THE DIRECTION OF MOVEMENT. The direction of movement is not always exactly parallel to the steering current, but has a component toward high pressure that varies inversely with the speed of the current, ranging from almost 0 with rapid movement to as much as 20 with speeds under 20 knots. In westward moving storms, a component of motion toward high pressure could result from the poleward acceleration arising from the variation of the Coriolis parameter across the width of the storm. This would indicate that, to the extent that this effect accounts for the component of motion toward high pressure, northward moving storms would fit the direction of the steering current more closely than westward moving ones. The tendency for poleward drift would be added to the speed of forward motion in the case of a northward moving storm so that it would approach more closely the speed of the steering current. Empirical evidence supports this hypthesis.

Corrections for both direction and rate of movement should be made when this is indicated by the windflow downstream in the region into which the storm will be moving. For prediction beyond several hours, changes in the flow pattern for a considerable distance from the storm must be anticipated. It should also be remembered that intensification or decay of a storm may call for use of a higher or lower level, respectively, to estimate the future steering current.

THERMAL STEERING. A number of efforts have been made to correlate hurricane movement with thermal patterns. For example, one writer suggests that that the warm tongue sometimes has more than one branch and it is questionable as to which is the major axis.

One of the fundamental problems of forecasting the movement of tropical cyclones is that of recurvature. Will the cyclone move along a relatively straight line until it dissipates, or will it follow a track that curves poleward and eastward? When recurvature is expected, the forecaster must next decide where and when it will take place. Then, he or she is faced with the problem of forecasting the radius of the curved track. Even after the cyclone has begun to recurve, there are a great variety of paths that it may take. At any point, it may change course sharply.

The most common recurvature situation arises when an extratropical trough approaches a storm from the west or when the storm moves west to northwest toward a stationary or slowly moving trough.

This information is now available on CD in Adobe PDF Printable Format

Privacy Statement - Press Release - Copyright Information. - Contact Us - Support Integrated Publishing