Digital remotely sensed observations of the atmosphere, particularly from radars and satellites, have become increasingly available over the last 15 years. Together with developments in computer technology this has stimulated the design and operation of a variety of systems to exploit these data in weather forecasting. Growing awareness of the importance of detailed site specific weather information and forecasts from zero to a few hours ahead has led to the emergence of a particular kind of forecasting called nowcasting which depends on the exceptionally detailed knowledge of the current pattern of weather that remote sensing can provide. This type of weather forecasting is reviewed, with emphasis on the measurement and extrapolation up to about 2 hours ahead of fields of various weather parameters, especially rainfall. Trends in the design of nowcasting systems are discussed, and potential benefits summarized. tion of the weather, have been available to forecasters in real time and in a digital form amenable to manipulation. Although the preparation of forecasts by extrapolating the observed fields might be regarded as a simple extension of what forecasters have done for generations, limitations in observing capability and understanding of mesoscale phenomena, together with the practical difficulties of handling vast quantifies of data quickly, make this a challenging task in practice.Since extrapolation is the basis of nowcasting systems, the quality of the resulting forecasts depends upon the time ahead for which linear extrapolation is valid. This varies for different weather systems as shown in Table 1. Frontal rainbands, for example, persist for many hours. However, severe weather caused by individual convective systems is short-lived, and linear extrapolation on its own is unlikely to provide good forecasts beyond an hour or so. The larger-scale systems with greater persistence, such as fronts, often act as the trigger for the smaller convective Published in 1989 by the American Geophysical Union. ß 345 ß Reviews of Geophysics, 27, 3 / August 1989 pages 345-370 Paper number 89RG01134 346 ß Browning and Collier: NOWCASTING OF PRECIPITATION SYSTEMS 27, 3 / REVIEWS OF GEOPHYSICS storm/blizzard High wind gusts 5-30 min very limited accompanying shallow showers Hurricane many hours fair Frontal passage many hours fair to good After Zipser [ 1983]. (Part of this table is from Doswell [ 1986].) 27, 3 /REVIEWS OF GEOPHYSICS Browning and Collier: NOWCASTING OF PRECIPITATION SYSTEMS ß 347 when and where it will be triggered, unless there is some well-def'med topographical forcing, for example. We shall not discuss the nonlinear methods further in this review; rather we concentrate on the conceptually simple linear nowcasting techniques that have been developed to characterize, track, and extrapolate the motion of existing weather features detected by mdars and satellite. At present, nowcasting systems in several countries are providing data for severe weather warning, hydrological forecasting, snow clearance on roa...