Formation Forecasting Principles (Cutoff Lows)

Another task in forecasting is that of the formation of cutoff lows. Some of the indicators are as follows:

. They generally form only off the southwestern coast of the United States and the northwestern coast of Africa.

. The upstream ridge intensifies greatly. This intensifying upstream ridge contains an increasing, strong, southwesterly flow.

. Strong northerlies on the west side of the trough.

. Height falls move south or southeastward.

. Strong cold air advection occurs on the west side of the upper trough.

The constant pressure prognostic chart is about to take form. The forecasted position of the long wave troughs and ridges have been determined and depicted on the tentative prognostic chart. The position of the highs, lows, and cutoff centers were then determined and depicted on the tentative prognostic chart. Short waves were treated in a similar fashion. Contours are then depicted. The height values of the contours are determined by actual changes in intensity of the systems. The pattern of the contours is largely determined by the position of the long waves, short waves, and closed pressure systems. Contours are drawn in accordance with the following eight steps:

1. Outline the areas of warm and cold advection in the stratum between 500 and 200 hPa, and move the thickness lines at approximately 50 percent of the indicated thickness gradient in the direction of the thermal wind.

2. Tentatively note, at several points on the chart, the areas of height changes on the constant pressure surface above the existing height values.

3. Move the areas of 24-hour height rises and falls at the speed of the short waves, and note at several key points the amount and direction of the height change from the current chart.

4. Adjust the advected height changes, and, in turn, adjust these for positions of the long waves, pressure systems, and short waves.

5. Extrapolate heights for selected points at 500 hPa on the basis of the 24-hour time differential indications and advection considerations, provided that they are justified by the indications of high-level convergence and divergence. When the contributions from advection and time differentials are not in agreement with convergence and divergence, adjust the contribution of each and use accordingly.

6. Adjust the height values to the forecasted intensities of the systems. These adjustments can lead to the following:

. All factors point toward intensification (deepening of lows and filling of highs).

. One factor washes away the contribution made by another, and the system remains at or near its present state of intensity.

. All factors point toward weakening of the system.

7. Sketch the preliminary contours, connecting the forecast positions of the long waves, short waves, and the pressure systems with the values determined by steps 1 through 5 above.

8. The last step in the construction of a constant pressure prognostic chart is to check the chart for the following points:

. The chart should follow continuity from the existing pattern.

. The chart should be vertically consistent and rational in the horizontal.

. The chart should not deviate from the seasonal pattern unless substantiated beyond a doubt.

. Unless indicators dictate otherwise, it should follow the normal patterns.

Now draw the smooth contours, troughs, ridges, highs, and lows; and adjust the gradients.

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