Weather Forecasting ... On-Line

How to Make a Weather Forecast


The purpose of this first web page is to describe, in a conceptual manner, how weather forecasts are prepared.

How Do You Prepare a Weather Forecast?

If you ask 10 meteorologists how they prepare their forecasts, you will likely get ten different answers. Nevertheless, there will be many commonalities among the ten responses. The diagram below is a conceptual description of how a forecast can be prepared. All forecasts start with weather data. At that point the process breaks into two branches. The right-hand branch make extensive use of computer processing to objectively analyze data, feed these data into numerical weather prediction (NWP) models, which in turn produce a variety of output parameters and statistical forecasts.

The left-hand branch makes use of human analysis skills and experience. After an initial review of data, including objective analysis information, an initial synthesis produces an understanding of what is happening in the atmosphere and, more importantly, why. At that point the initial picture of the atmosphere is used to evaluate the computer forecasts. After a final synthesis of all available information, a final forecast is prepared.

In the following sections, each of these pieces of the process will be discussed in more detail. Remember that forecast preparation is not a "cook book" process. Even though you may start your forecast process in a similar way each day, at some point, the current weather situation will dictate what you examine and how you use this information.

Weather Data
Analysis < < Objective Analysis
Initial Synthesis   NWP Models
Evaluate NWP <<< Computer Forecast
Final Synthesis  

It All Starts with Data!

All weather forecasts start with real weather data. These data are collected from a variety of sources including human surface observations, automated surface observations, rawinsonde observations, wind profilers, aircrfaft, satellites, and radar. Each observation platform provides one piece of the puzzle that makes up a total picture of the atmosphere at some point in time. Past observations provide a time series of information to evaluate changes in the state of the atmosphere.

Real data tell you what is actually happening in the atmosphere. In a mathematical sense, forecasting is an initial value problem. If you don't have a good picture of where the atmosphere is starting, it can be difficult to accurately determine its future state. This is particularly important for forecasts of short time durations (0 to 6 hours). Be sure the forecast follows from what you see outside the window!


In order to make sense of the vast amount of data available, you must "analyze" it. What does analysis mean? The American Heritage Dictionary of the English Language dictionary defines analysis as:

The separation of an intellectual or substantial whole
into constitutents for individual study

The Glossary of Meteorology defines it as follows:

a detailed study of the state of the atmosphere based on actual observations,
usually including a separation of the entity into its component patterns and
involving the drawing of families of isopleths for various elements

This process usually consists of drawing and interpreting the patterns of winds, pressure, pressure change, temperature, humidity, clouds, and hygrometers. Each element is examined separately in order to provide the analyst with a better understanding of the current state of the atmosphere and on-going physical processes.

Part of this analysis process is the use of derived parameter, that is, quantities calculated from observational data via objective computer analysis. Derived parameters include such things as moisture flux divergence or temperature advection. These parameters can be evaluated qualitatively, but objective analysis often gives a more detailed picture.

Initial Synthesis

Once you have taken things apart, you need to put them back together using synthesis. Synthesis is defined as:

the combining of separate elements or substances
to form a coherent whole

The overall purpose of the analysis-synthesis process is to develop a complete understanding of why the atmosphere is doing what it's doing. Why is the current weather occurring?

There are several processes that can be used to accomplish this goal:
  • pattern recognition: this refers to the fact that specific patterns are routinely associated with specific types of weather; for example, if you recognize that a cold front will pass through your area in the next six hours, you need to further evaluate whether there is a potential for showers or thunderstorms

  • parameter evaluation: this refers to the fact that specific weather parameters often provide a clue to the occurrence of a specific type of weather; for example, Lifted Index values of minus six or less is one indicator of severe weather potential

  • recognition of physical processes: by examining the various weather elements you should be able to determine why specific weather elements are occurring, that is, what physical process is causing the weather; for example, why are there clouds over Topeka, Kansas?

  • recognition of trends and changes: how a weather element is changing over the last six to twelve hours can sometimes tell you what to anticipate during the next six to twelve hours; pure extrapolation is not the best way to develop a forecast, but extrapolation combined with changes dues to physical processes, pattern influences, and parameter changes do lead to accurate short term forecasts

  • comparison with analysis models: schematic models of atmospheric patterns and features provide typical atmospheric structures and weather associated with the type of phenomenon modeled; if you recognize a particular model in the data during your analysis, you have a good lead as to what to anticipate from this situation

  • impact of climatology: although climatology, by itself, is not usually an very accurate or detailed forecast, it can give you a fairly large window within which your forecast is likely to occur; for example, if the record low for tonight is 20°F, and you come up with 15°F, be sure your forecast is realistic and in tune with some much colder-than-normal air moving into your area
  • Objective Analysis

    Running parallel to the human analysis-synthesis process is the computer processing of weather data. In most cases this processing starts with objective analysis or OA. The purpose of OA is to transform weather observations that are irregularly spaced onto to a regularly spaced grid. These gridded data sets are used to initialize numerical forecast models and also allow the analyst to calculate "derived parameter" such as temperature advection.

    There are several methods used to perform OA. They will not be covered here in detail but a brief description is in order:

  • function surface models: this OA process fits a continuous mathematical function or surface to data

  • empirical interpolation: the parameter value at a grid point is the weighted average of the data surrounding the point

  • statistical interpolation methods: an ensemble special correlation structure of the entire field is determined
  • NWP Models

    As noted above, the objective analysis provides an initial set of data that are used by numerical models. Numerical models are mathematical representations of atmospheric parameters and processes. Most models use some form of the hydrodynamic equations to predict wind, temperature, moisture or other atmospheric parameters.

