FORECASTING THE MOVEMENT OF SURFACE PRESSURE SYSTEMS

LEARNING OBJECTIVES Forecast the movement of surface low- and high-pressure systems by extrapolation, isallobaric indications, relation to warm sector isobars, relation to frontal movement, thickness lines, relation to the jetstream, and statistical techniques.

Whether you move the high- or low-pressure areas first is a matter of choice for the forecaster, Most forecasters prefer to move the low-pressure areas first, and then the high-pressure areas.

MOVEMENT OF LOW-PRESSURE SYSTEMS

Lows determine, to a large extent, the frontal positions. The y also determine a portion of the isobaric configuration in highs because gradients readjust between the two. As a result of knowing the interplay of energy between the systems, meteorologists have evolved rules and methods for progging the movement, formation, intensification, and dissipation of lows. Extrapolation 

First and foremost in forecasting the movement of lows should be their past history. This is a record of the pressure centers, attendant fronts, their direction and speed of movement, and their intensification/weakening. From this past history, you can draw many valid conclusions as to the future behavior of the systems and their future motion. This technique is valid for both highs and lows for short periods of time.

The general procedure for the extrapolation of low-pressure areas is outlined below. Although only movement is covered, the central pressures with anticipated trends could be added to obtain an intensity forecast.

1. Trace in at least four consecutive past positions of the centers.

2. Place an encircled X over each one of these positions, and connect them with a dashed line, (See fig. 3-3.) If you know the speed and direction of movement, as obtained from past charts, the forecasted position can be calculated. One word of caution, straight linear extrapolation is seldom valid beyond 12 hours. Beyond this 12-hour extrapolated position, deepening/filling, acceleration/deceleration, and changes in the path must be taken into consideration. It is extremely important that valid history be followed from chart to chart, Systems do not normally appear out of nowhere, nor do they just disappear

3. An adjustment based on a comparison between the present chart and the preceding chart must be made, For example, the prolonged path of a cyclone center must not run into a stationary or quasi-stationary anticyclone, notably the stationary anticyclones, over continents in winter. When the projected path points toward such anticyclones, it will usually be found that the speed of the cyclone center decreases and the path curves northward. This path will continue northward until it becomes parallel to the isobars around the quasi-stationary high. The speed of the center will be least where the curvature of the path is greatest. When the center resumes a more or less straight path, the speed again increases.

Figure 3-3.-Example of extrapolation procedure. X is the extrapolated position.

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