The semipermanent Icelandic and Aleutian lows undergo little movement. These semipermanent lows will decrease or increase in area of coverage; occasionally split, or elongate east-west during periods of high zonal index. North-south displacements are due primarily to seasonal effects. The movement of these semipermanent lows is derived primarily from extrapolation.

EXTRAPOLATION. Extrapolation can be used at times to forecast both the movement and the intensity of upper closed lows. This method should be used in conjunction with other methods to arrive at the predicted position and intensity. Figure 2-2 shows some examples of simple extrapolation of both movement and intensity. Remember, there are many variations to these patterns, and each case must be treated individually.

Figure 2-2, view (A), illustrates a forecast in which a low is assumed to be moving at a constant rate and filling. Since the low has moved 300 nautical miles in the past 24 hours, it maybe assumed that it will move 300 nautical miles in the next 24-hour period. Similarly, since the central height value has increased by 30 meters in the past 24 hours, you would forecast the same 30 meter increase for the next 24 hours. While this procedure is very simple, it is seldom sufficiently accurate. It is often refined by consulting a sequence of upper air data to determine a rate of change.

This principle is illustrated in figure 2-2, view (B). By consulting the previous charts, we find the low is filling at a rate of 30 meters per 24 hours; therefore, this constant rate is predicted to continue for the next 24 hours. However, the rate of movement is decreasing at a constant rate of change of 100 nautical miles in 24 hours. Hence, this constant rate of change of movement is then assumed to continue for the next 24 hours, so the low is now predicted to move just 200 nautical miles in the next 24 hours.

Figure 2-2.-Simple extrapolation of the movement and Intensity of a closed low on the 700-mb chart. (A) Constant movement and filling (B) constant rate of change, (C) percentage rate of change.

Frequently neither of these two situations exist, and both the change in movement and the height center change occur at a proportional rate. This is illustrated in figure 2-2, view (C). From a sequence of charts 24 hours apart, it is shown that the low is filling at a decreasing rate and also moving at a decreasing rate. The height change value is 50 percent of the value 24 hours previously on the successive charts, and the rate of movement is 75 percent. We then assume this constant percentage rate to continue for the next 24 hours, so the low is forecast to move 225 nautical miles and fill only 15 meters.

Accelerations may be handled in a similar manner as the decelerations shown in figure 2-2. Also, a sequence of 12-hour charts could be used in lieu of 24-hour charts to determine past trends.

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