MOVEMENT OF HGH-PRESSURE SYSTEMS

In general, the methods for extrapolation of low-pressure areas are applicable to the movement of high-pressure areas.

The following are general considerations in forecasting the movement of high-pressure systems:

. A surface high, or that portion situated under a blocking high aloft, remains very nearly stationary.

. A high situated under or very near a jetstream is steered by the current aloft.

. Cold, shallow highs are steered more easily than the larger ones. The Canadian and Siberian highs move little when there is no jet max in their vicinity or above them, and they move rapidly when the jet max is present.

. Progressive warm highs move with a speed consistent with that of the major ridges aloft. . With straight westerly currents aloft, surface highs are displaced equatorward.

. Highs tend to move in the direction of, and with the speed of, the isallobaric centers; however, this rule is not very reliable because the isallobaric rises often follow the low rather than lead the high.

Steering is not used for high-pressure systems as widely as for lows because high-pressure cells do not have as great a vertical extent as low-pressure systems. However, steering seems to work about 75 percent of the time for cold highs.

FORECASTING THE INTENSITY OF SURFACE PRESSURE SYSTEMS

LEARNING OBJECTIVES: Forecast the intensity of surface low- and high-pressure centers by using extrapolation, isallobaric indications, relation to frontal movement, aloft indications, weather type, and in relation to normal storm tracks.

The changes in intensity of pressure systems at the surface are determined, to a large extent, by events occurring above the system.

The 3-hour pressure tendencies reported in a synoptic plot indicate the sum of the pressure change due to movement of the system, plus that due to deepening and filling. If the exact amount of pressure change due to movement could be determined, it could be assumed that the system would continue to deepen or fill at that rate. However, it is not normally prudent to assume that the current rate of change will continue, nor just how much of the pressure change is due to movement.

Isallobaric analyses at the surface show the following relationships between the isallobars and the change in intensity of pressure systems:

. When the 3-hour pressure falls extend to the rear of the low, the low is deepening.

. When the 3-hour pressure rises extend ahead of the low, the low tends to fill.

. When the 3-hour pressure rises extend to the rear of the high or ridge, the high or ridge tends to fill.

. Since low-pressure systems usually move in a direction parallel to the isobars in the warm sector, and since the air mass in the warm sector is homogeneous, it is possible to assume that the pressure tendencies in the warm sector are an indication of the deepening or filling of the system. The effects of frontal passages must be removed. Therefore, if a low moves parallel to warm sector isobars, the 3-hour pressure tendency in the warm sector is equal to the deepening or falling of the system.

Remember that when you use the present 3-hour pressure tendency values for any of the above rules, they arc merely an indication of what has been happening, and not necessarily what will be taking place in the future. Consequently, if you use the tendencies for indication of deepening or filling, you will need to study the past trend of the tendencies.

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