Nowcasting is a technique for very short-range forecasting that maps the current weather, then uses an estimate of its speed and direction of movement to forecast the weather a short period ahead — assuming the weather will move without significant changes.
Since it takes time to gather and map the weather observations, a short forecast is needed to even know what the weather is now.
Rainfall and associated severe weather, such as hail and lightning, are the most widespread and most advanced applications of nowcasting. In the UK, rainfall nowcasts can be useful up to three or four hours ahead in widespread rain bands in winter, but only one to two hours ahead for summer thunderstorms.
To extend the period of predictability nowcasts can be combined with output from numerical weather prediction models.
We use nowcasting for many weather variables including wind, temperature, snow and fog. Because it is a forecasting technique that can be applied quickly, either by human forecasters or by modest-sized computers, it is possible to update the forecasts frequently - every time there are new observations available. In the Met Office most nowcasts are updated every hour.
As computer models improve, the lead times will become shorter and, ultimately, these simple techniques may be used for instant forecasting, such as the immediate path of a tornado.
The Met Office has STEPS (Short Term Ensemble Prediction System), a state-of-the-art rainfall nowcasting system, developed in collaboration with the Australian Bureau of Meteorology.
In STEPS, the rainfall distribution is separated into different sizes of rainfall feature, so the large rainfall events (which are the more predictable) can be nowcast for longer, while the small events are only nowcast for a very short time.
Beyond this predictability limit, information is used from the NWP model for larger rainfall features, and the smaller features are filled in realistically using a random statistical method.
The 'ensemble' in the title refers to the fact that many forecasts are produced, with the rainfall areas moving at slightly different speeds, and with the small rainfall features represented by slightly different random statistics. Using this approach enables a realistic range of uncertainty to be estimated for flood forecasting.
Nowcasting is a very old technique. When Admiral FitzRoy first produced forecasts at the Met Office in the 1860s, he did it by collecting reports of storms from around the coast, and then sharing these reports with coastal ports that may be downwind, so that they knew there was bad weather coming. This was a simple form of nowcasting.
The term 'nowcasting' was actually coined in the 1980s by Met Office scientist Professor Keith Browning, to describe the process of extrapolating a sequence of radar images to produce a very short-range rainfall forecast.