Origin of this Applet
Wilson and Megenhardt (1997) examined data of convection that occurred over Florida under low vertical wind shear situations. The authors concluded that the boundary-relative speed (Ub)of a cell was an important factor in predicting cell evolution:
"For Ub values less than 3-5 m s-1 there was considerable storm initiation and relatively well organized, long-lived squall lines. When Ub values were less than 5 m s-1 and were observed to increase above 5 m s-1, the cells tended to decrease in number after about 15-30 min. For Ub values greater than 10 m s-1 storm initiation is likely relatively rare and storms will tend to be scattered and short lived." (Wilson and Megenhardt, 1997)
The study also introduced a chart to help forecasters determine preferential areas of development along a hypothetical circular outflow boundary if the cell velocity (speed and direction) and boundary speed were known. This applet was created to give forecasters an easy way to use the proposed schematic diagram in an operational environment.
Use the link at the bottom of this page to open the applet. The applet uses three input variables (cell speed, cell direction, and boundary speed; all speeds in m s-1) to compute the boundary-relative velocity at points all along the circular boundary. Change any (or all) of the input values in the text fields and press the "Calculate Boundary-relative Speed" button to update the schematic diagram. Boundary-relative speeds displayed in red are in the range of speeds favorable for continued cell development. Cells with the input speed and direction located near boundaries with these orientations and speeds should be monitored for further development.
What if you have a linear (not circular) boundary? Find the point on the circular boundary where your linear boundary will be tangent to the circular boundary. The speed(s) nearest the point where the linear and circular boundaries intersect would be the boundary-relative speed for the linear boundary. Whether a circular or linear boundary, this applet can help forecasters determine which cells along a boundary are likely to intensify.