Forecast Description
NCWF-2 produces rapidly-updating probabilistic forecasts of convection that has deemed hazardous to aviation. The system produces 30, 60, 90 and 120 min probabilistic forecasts are every 5 minutes. The frequent update cycle is required to capture rapidly changing nature of thunderstorms which evolve over very short time scales (order 30 min). The goal of this system is to continuously provide the most up-to-date information on convective activity across the CONUS for aviation users including aviation weather forecasters, airline dispatchers, general aviation, and FAA Traffic Management Units and Central Weather Service Units. Forecasts are produced through the following sequence of steps: (1) detection, (2) motion determination, (3) trending, (4) extrapolation, (5) conversion to probabilities and (6) dissemination and display.
Tracking Storms
A spatial average is performed on the NCHD data to remove perishable scales. Averaging is performed using an elliptical filter (filter dimensions = 48 X 12 km2) which is rotated 360 degrees to determine the angle of maximum coverage. The elliptical filter maintains the linear structure of a storm system while removing smaller scale cells that often have a different motion than the line itself. The filtering allows for a better depiction of the line’s propagation, removing contributions from the movement of short-lived cells within the line.
National Convective Hazard Detection before and after the elliptical filter is applied. Mouse over the image above to see how the smaller-scale features are removed using an elliptical filter on the line of cells depicted by the NCHD.
The storm shapes are then detected using the Thunderstorm Identification Tracking and Nowcasting (TITAN, Dixon and Wiener 1993) algorithm which is run on the smooth NCHD. TITAN is programmed to detect convective elements by looking for areas with VIP levels greater than 3 exceeding a contiguous area of 520 km2. TITAN matches current and previously determined shapes to determine storm motion (speed and direction).
Smoothed NHCD showing Titan detection with mouse over.
Titan shapes are obtained from the smoothed NCHD field at successive timesteps (see above). The shapes are matched to produce motion vectors for each tracked object.
Mouse over image to see successive Titan detections that are 1 hour apart. The successive detections are used to determine storm motion which is indicated by the black line segments and contours seen in Figure 5.2c.