Why are weather radars not much use in determining what weather is coming up in the next several days, but is able to pinpoint storms before they even form?

MIKE MOSS SAYS:      Merwa,     Weather radars are designed to show the recent and current location of precipitation areas and how those areas are moving, developing or dissipating. While the information from radars, when composited on a large scale, help in identifying broad weather patterns and in checking the performance of computer model rainfall estimates (which may assist in identifying models that are performing well, or poorly, with a given weather scenario), they are simply an added analysis tool and not a predictive system that tells us anything directly about the future.

On a smaller, local scale, radar trends can help us extrapolate the movement of precipitation areas and their intensity trends, along with information on winds within the radar beam (thanks to doppler technology) and the kinds of precipitation particles present within the beam (using dual-polarization techniques). Because many types of precipitation, especially those that are convective in nature, tend to change so rapidly in response to evolving large and small scale weather features, radar can only help with these kinds of extrapolations for anywhere from a few minutes to a few hours into the future, beyond which we depend on interpretations of trends evident in larger scale satellite loops and pressure pattern analyses and the projections of computer models.

As for pinpointing storms before they form, there are rare instances when radar imagery can provide clues as to the location of a storm cell before it even develops, but most of the time at least a rainshower has to have formed before it becomes visible on the radar display. Sometemis the precipitation is visible to radar aloft before any reaches the ground and we can estimate that rain will begin shortly. Also, there are situations where a storm cell is already active, and the combination of radar intensity and velocity signatures can give meteorologists a good idea of where a tornado could form in the next 15-30 minutes - that capability may be what you are referring to regarding storms before they form. This works best in well developed supercell thunderstorms, which are fairly rare in our area, with less likelihood of significant lead time for weaker, short-lived tornadoes that are more common in this part of the country.