Weather Analysis and Forecasting
(Adopted by the AMS Council on 17 August 1998)
Bull. Amer. Met.Soc., 79, 21612163
The goal of the American Meteorological Society's (AMS) statement on weather forecasting is to provide an overall summary of current weather forecast capabilities to a broad range of users. More specific information can be found in the AMS statements on flash floods, (Bull. Amer. Meteor. Soc., 74, 1377), hurricane tracking and forecasting (Bull. Amer. Meteor. Soc., 74, 1377), and tornado forecasting and warning (Bull. Amer. Meteor. Soc., 78, 2659).
There has been a revolution in the accuracy and utility of weather forecasts in the past several decades. Weather forecasts at all time ranges have improved markedly as a direct outgrowth of technological developments, basic and applied research, and increased forecaster understanding of how the atmosphere works. The last decade has brought major advances in observing systems, computing technology, and in the scientific understanding of the physics that underlie various types of weather phenomena.
Weather forecasts and warnings, especially of severe storms and other extreme weather conditions, may be the most important services provided by the meteorological profession. The impact of the weather and of weather forecasts and warnings is difficult to overstate. Eighty percent of the emergencies declared by the Federal Emergency Management Agency have been weather related. In a similar vein, the Department of Transportation has documented that more than 6000 fatalities per year can be directly or indirectly attributed to weather. Heat waves kill an average of 1000 people each year in the United States. Flood-related damages have risen steadily over the past three decades. The 1993 Mississippi Valley Flood alone cost $15–$20 billion and killed 48 people. As the population continues to grow, the weather's economic impact also continues to escalate. During the 10-year period from 1986 through 1995, the property damage insurance claims due to wind, hail, snow, and tornadoes have increased by a factor of 5. Weather impacts virtually every citizen and industry in the country. An estimated 90% of the public consult weather forecasts once or twice a day.
Weather and hydrologic forecasts and warnings help the general public and industry plan a wide range of daily activities that are sensitive to the weather. Predictions, including watches and warnings relating to severe and hazardous weather and flood conditions, are utilized by emergency managers to protect lives and property and by industry to improve the efficiency of weather-sensitive enterprises. For example, the death toll due to hurricanes, lightning, and tornadoes has decreased steadily due to improved weather forecasts and warnings. The U.S. economy benefits from weather forecasts in a number of ways, ranging from helping the energy industry to anticipate their requirements for heating fuel to providing guidance to the farmer as to which day to harvest crops. The large demand for accurate specialized forecasts from various economic sectors has led to the rapid growth of private meteorological firms.
1. Very short-range forecasts (0–12 hour)
These forecasts have shown considerable skill and utility, especially for predictions of the evolution and movement of large- and medium-sized weather systems. However, the accuracy of the forecasts decreases rapidly as the scale of the weather features decreases and the time range of the forecasts increases. Forecasting the evolution and movement of smaller-scale, short-lived, often intense weather phenomena such as tornadoes, hail storms, and flash floods is less mature than for predictions of larger-scale weather systems. The difficulty in forecasting small-scale systems is due to insufficient computational ability, inadequate observational capabilities, and limited understanding of the physical processes that are taking place during these events. General areas where these systems are likely to form can often be predicted up to 3 days in advance but the precise location that such a small-scale storm will form cannot usually be forecast reliably with much lead time. However, forecasts of small-scale features have improved markedly in regions where weather-related phenomena are generated or modulated by fixed terrain features, land-sea contrasts, and land-use characteristics.
Despite the difficulties in predicting these small-scale phenomena, the lead time of watches and warnings has increased. For example, the lead time for tornado warnings has more than doubled in the last decade due to the improved observing systems provided by the NWS operational Doppler radar network and satellite imagery. These warnings and watches rely heavily on observing and detecting when conditions are favorable for the development of severe convection and then monitoring each storm's evolution. Forecaster interpretation of radar and satellite imagery and local spotters play a critical part in these very short-term forecasts.
2. Short-range forecasts (12–72 hour)
The accuracy of short-range forecasts (12-72 hour) has continued to increase during the past decade. Improvements in observing systems and in how the data are assimilated into the computer models have resulted in steady improvement in the ability to predict the evolution of major, larger-scale weather systems. Accurate predictions of the development and movement of large-scale weather systems and the associated day-to-day variations in temperature, precipitation, cloudiness, and air quality are made regularly throughout this time range.
