Surface pressure systems move with the upper-level steering current. his principle is based on the concept that pressure systems are moved by the external forces operating on them. Thus, a surface pressure system tends to be steered by the isotherms, contour lines, or streamlines aloft, by the warm sector isobars, or by the orientation of a warm front. This principle is nearly always applied to the relationship between the velocity of a cyclone and the velocity of the basic flow in which it is embedded.

The method works best when the flow pattern changes very slowly, or not at all. If the upper flow pattern is expected to change greatly during the forecast period, you must first forecast the change in this pattern prior to forecasting the movement of the surface pressure systems. Do not attempt to steer a surface system by the flow of an upper level that has closed contours above the surface system. When using the steering method, you must first consider the systems that are expected to have little or no movement; namely, warm highs and cold lows. Then, consider movement of migratory highs and lows; and finally, consider the changes in the intensity of the systems.

Figure 3-6.-Movement of lows in relation to warm sector isobars. (A) Movement of warm sector lows; (B) movement of old occluded cyclones.

Studies that used the steering technique have found, inmost cases, that there was a displacement of the lows poleward and the highs equatorward of the steering current. Therefore, expect low-pressure centers, especially those of large dimension, to be deflected to the left and high-pressure areas deflected to the right of a westerly steering current. Over North America the angle of deflection averages about 15, although deviations range from 0 to 25 or even 30.

The steering technique should not be attempted unless the closed isallobaric minimum is followed by a closed isallobaric maximum some distance to the rear of the low.

THE STEERING CURRENT. The steering flow or current is the basic flow that exerts a strong influence upon the direction and speed of movement of disturbances embedded in it. The steering current or layer is a level, or a combination of levels, in the atmosphere that has a definite relationship to the velocity of movement of the embedded lower level circulations. The movement of surface systems by this flow is the most direct application of the steering technique. Normally, the level above the last closed isobar is selected. This could be the 700-, 500-, or 300-hPa level. However, in practice, it is better to integrate the steering principle over more than one level. The levels most often used are the 700- and 500-hPa levels.

For practical usage, the present 700- or 500-mb chart should be used in conjunction with the 24- or 36-hour prognostic charts for these levels. In this way, changes both in space and time can be considered. For a direct application for a short period of time, transfer the position of the low center to the concurrent 700-mb chart. For direction, move the center in the direction of the contours downstream and slightly inclined to the left for low-pressure areas. Experience with moving systems of this type will soon tell you how much deviation should be made. For speed of the surface cyclone, average the basic current downstream over which the cyclone will pass (take into consideration changes indirection and speed of flow over the forecast period). Take 70 percent of this value for the mean speed for 24 hours. Move the low center along the contours, as described above, for this speed for 24 hours.

This should be your position at that time. For the 500-mb chart, follow the same procedure, except use 50 percent of the wind speed for movement. If these two are not in agreement, take a mean of the two. There may be cases where the 500-mb chart is the only one used. In this case, you will not be able to check the movement against the 700-mb chart.

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