example, intensification is caused by high-level coldadvection and/or mass convergence.l Under low zonal index situations, a blockinghigh will normally exist at a northern latitude and willhave a pronounced effect on the systems in that area; ingeneral it will slow the movement. Under high zonal index situations, there is astrong west to east component to the winds, and systemswill move rapidly.Movement of Closed LowsThe semipermanent Icelandic and Aleutian lowsundergo little movement. These semipermanent lowswill decrease or increase in area of coverage;occasionally split, or elongate east-west during periodsof high zonal index. North-south displacements are dueprimarily to seasonal effects. The movement of thesesemipermanent lows is derived primarily fromextrapolation.EXTRAPOLATION.— Extrapolation can be usedat times to forecast both the movement and the intensityof upper closed lows. This method should be used inconjunction with other methods to arrive at the predictedposition and intensity. Figure 2-2 shows some examplesof 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 whicha low is assumed to be moving at a constant rate andfilling. Since the low has moved 300 nautical miles inthe past 24 hours, it maybe assumed that it will move300 nautical miles in the next 24-hour period. Similarly,since the central height value has increased by 30 metersin the past 24 hours, you would forecast the same 30meter increase for the next 24 hours. While thisprocedure is very simple, it is seldom sufficientlyaccurate. It is often refined by consulting a sequence ofupper 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 isfilling at a rate of 30 meters per 24 hours; therefore, thisconstant rate is predicted to continue for the next 24hours. However, the rate of movement is decreasing ata constant rate of change of 100 nautical miles in 24hours. Hence, this constant rate of change of movementis then assumed to continue for the next 24 hours, so thelow is now predicted to move just 200 nautical miles inthe 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 andfilling (B) constant rate of change, (C) percentage rate of change.2-6
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