Forecasts of how much precipitation will fall in the 36-60-hour time frame are now more accurate than 12-36-hour predictions were during the late 1970s. However, the details of precipitation patterns are often tied to smaller-scale structures such as fronts, thunderstorm outflow boundaries, and mesoscale convective systems that are still difficult for the current generation of numerical models to simulate. High-resolution computer models with more advanced physics show promise for being able to simulate these small-scale features, and this, together with the advances in observing systems, gives reason to believe that further improvement in forecasts of precipitation is likely.
3. Medium-range forecasts (3–7 days into the future)
Medium-range forecasts have shown significant improvement in the last two decades. Large-scale events like the East Coast blizzards of 1993 and 1996 are now often forecast days in advance of the first flake of snow, allowing emergency managers the opportunity to make plans to mitigate potential life threatening situations that might develop. Three-day forecasts of major low pressure systems that determine the general evolution of the weather are more skillful today than 36-hour forecasts were 15 years ago. In the late 1970s, day 5 forecasts of precipitation were no more accurate than climatology. Since then, skill of day 5 forecasts has more than doubled, with predictions of major cyclones now being as skillful as day 3 forecasts were a decade ago. Temperature forecasts have also improved and now show considerable skill on day 3, with the skill decreasing with time until generally only marginal skill remains by day 7. However, there is reason to believe skillful day 7 forecasts will be possible in the future given the steady improvements in computer models, observational approaches, and forecast strategies.
4. Extended-range forecasts (week 2)
The predictability of the day-to-day weather for periods beyond day 7 is usually small. Operationally, forecasts at these time ranges have taken the form of 6-10-day mean temperature and occurrence of precipitation departures from normal. The accuracy of these 5-day mean temperature and precipitation forecasts has more than doubled since the 1970s. The accuracy of precipitation forecasts is less than that for temperature, even though the skill of both has increased at about the same rate. Advances in observing systems, computer models, and statistical techniques may allow skillful forecasts of the mean conditions for the 8-14-day period ("week 2") in the near future.
5. Monthly and seasonal forecasts
As a result of research over the last decade, monthly and seasonal forecasts of mean temperature and precipitation are now useful for specialized applications in major economic sectors, such as agricultural and energy interests, if utilized over a long period. There is reason for optimism that the utility of these long-range forecasts can be improved as computer models and statistical methods become more sophisticated. These new techniques are expected to improve monthly and seasonal outlooks, especially when the relatively strong signals associated with El Niño and La Niña events are present. Notwithstanding these advances, no verifiable skill exists or is likely to exist for forecasting day-to-day weather changes beyond two weeks. Claims to the contrary should be viewed with skepticism.
6. Opportunities for future improvement
Opportunities exist for increasing the skill and utility of forecasts at all time ranges. Short- and medium-range forecasts should improve as new observing systems are deployed to measure the vertical and horizontal structure of the atmosphere. These new datasets and improved methods of incorporating the data into computer models should improve forecasts by providing models with a more realistic starting point for their forecasts.
One of the main causes of forecast deterioration is the uncertainty in the initial state of the atmosphere. Techniques have been developed to assess the impact of initial conditions on the subsequent forecast. Such techniques provide forecasters with increasingly reliable information on the probability of disruptive weather phenomena occurring. Weather-sensitive industries should benefit from these probability-based forecasts. Equally important is the continued evolution of numerical models based on increased model resolution and more realistic handling of the physics of the atmosphere. Rapid communications systems and more powerful interactive workstations are needed to synthesize and manipulate quickly the large volumes of data that are now becoming available to forecasters.
The theoretical limit for being able to predict the daily evolution of weather systems is thought to be about 2 weeks, suggesting there is considerable opportunity to improve forecasts, especially in the 6-14-day range. To realize this opportunity will require further improvements in observing and measuring the initial state of the atmosphere, improved model physics (e.g., more accurately handling the water cycle), improved access to powerful supercomputers, and research on the factors that govern the predictability of weather systems. Applied and basic research are also needed to better our understanding of the atmosphere, especially of small-scale, relatively rare phenomena.
In summary, weather forecasts are more accurate and useful than ever before. The American Meteorological Society encourages the expanded use of weather forecasts, not only as a defense against hazardous weather, but also in planning day-to-day operations of private enterprise and governments, and by individuals to enhance their quality of life. While much has been accomplished in improving weather forecasts, there remains much room for improvement. The Society therefore encourages continued investment in research, observational technology, high-performance computing, and education to foster continued improvement in the accuracy and utility of forecasts.