EXTRAP

This interactive applet enables you to extrapolate the flow of weather to a particular location by selecting a feature that is representative of the flow of other features that may be heading your way. E.g., Find an easily selectable feature such as a dry line. Select the dry line in a couple of frames, then click control (Ctrl) and right mouse button over a location, like Madison. The line extending from the location that you selected will have times on it. Features like storm cells or dry lines along the line will reach your location at the time that is stamped. Please follow the directions below.

Unlike other tracking programs, this applet lets you know what sort of weather will arrive at a particular location and when. You are not necessarily trying to track the motion of a particular storm.

Instructions:

• Enable/disable images using check boxes at top
• Click left mouse button to step through images
• Click or drag mouse with right mouse button (or on some machines: hold shift key and click or drag left button) to mark & refine the location of a feature on each image which represents the general motion (or component of motion)
• Hold control (Ctrl) and click left button over your location of interest to perform extrapolation to that location.

When you mark a feature, you may find it easier to use one of the endpoints of the marker cross-hair, rather than the center which may obscure the feature or important nearby clues.

When you "forecast" by moving the mouse pointer over a location and click the button while holding the Control (Ctrl) key down, the line that is drawn on the screen is forward in time but backwards in space, so that you see (given the movement you defined) what will arrive at the destination point and when....assuming persistence, of course!

When you use this, you are not extrapolating the movement of the feature you track; rather, you are using that feature to obtain a “ representative motion” of everything in the field. The vector that is drawn when you want to forecast will show you (based on linear extrapolation) what, if anything, will arrive at the position you put the cursor at; it may, or may not, have any thing to do with the feature you tracked (except for the motion). The idea is to find out what will arrive at a particular point (like your city) and when. It is not to answer questions like “where is that thunderstorm going?” (although, it can be used for that).

Disclaimer: use at your own risk. We make no claims as to the accuracy or suitability of this applet for your particular application. By using this, you agree to hold us harmless. The Extrap applet is Copyright(c) 1998 by Tom Whittaker.