THE TREND CHART AS AN EXTRAPOLATION AID

The trend chart can be a valuable forecasting tool when it is used as a chronological portrayal of a group of related factors. It has the added advantage of helping the forecaster to become "current" when coming on duty. At a glance, the relieving duty forecaster is able to get the picture of what has been occurring. Also, the forecaster is able to see the progressive effect of the synoptic situation on the weather when the trend chart is used in conjunction with the current surface chart.

The format of a trend chart should be a function of what is desired; consequently, it may vary in form from situation to situation. It should, however, contain those elements that are predictive in nature.

The trend chart is a method for graphically portaying those factors that the forecasters generally attempt to store in their memory. Included in this trend chart is a list of key predictor stations. The forecaster uses the hourly and special reports from these stations as aids in making short forecasts for his/her station. Usually, the sequences from these predictor stations are scanned and committed to memory. The method is as follows:

1. Determine the direction from which the weather will be arriving; i.e., upstream.

2. Select a predictor station(s) upstream and watch for the onset of the critical factor; for example, rain.

3. Note the effect of this factor on ceiling and visibility at predictor station(s).

4. Extrapolate the approach of the factor to determine its onset at your station.

5. Consider the effect of the factor at predictor station(s) in forecasting its effect at your station.

The chief weakness of this procedure is its subjectivity. The forecaster is required to mentally evaluate all of the information available, both for their station and the predictor station(s).

A question posed, "How many trend charts do I need"? The answer depends on the synoptic situation. There are times when keeping a graphic record is unnecessary; and other times, the trend for the local station may suffice. The trend chart format, figure 4-13, is but one suggested way of portraying the weather record. Experimentation and improvisation are encouraged to find the best form for any particular location or problem.

In the preceding sections of this chapter, several methods have been described for "keeping track of the weather" on a short-term basis. Explanations of time-distance charts, isochrone aids, trend charts, etc., have been presented. It is usually not necessary to use all, or even most of these aids simultaneously. The aid described in this section is designed for use in combination with one or several of the methods

previously described. Time-liners are especially useful for isochrone analysis and follow-on extrapolation. Inasmuch as a majority of incorrect short-range forecasts result from poor timing of weather already upstream, an aid, such as described below, may improve this timing.

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