cooler in the surface layers than the air normally present
over the tropical oceans in summer. This also holds true
in air with a trade inversion and upper dry layer present.
Surface ship temperature and dew point reports, along
with upper air data, are extremely valuable in
determining whether the surface layers are truly tropical
or whether they contain old polar air not yet completely
THE DYNAMICS OF TROPICAL
Dynamically, storms (existing and potential) are
subject to influences from the surrounding areas. In the
Tropics, we usually encounter two layers in the
troposphere, and these levels have very different
characteristics. Most of the time, a steady trade wind
blows in the lower levels (surface to 500 hPa), while a
succession of large cyclonic and anticyclonic vortices
are present in the upper troposphere in the 400- to
150-hPa strata. In some regions, such as the area
between the Mariana Islands and the South China Sea
during midsummer, intense eddy activity also takes
place near the surface. It is not possible to deduce from
charts drawn in the lower layers whet is taking place
above 500 hPa. Therefore, it is necessray to keep track
of events in both layers.
Aside from the surface chart, the 700-hPa level
chart is very helpful in determining low-level flow
patterns. The 200-hPa level is representative of the
upper layer, whereas the 500-hPa level, often located in
the transition zone, is of much less use in tropical than
in extratropical forecasting.
However, forecast considerations should not be
limited to the two layers in the Tropics. Middle latitude
weather also influences and shares in the control of
weather changes in low latitudes. The position and
movement of troughs and ridges in the westerlies affect
both the formation and motion of tropical storms.
Therefore, middle latitude analysis is important. Since
forecasts generally run from 1 to 3 days, there should
also be a hemispheric analysis.
The climatological approach to the forecast
problem should also be taken into consideration. It is
obvious that a forecaster must be familiar with regional
and seasonal changes; for instance, areas of frequent
tropical cyclone formation, mean storm tracks, the
scattering of individual tracks about the mean, and the
month to month variations of all of the above. Mean
tracks and other data on Pacific storms are available in
several publications. These maps and charts reveal that
the climatological approach gives some useful
information, but it cannot be relied on in anyone specific
area, or in the case of any particular storm to the
exclusion of synoptic indications for the forecast. These
probability considerations are used mainly for
long-range planning and when data are unavailable.
Therefore, we reach the conclusion that climatological
information should be treated as a weighing factor to be
included after evaluating synoptic data.
A full-blown typhoon/hurricane cannot be forecast
as such when there are no indications of any type of
irregularity on the charts or aids used in the tropical
analysis. Only when the incipient stage frost appears
among the data can we begin to think in terms of an
actual typhoon/hurricane, and often not even then. For
the most part, the forecast evolution from area of
disturbed weather to typhoon/hurricane is a matter of
step-by-step progression. Assume the term formation
means formation of a potential typhoon/hurricane.
Empirical rules or checks have evolved through the
years that enjoy a measure of reliability to warrant their
use, and of course, the more signs that point to
formation, the more likely formation will occur.
Synoptic Conditions Favorable for
The following list contains conditions favorable for
development of tropical cyclones. In this list, no
attempt is made to separate the surface from the upper
air indications, as the two are most often occurring
simultaneously and are interrelated.
l Marked cyclonic turning in the wind field.
. A shift in the low-level wind direction when
easterlies are normally present. Wind-speeds generally
10 knots or more.
. Greater than normal cloudiness, rainfall, and
pressure falls. Cloudiness that increases in vertical as
well as horizontal extent.
. Moderate to strong outflow aloft.
l Sea level pressures lower than normal. The value
is dependent upon the region analyzed; however, a
deviation of greater than 3 hPa is the normal criterion.
. Easterly wind speeds 25 percent or more above
normal in a limited area, especially when the flow is