A policy statement of the American Meteorological Society as adopted by the Council on 13 January 1991
|Weather forecasts and warnings are the most
services provided by the meteorological profession. Forecasts are used
by government and industry to protect life and property and to improve
the efficiency of operations, and by individuals to plan a wide range
daily activities. This summary of present-day weather forecasting
is intended to provide general guidance to a broad constituency of
The American Meteorological Society Statements on Flash Floods (Bull.
Amer. Meteor. Soc., 66, p. 858), Hurricane Detection,
and Forecasting (Bull. Amer. Meteor. Soc., 67, p. 1508),
and Tornado Forecasting and Warning (Bull. Amer. Meteor. Soc., 72,
p. 1270) assess the current state-of-the-art pertaining to some of the
most hazardous weather events that require warnings to protect people,
commerce, and industry. Topics related to prediction of climate change
are addressed in the Statement on Global Climate Change (Bull. Amer.
Meteor. Soc., 72, p. 57).
As practiced by the professionally trained meteorologist, weather forecasting today is a highly developed skill that is grounded in scientific principle and method and that makes use of advanced technological tools. The notable improvement in forecast accuracy that has been achieved since the 1950s is a direct outgrowth of technological developments, basic and applied research, and the application of new knowledge and methods by weather forecasters. High-speed computers, meteorological satellites, and weather radars are tools that have played major roles in improving weather forecasts.
The most impressive gain in forecast accuracy in recent years has been in prediction for the 1 to 5 day range. A number of factors have contributed to the increase in accuracy. Foremost among these has been the further development of numerical prediction models, based on the laws of physics, that are able to forecast the formation and movement of the large high and low pressure systems that govern day-to-day weather changes in middle and high latitudes. These models have undergone steady improvement since their introduction more than a quarter century ago. The improvement has been made possible in large measure by the growth in the capacities of the computers that are required for carrying out the vast number of calculations involved in a numerical forecast.
Several other factors have contributed significantly to this increase in forecasting accuracy. One is the development of statistical methods for enhancing the scope and accuracy of model predictions. Another is the improved observational capability afforded by meteorological satellites. A third primary reason for the increase in accuracy is the continued improvement of the initial conditions prepared for the forecast models. Statistical methods allow a wider variety of meteorological elements to be predicted than do the models alone, and they tailor the geographically less precise model forecasts to specific locations. Satellites now provide the capability for nearly continuous viewing and remote sensing of the atmosphere on a global scale. The improvement in initial conditions is the result of an increased number of observations and better use of the observations in computational techniques.
The numerical prediction models that have proved successful in 1 to 5 day prediction have at present only limited, and as yet unproved, capabilities for making predictions for periods beyond 10 to 14 days. Forecast methods and tools for monthly outlooks have been a mixture of dynamical and statistical techniques. Recent improvements in numerical weather prediction models have led to a greater emphasis on the dynamical part of the forecast input, especially for the first 10 days of the month. For seasonal outlooks, no dynamical technique is used and the forecast is based entirely on a variety of statistical tools. At monthly and seasonal ranges, day-to-day weather changes are not predictable, either in theory or in practice, and forecasts are concerned instead with the probability that average temperatures and total precipitation for the forecast period, typically a month or a season, will be above or below normal. Improvements in statistical forecasting methods have resulted in some skill in monthly and seasonal prediction thus making the information potentially beneficial to certain users. The skill of both temperature and precipitation forecasts varies with season, location, and situation, however. In some regions, the seasonal forecasts remain no better than a forecast based on a continuation of the weather observed at the beginning of the season or a forecast based on average conditions.
The cause of the large circulation anomalies that produce persistent pattens of abnormal warmth and cold and wetness and dryness are not well-understood. There is, however, growing evidence that surface conditions (i.e., snow and ice cover, sea surface temperature, and soil moisture) play an important role in these anomalies. Numerical models now under development provide the basis for an improved understanding of long-range weather variations and for an enhanced ability to predict them.
