Unified Model (UM) as different turbulent gas on a rotating Earth. configurations of the same model are Numerical Weather Prediction (NWP) used for both weather and climate models replicate the behaviour of this prediction. gas with mathematical equations. Supercomputers Details of the wind speed and direction, Numerical weather prediction involves temperature, pressure, moisture and billions of mathematical calculations. cloud in each grid box are then stored in The process of turning observational a computer. data into a numerical representation of the atmosphere takes the supercomputer Equations describing all of the relevant longer than it does to actually make the atmospheric processes are solved for forecast. We use powerful supercomputers each grid box to predict the values at to do these calculations as quickly as that point several minutes later. possible.

Weather forecasting — the big picture 03 Human intervention is key

FACT 3 — Human forecasters check and improve our forecasts.

Our expert forecasters use their To ensure that people and critical knowledge to compare the predictions operations will not be put at risk by of computer models against actual unexpected weather, the Met Office observations. If necessary, they can Operations Centre operates round the respond quickly and amend a computer clock throughout the year, even on forecast if it is going wrong in some way Christmas Day. – for example, computer models sometimes have problems forecasting Nowcasting thunderstorms and other small­scale Nowcasting is a technique for very short­ details a short period ahead. In such a range forecasting that maps the current situation, rainfall radar observations and weather, and then uses an estimate of its satellite pictures can be very useful when speed and direction of movement to making predictions over short periods of forecast the weather a short period ahead time. This is an example of forecasters – assuming the weather will move adding value to computer guidance. without significant changes.

Another example of forecasters adding The Met Office uses nowcasting for value to computer guidance is by many weather variables including wind, correcting for known biases or temperature, snow and fog. weaknesses in computer model As a forecasting technique it can be predictions. Through skilful interpretation applied quickly, either by human of computer output, forecasters also forecasters or by modest­sized decide on the correct emphasis in computers, so it is possible to update the weather forecasts. Forecasters also tailor forecasts frequently – every time there forecast information to the interests of are new observations available. different customer groups which can include for example, the general public, Most Met Office nowcasts are updated emergency responders or airlines. every hour. Sifting through the vast quantities of forecast data from the NWP system and identifying which are of particular significance to particular customersona particularday isa risk assessment process, considering the vulnerability of each user to all aspects of the weather.

Weather forecasting — the big picture 04 We use a variety of techniques

FACT 4 — To produce weather forecasts for one to 15 days ahead, we use a variety of techniques. Ensemble forecasting Probability and decision­making One example of the range of techniques By assessing the uncertainty in a forecast, we use to forecast the weather is ensemble forecasting helps decision­ ensemble forecasting. The risk of makers judge the appropriate level of particular weather events can be response to a risk of high­impact estimated using ensemble forecasting. weather.

An ensemble forecasting system samples When the predicted risk of a severe the uncertainty inherent in weather weather event is high it can give prediction to provide more information forecasters the confidence to issue earlier about possible future weather warnings. If there is a smaller probability conditions. Multiple forecasts are of a severe event then emergency produced by making small alterations to planners and vulnerable businesses can either the starting conditions or the still be alerted to protect their vulnerable forecast model, or both. assets via a special advisory service.

Weather happening many thousands of The output from the ensemble systems miles away today will affect the weather allows the uncertainty of the forecast to over the UK in a few days time. In be quantified, and the risk of a particular medium­range forecasts of two to 15 weather event occurring can be days ahead, the use of ensemble assessed. This can aid decision­making forecasting is essential as the uncertainty for those who are at risk from certain in the large­scale weather patterns weather events. becomes greater.

Copyright 2009 EUMETSAT

Weather forecasting — the big picture 05 We are always improving our forecasts

FACT 5 — We are always improving our forecasts.

We measure the accuracy of all our Another way of monitoring the accuracy forecasts. Over the years, there has been of our forecasts is by setting targets for: a steady pattern of improvement. For example, our four­day forecasts are now � Next­day rainfall forecasts for 11 places as accurate as our one­day forecasts were within the UK.

30 years ago. 2008 � Next­day temperature forecasts (max

accuracy Improvement in forecast accuracy is a and min) for 11 places within the UK. result ofinvestment in research,faster ahead 1978 � The overall forecasting skill of our 2008 supercomputers and greater coverage computer models assessed by the Forecast and the targeting of observations. 1­day ahead NWP Index that takes into account ahead There are a number of ways of computer forecasts for several 1­day measuring the accuracy of a forecast. weather elements which we can 4­day One method used at the Met Office is compare with other forecasting called the Numerical Weather Prediction centres around the world. (NWP) index. This combines the accuracy of a number of different elements into a single measure of overall accuracy.

Weather forecasting — the big picture 06 The atmosphere is chaotic

FACT 6 — The atmosphere is chaotic.

Despite the expertise of our researchers To further improve the accuracy of our and forecasters, and the sophistication of forecasts we are investing in further our computers and satellites, our forecasts research to reduce the likelihood of are sometimes inaccurate. This is due to errors. In addition to improving the the complexity of the atmosphere and a accuracy of the one­ to five­day forecast, lack of observational data. In fact, the research has enabled us to make atmosphere is a chaotic system: forecasts that were previously impossible.

� The atmosphere is often in a state We can now forecast further into the where small disturbances reinforce future allowing regular seasonal forecasts themselves, systems such as to be produced, and predictions of thunderstorms or depressions. climate change are continually improving.

