Figure 11-3.-Surface flow pattern (dashed line) and 300- to
200-hPa flow pattern (solid lines). (A) Corresponds to track
(A) in figure 11-2; (B) Corresponds to track (B) in figure
11-2.
. Tropical cyclones move toward the area where
the surface pressure falls increase most rapidly with
time. For calculation, 3-hourly reports are necessary.
Take, for example, the 24-hour pressure change at 1800
and subtract it from the 24-hour pressure change at
1500. This gives the acceleration of the pressure fall. If
this quantity is computed for a network of stations and
isolines are drawn for a suitable interval, the negative
center helps to pinpoint the expected cyclone position.
This rule is used at intervals of 12 hours or less, and is
especially helpful in determining the precise place of
landfall for a storm that is just offshore.
CHANGES IN INTENSITY DURING
MOVEMENT OUT OF THE TROPICS
During movement out of the Tropics the storm
comes under influences different from those in its origin
and tropical path. The following rules and observations
can be helpful in forecasting the storm’s changes in
intensity:
l A storm drifting slowly northward with a slight
east or west component will preserve its tropical
characteristics.
. Storms moving northward into a frontal area or
area of strong temperature gradients usually become
extratropical storms.
In this case the concentrated
center dies out rapidly while the area of gale winds and
precipitation expands.
The closed circulation aloft
gives way to a wave pattern and the storm accelerates
to the northeast.
l If the tropical cyclone recurves into a trough
containing a deep, slowly moving surface low, it
normally will overtake this low and combine with it.
Temporary intensification may result.
l If the tropical cyclone recurves into a trough
containing an upper cold core low, it appears to move
into the upper low in most cases.
SHORT-RANGE PREDICTION BY
OBJECTIVE TECHNIQUES
As in all so-called objective techniques, no one
method will serve to produce an accurate forecast for all
tropical storms or extratropical systems. In the
following section several techniques that have been
developed in recent years are discussed. It should be
mentioned that objective techniques should not be
considered as the ultimate forecast tool and that all other
rules, empirical relationships, and synoptic indications
should be integrated into the final forecast. Wholly
inaccurate forecasts may result if objective methods are
the only ones considered in preparing a forecast.
Statistical Methods
One of the most prominent and widely used of these
methods was devised by Veigas-Miller to predict the
24-hour displacement of hurricanes based primarily on
the latest sea-level pressure distribution. Sea-level
pressures were used rather than upper air data due
primarily to the longer available record of sea-level data,
and also because of the advantage of denser areas and
time coverage, and the more rapid availability of the
data after observation time. In addition to the sea-level
pressure field, this method also incorporates the past
24-hour motion and climatological aspects of the storm.
Pressure values are read from predetermined points
located at intersections of latitude and longitude lines
11-8