For very short prediction periods (0 to 6 hours) principle interest centers on small-scale, short-lived, often violent phenomena, such as thunderstorms, tornadoes, as flash floods, and on localized nonviolent weather such as dense fog and freezing rain. Although a growing capability exists for mathematically modeling small-scale weather systems, practical application of the models in forecasting specific events is not yet feasible because of the difficulty and expense of observing the atmosphere in the required detail. An exception to this statement is the prediction of local wind systems, such as sea breezes and mountain winds, that are induced by topographical features. Numerical prediction of such systems is feasible and is presently under development.
In view of the difficulty in applying physical methods in the forecasting of small-scale weather, the emphasis at present is on detecting and tracking these features and determination by forecasters of their development and movement for short intervals into the future. Weather radar and geostationary satellites are particularly valuable tools for this purpose, and their capabilities and usefulness have been and are being steadily improved. Rapid communication of warnings and the establishment of adequate warning networks are also essential elements in very short- range prediction. Progress has been made in these areas too.
The tropical storm or hurricane is a phenomenon that is of special importance in short- range prediction. With the help of weather satellites these systems can now be detected and tracked over data-sparse ocean regions and estimates of their intensities can be made. Aircraft reconnaissance and ground radar permit increasingly accurate tracking and a reliable determination of intensity as the storms approach inhabited coastal areas.
Progress in forecasting the movement and changes in intensity of tropical cyclones has not kept pace with advances made in forecasting the behavior of extratropical cyclones, although predictive skill has begun to increase in recent years through the use of combined statistical and numerical prediction methods. A principle reason for the slow progress has been the lack of adequate data for use in numerical prediction models. Because of the limited application of these models, human judgement also weighs heavily in hurricane prediction.
The methods currently employed yield useful warnings of storm danger a day or two in advance. They are less successful in providing timely warnings of narrow within which major damage is likely to occur and within which special precautionary measures should be taken.
The usefulness of the present imperfect forecasts has been increased by application of sophisticated statistical methods to aid decision making when evaluating various preparedness actions. Additional information regarding current skill in tropical cyclone forecasting can be found in the AMS Statement on Hurricane Detection, Tracking, and Forecasting (Bull. Amer. Meteor. Soc., 67, p. 1508).
The foregoing overview offers a brief account of the present state of the forecasting practice and highlights some of the important accomplishments of recent years. It is now desirable to state more fully the current levels of accuracy and skill. The statements that follow pertain to land areas in temperate and high latitudes of the Northern Hemisphere. Skill levels are lower over the northern oceans and the Southern Hemisphere owing to the reduced numbers of observations in these areas. Forecasting skill can be determined objectively by comparing the accuracy of a given set of forecasts with the accuracy of a corresponding set produced by simple procedures, such as predicting that the weather will remain unchanged (persistence or assuming that the weather will correspond to average conditions (climatological forecast). Unless forecast accuracy exceeds levels achieved by these simple methods, predictive skill cannot be said to exist.
1) For the period 0 to 12 hours: The
weather forecasts in this time range depends very much on the
required in the forecast and varies with the weather situation.
of general weather conditions and trends in this time periods show
skill and utility. However, the spatial and temporal detail that can be
included in the forecast decreases as the period increases. The
of small, short-lived, severe local storms is predictable only for
of the order of several minutes to an hour. Recent observational
of severe convective storms combined with numerical simulations of
systems (approximately 1 to 100 km in horizontal extent) have provided
the forecaster with an increased ability to anticipate thunderstorm
and make very short-range forecasts of the evolution of the convective
system. The behavior of larger features, such as squall lines, fronts,
and organized areas of heavy precipitation, is often predictable for
of up to 6 to 12 hours. Weather changes associated with large cyclonic
storms are well forecast for this time range. Strong downslope winds,
winds, and other features induced by irregular terrain and surface
can often be predicted for periods up to several hours ahead or longer.
The utility of forecasts can be increased by
the user of the estimated or otherwise-determined probability of
of an event as is done, for example, in daily precipitation forecasts
in some monthly and seasonal outlooks. Probability statements offer
benefits to those engaged in weather-sensitive operations, if
costs and losses can be evaluated.
|© 1996 American Meteorological Society|