Nowcasting is very short-range analysis and forecasting, usually from the present out to about six hours, driven mostly by current observations and rapidly updating data rather than by longer-range numerical models. It sits between real-time analysis and traditional numerical weather prediction.
What it draws on
A nowcast is built from what the atmosphere is doing right now. Radar volumes, satellite imagery, lightning networks, surface mesonet reports, and short-fuse model output all feed it. The forecaster or algorithm looks for the trend across the last few frames and extends it forward.
For convection, that means watching cell splits, boundary intersections, updraft strengthening, and storm motion changes as they happen. Rapidly updating models such as the Rapid Refresh and the High-Resolution Rapid Refresh support the human nowcast by ingesting the latest observations each hour.
How it is used operationally
National Weather Service forecasters use nowcasting to write warnings, mesoscale discussions, and short-fuse updates. The Storm Prediction Center issues mesoscale discussions when the near-term picture is changing fast enough that outlooks alone cannot keep up.
In the field, nowcasting is what a chaser is doing when they weigh the last three radar scans, the newest surface obs, and the visual clues from the cell in front of them, then decide whether to hold, reposition, or drop the storm.
Where nowcasts mislead
Extrapolation works only when the atmosphere keeps behaving the way it just did. Boundary passages, cold pool surges, a supercell right turn, and rapid destabilization can all change storm behavior faster than a trend line can follow.
The useful lead time also shrinks as the phenomenon gets smaller and shorter-lived. A large mesoscale convective system may be nowcastable for a couple of hours, while a single mesocyclone cycle or a tornado touchdown may only be nowcastable for minutes.
Treat a nowcast as a working estimate, not a promise. When the newest observation disagrees with the extrapolation, the observation wins, and the nowcast is redrawn.