hurricane, their analysis is necessarily rather subjective.
Some forecasters claim dependable results in using this
concept when data were available to high levels near the
storm. The technique is not based on the assumption
that wind at any single level is responsible for steering
the storm, since the forces controlling movement are
active through a deep layer of the atmosphere.
However, as successive levels are analyzed, a level is
found at which the closed cyclonic circulation of the
storm virtually disappears. This steering level
coincides with the top of the warm vortex and varies in
height with different stages and intensities of the storm.
It maybe located as low as 20,000 feet or, in the case of
a large mature storm, as high as 50,000 feet. It has been
found in analysis that most weight should be given to
the winds in advance of the storm within a radius of 200
to 300 miles in preference to those in the rear quadrants.
The hurricane generally moves with a speed of 60 to 80
percent of the current at the steering level.
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
a storm will move along tongues of warm air in the layer
from 700 to 500 hPa that often extends out to 1,000 miles
ahead of the storm. The orientation of the axis of the
tongue then may be regarded as a reliable indicator of
storm movement for the next 24 hours. A considerable
difficulty in applying this technique is created by the fact
that the warm tongue sometimes has more than one
branch and it is questionable as to which is the major
axis.
RECURVATURE
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.
Indicators of Possible Recurvature
Some of the indicators of possible recurvature are
as follows:
. If the base of the polar westerlies lowers to
15,000 to 20,000 feet west of the storm’s latitude, and
remains in this position, recurvature may then be
expected to occur.
. However, if there is the building of a dynamic
high or an eastward movement of this high to the rear
of the advancing trough, and the westerlies dissipate in
the low latitudes, the storm will move past the trough to
its south and continue its westward path.
l The above rule also holds true in cases where the
polar trough moves from the west against a blocking
high. The higher latitude portion of the trough continues
to move eastward while the southern segment of the
trough is retarded and is no longer connected with the
upper portion of the trough.
l Recurvature may be expected when an anchor
trough is about 500 miles west of the storm and when
the forward edge of the westerlies is from 500 to 700
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