miles to the west. Correlated with this parameter, R. J.
Shafer found that a spot value of the thickness between
850 to 500 hPa 7.5 degrees of latitude to the northwest
of the storm was one of the most valuable of all
parameters on recurvature or as an indication of future
movement of Atlantic hurricanes. This thickness
reflects the relative strength of cold troughs to the west
essential for recurvature. Low values of thickness,
14,000 feet (approximately 4,270 meters) or less, almost
always indicate recurvature; and high values, 14,200
feet (4,330 meters) or more, generally indicate
continued westward motion.
. The major trough west of the storm (in the
westerlies) is slowly progressive.
l
l
aloft.
l
Long waves are stationary or slowly progressive.
There is a rapid succession of minor troughs
The climatological mean track indicates
recurvature (use with caution).
l When the neutral point at the southern extremity
of the trough in the westerlies at the 500-hPa level lies
at or equatorward of the latitude of the cyclone,
recurvature into the trough will usually occur. In this
situation, the cyclone would normally be under the
influence of southerly winds from the upper limits to a
level well below the 500-hPa level while approaching
the trough.
. When the subtropical ridge at the 500-hPa level
is broad and consists of large anticyclones, recurvature
usually occurs.
This case represents a low index
situation in which the cyclone remains under the
influence of a single, large, slow-moving anticyclone
for a relatively long time.
. Weak troughs exist between two separate
subtropical high cells. Sometimes tropical storms move
northward through these very weak breaks in
subtropical highs.
Nonrecurvature
The following flow patterns are associated with
nonrecurvature:
. Strong subtropical anticyclone or ridge to the
north of the storm with the mean trough in the westerlies
located far to the west of the longitude of the storm. If
this pattern develops strongly over the western oceans
or continents, a storm will generally be driven inland
and dissipate before it recurves into the western end of
the ridge.
. Flat (small-amplitude waves) in the westerlies at
latitudes near or north of the normal position. A narrow
subtropical ridge separates the westerlies from the
tropical trough. In many cases the mean trough in the
westerlies may be located at the same longitude of the
storm.
Motion During Recurvature
The following rules apply during recurvature only:
. When the radius of recurvature of a storm is
greater than 300 miles, it will not decelerate and may
even accelerate. The storm will slowdown if the radius
of recurvature is less than 300 miles. In general, when
the radius of recurvature is large, it is usually very
uniform. A small radius will occur along a brief portion
of the track. (See fig. 11-2.)
. A large radius of recurvature is to be expected if
the high northeast of the tropical cyclone has a vertical
axis (fig. 11-3 (A)). This will occur when long waves
are stationary.
. When the high slopes south to southeast with
height (fig. 11-3 (B)), the cyclone is transferred rapidly
from the influence of upper easterlies to that of the upper
westerlies and the track has a short bend. This occurs
when long waves are progressive.
The following rules refer to short-term (24 hours or
less) forecasting:
. Tropical cyclones move toward the area of
greatest surface pressure falls (12- to 24-hour pressure
change).
Figure 11-2.-(A) Recurvature at a constant speed; (B) First
decelerating and then accelerating.
11-7