    Several numerical prediction models are run operationally and are available for forecaster use. Check the National Centers for Environmental Prediction website for the latest information on currently available forecast models.

    Computer Forecasts

    Numerical forecast models produce a wide variety of output. Some forecasts are basic weather charts such as 500 mb or 300 mb contour, temperature, and/or isotach fields. Derived parameters such as divergence are also available. Basic model data are processed via statistical methods to produce products such as Model Output Statistics (MOS). Further processing of these data are now used to produce worded forecasts such as the Interactive Forecast Preparation System (IFPS) products.

    One very valuable aspect of today's computer forecast models is the gridded data sets that are routinely available. These data sets allow anyone with the proper software (e.g., GEMPAK) to manipulate the model forecast data into any form that can be represented by equations or algorithms. In some ways, what you, as a forecaster, examine is limited only by your imagination.

    Evaluate NWP

    The purpose of this step in the conceptual forecast process is to bring together real data interpretation with the computer products. At this point you need to ask yourself questions like:

  • Do the initial computer OA fields look like the observed data fields? Significant differences, particularly in data sparse areas, should raise red flags.

  • Do the short term implications of the analysis-synthesis process correlate with the short term computer forecasts?

  • How do different model forecasts compare? Do they handle a specific situation similarly or are there variations?
  • influence of data vs NWP

    Another part of the NWP evaluation involves the relative influence on the forecast of observations versus model output. Beyond 18 hours from issue time, numerical forecast model output is the primary basis for the forecast. The main issue at this point in time is to determine which model will have the most influence on your forecast.

    On the other hand, within the first 12 hours from issue time, observations and the conclusions drawn during the analysis-synthesis process should have an impact on the forecast. The diagram above is intended to approximate the relative influence of observations versus model output on the forecast during the first 12 hours of the forecast period. As time progresses into the forecast, the influence of "what's outside the window" slowly diminishes while the influence of the model increases. In any given situation you have to judge the relative importance of these two factors.

    Some people would say that the information provided by the (computer forecast) phase described above is all you need to issue a forecast. It is true that there is sufficient model output that one can issue a forecast based only on these output with little or no intepretation of any other information or data.

    However, a real meteorologist needs to be able to understand what the atmosphere is doing and why things are going on, not just be able to interpret computer output. If forecasts become no more than a repetition of what the computer says, why do we need people (meteorologists) in the middle?

    Final Synthesis

    This step brings together the real data and the computer forecasts for one final review and synthesis. You have to ensure that you have a good understanding of what has happened and what will happen. You need to ensure that the temporal flow of your forecast is consistent. The methods you applied during the analysis-synthesis process will also be useful here.

    The Forecast

    The forecast brings together in words (or code) what you want to tell your users. It provides the temporal and spacial details consistent with the specific forecast product being issued. The idea here is to convey useful information within the state of the art and provide as much accuracy as possible without hedging any more than necessary.

    In Summary

    A nice way to summarize the weather analysis and forecast process is stated by the following six questions:

    a. What happend?
    b. Why did it happen?
    c. What is happening?
    d. Why is it happening?
    e. What is going to happen? 
    f. Why is it going to happen?
    Lance Bosart, SUNY-Albany

    In discussing these six questions in his article in Weather and Forecasting (2003), Dr. Bosart reinterates what was stated above. With the availability of computer forecasts, it is easy to go straight to "e" and not concern youself with the "why" questions. However, real meteorologists need to answer all six questions as they prepare their forecasts. A large part of a meteorologist's experience is gained by watching the weather "over the years" and answering the "why" questions. If you skip these questions and rely on computers to do all your thinking, your experience level with atmospheric processes will be weak and your claim to the title "meteorologist" will be questionable.

    One last thought, perhaps that of person who has examined a lot of data and analyzed a lot of maps and believes that this information tells you a lot about the future state of the atmosphere:

    a meteorologist worth his/her salt
    should be able to make a 12 hours forecast
    without ever looking at
    numerical forecast model output

    Review Questions

    Instructions: Place the cursor over the answer of your choice. If you are correct, it will be highlighted in green; if you are incorrect, it will be highlighted in red.

    The weather forecast process starts with:

    1.  analysis

    2.  synthesis

    3.  weather data

    4.  computer forecast

    The step labeled weather data means:

    1.  collecting and examining weather observations

    2.  checking time series of past information

    3.  looking out the window

    4.  all of the above

    Derived parameters refer to quantitues calculated from observational data via computer objective analysis.

    1.  true

    2.  false

    The overall purpose of the analysis-synthesis process is to develop a complete understanding of why the atmosphere is doing what it is doing.

    1.  true

    2.  false

    Numerical weather prediction is based on:

    1.  mathematical representation of atmospheric processes

    2.  observation trends

    3.  statistical analysis of observations

    4.  computer operating system biases

    You should never compare short term forecasts based on observed data to short term computer forecasts.

    1.  true

    2.  false

    As "time after forecast issuance" increases, the influence of observations on a forecast normally:

    1.  is not considered

    2.  decreases

    3.  increases

    Real meteorologists can answer the question: Why is today's weather occurring?

    1.  true

    2.  false

    Return to the Operational Weather Topics page

    last updated on 3/06/10