FORECASTING THE MOVEMENTS AND INTENSITY OF FRONTS

Figure 3-13.-Infrared, local midnight, second day. · When high or middle clouds completely surround the vortex center, the cyclone has reached maturity and can be expected to fill. This generally indicates the advection of cold, dry air into the cyclone has ceased. FORECASTING THE MOVEMENTS AND INTENSITY OF FRONTS LEARNING OBJECTIVES Forecast the movement and intensity of fronts by using extrapolation and the geostrophic wind method. The following text discusses various rules for the movement of fronts, as well as the effects upper air features have on fronts. MOVEMENT OF FRONTS Fronts are ordinarily forecasted after the pressure systems have been forecasted. However, there may be cases for short-range forecasting where movement of all of the systems is unnecessary. Extrapolation is perhaps the simplest and most widely used method of moving fronts for short periods. When moving fronts for longer periods, other considerations must be taken into account. The following text discusses a simple extrapolation method, as well as the geostrophic wind method, and considerations based on upper air influences. Extrapolation When fronts are moved by extrapolation, they are merely moved based on past motion. Of course such factors as occluding, frontogensis, frontolysis, change in position, intensity of air masses and cyclones, and orographic influences must be taken into account. Adjustments to the extrapolation frontal positions are made on the basis of the above considerations. You should keep in mind that the adjacent air masses and associated cyclones are the mechanisms that drive the fronts. You should also keep in mind the upper air influences on fronts; for example, the role the 700-hPa winds play in the movement and modification of fronts. Finally, you should remember that past motion is not a guarantee of future movement, and the emphasis should be on considering the changes indicated, and incorporating these changes into an extrapolated movement. Geostrophic Wind Method Frontal movement is forecasted by the geostrophic wind at the surface, and at the 700-hPa level at several points along the front. The basic idea is to determine the component of the wind at the surface and aloft, which is normal (perpendicular) to the front and, therefore, drives the front. Determination of the component normal to the front is made by triangulation. THE PROCEDURE.— The steps are as follows: 3-17