As a further aid, satellite imagery can also be used in preparing prognostic charts. The availability of useful satellite data will vary with time and area.

OBJECTIVE FORECASTING TECHNIQUES

LEARNING OBJECTIVES: Evaluate various objective forecasting techniques, including extrapolation and isotherm-contour relationships for the movement of troughs and ridges. Forecast intensity of troughs and ridges. Forecast the movement of upper level features. Forecast the intensity of upper level and associated surface features. Lastly, forecast the formation of upper level and associated surface features.

Experience in itself is not always enough to forecast the movement and/or intensity of upper air systems, but, couple the forecasters experience with basic objective techniques and a more accurate product will be prepared.

FORECASTING THE MOVEMENT OF TROUGHS AND RIDGES

Techniques covered in this section apply primarily to long waves. Some of the techniques will be applicable to short waves as well. A long wave is by definition a wave in the major belt of westerlies, which is characterized by large length and significant amplitude. (See the AG2 TRAMAN, volume 1, for a discussion of long and short waves.) Therefore, the first step in progging the movement and intensity of long waves is to determine their limits. There are several basic approaches to the progging of both long and short waves. Chiefly, these are extrapolation, isotherm-contour relationship, and the location of the jet maximum in relation to the current in which it lies.

The past history of systems affecting an area of interest is fundamental to the success of forecasting. Atmospheric systems usually change slowly, but, continuously with time. That is, there is continuity in the weather patterns on a sequence of weather charts. When a particular pressure system or height center exhibits a tendency to continue without much change, it is said to be persistent. These concepts of persistence and continuity are fundamental forecast aids.

The extrapolation procedures used in forecasting may vary from simple extrapolation to the use of more complex mathematical equations and analog methods based on theory. The forecaster should extrapolate past and present conditions to obtain future conditions. Extrapolation is the simplest method of forecasting both long and short wave movement.

Simple extrapolation is merely the movement of the trough or ridge to a future position based on past and current movement and expected trends. It is based on the assumption that the changes in speed of movement and intensity are slow and gradual. However, it should be noted that developments frequently occur that are not revealed from present or past indications. However, if such developments can be forecast by other techniques, allowances can be made.

Extrapolation for short periods on short waves is generally valid. The major disadvantage of extrapolating the long period movement of short waves or long waves is that past and present trends do not continue indefinitely. This can be seen when we consider a wave with a history of retrogression. The retrogression will not continue indefinitely, and we must look for indications of its reversal; that is, progressive movement.

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