� Small initial state uncertainties or New research is expected to provide model errors may rapidly lead to large further significant improvements in our forecast errors. ability to forecast heavy thunderstorms a few hours in advance. � Such states mostly arise from uneven heating, for example the equator to pole variation in solar intensity; land / sea differences in absorption of solar radiation; variations in land use, cloud cover, melting and evaporation.

Weather forecasting — the big picture 07 Size matters – small is less predictable

FACT 7 — Size matters – small is less predictable.

There are many different sizes of � Much of our severe weather comes disturbances in the atmosphere. from disturbances of the size of a The lifetime of a disturbance is related thunderstorm – about 10 km – lasting to its size. an hour or so. We can predict these up to about three hours ahead. Their predictability is related to their lifetime and size. � Very small disturbances such as tornadoes may be only 1 km across, � The largest disturbances are planetary last for about 10 minutes and are waves (known as Rossby waves, as predictable for only half an hour depicted in the sculpture on the right), ahead. best seen in the meanderings of the jet streams, at aircraft cruising � While individual storms causing altitudes, which guide the severe weather may only be development and movement of predictable for a few hours ahead, the weather systems around the globe. likelihood and intensity of storms in They have a length scale of around an area can often be forecast 10,000 km, evolve on a timescale of much further ahead. several days to a week, predictable for one to two weeks ahead.

� Bands of weather associated with depressions and fronts are typically 100 km across, evolve on a timescale of 12 hours and are predictable up to about one or two days ahead.

Weather forecasting — the big picture 08 Seasonal forecasting

FACT 8 — Seasonal forecasts provide information on how weather, is expected to vary from normal. Seasonal forecasts incorporate more of a to forecast individual events so far ahead. global view to look out a month or more Because of uncertainty in forecasting at into the future. They factor in details such long range, seasonal forecasts are generally as the current average state of the expressed in terms of probabilities. atmosphere and the ocean at distances often thousands of miles away from the Sea­surface temperature specific location of interest. Natural variations of the Earth’s climate that vary over long periods of time can The same computer models of the influence weather patterns. Fluctuations atmosphere that are used to make daily in the surface temperature of the global weather forecasts are used for seasonal oceans are particularly influential. forecasts, with some differences: The influences are not easily noticed in � They are run forward in time up to day­to­day weather events but become many monthsahead,ratherthanjustfor evident in long­term weather averages. afewdays. Slow fluctuations of sea­surface � They include active oceanic, as well as temperature (SST) can be predicted, to atmospheric, components. some extent, at least up to six months ahead. � They are run many times, with slight variations to represent uncertainties in The links between SST and weather can the forecast process. be represented in computer models of the atmosphere and ocean. Computer Long­range forecasts estimate only the models developed at the Met Office, average weather, not specific weather like those used in making both daily events. Therefore, in seasonal forecasts, forecasts and long­term climate change phrases such as ‘warmer than normal’ predictions, form the basis of our and ‘wetter than normal’ are common. seasonal prediction systems. Seasonal forecasts are indications of an overall picture because, unlike short and medium range forecasts, it is impossible

Weather forecasting — the big picture 09 We don’t just forecast ‘bad’ weather

FACT 9 — We don’t just forecast ‘bad’ weather.

It is our duty to warn people of adverse Many forecasts are provided to our (or high impact) weather conditions. customers in this time range, for On occasion, we have been accused of example to those who have to respond being too pessimistic in our forecasts – to emergencies, such as flooding, like the often when we are forecasting severe Environment Agency. weather events such as heavy rain or snow. In addition to short period ‘Flash However, it is also our duty to tell it like it warnings’ we also issue warnings of is – we aim to provide accurate forecasts potentially severe weather up to several that give people the facts so they can days ahead via NSWWS ‘Early Warnings’ make the most informed decisions. In and ‘Advisories’. some cases, warnings criteria have been agreed with customer groups based on These services differentiate between two impacts that affect them specifically. levels of severity and three levels of likelihood, with the highlight advice We aim to achieve a balance between given as a traffic light code indicating: not missing important weather events while not ‘over­warning’. � Green (no severe weather likely)

National Severe Weather Warning � Yellow (for information) Service Orange (be prepared) We warn people of severe weather � under the National Severe Weather � Red (take action now) Warning Service (NSWWS). We use Nowcasting to decide when a ‘Flash Not only do these warnings inform the warning’, which indicates a high general public via our website, confidence of severe weather in the next emergency planners also use them to few hours, needs to be issued. make strategic decisions.

Weather forecasting — the big picture 10 Collaboration is essential

FACT 10 — Collaboration is essential for accurate weather forecasts.

Weather takes no notice of boundaries As the Met Office has a reputation as one between countries and continents so it’s of the best weather forecasting services vital that our forecasters and researchers in the world we are a partner of choice work with other national weather for many organisations around the world. services to make the most use of the information about the atmosphere. To improve our forecasts further, we’re actively involved in a number of This is true for operational forecasting, but international programmes, projects and also for the research that underpins our initiatives, working with other partners to forecasts. Our scientists work closely with improve our joint capability and organisations from all around the world. understanding of science.

We strive to build academic and research relationships with other centres of excellence, by combining skills, capabilities, and aligning research and development programmes.

Weather forecasting — the big picture 11 Met Office Tel: 0870 900 0100 Produced by the Met Office FitzRoy Road, Exeter Fax: 0870 900 5050 © Crown copyright 2009 08/0255 Devon, EX1 3PB [email protected] Met Office and the Met Office Logo United Kingdom www.metoffice.gov.uk are registered trademarks