CHAPTER 5
FORECASTING SEVERE WEATHER FEATURES
The paramount responsibilities of the forecaster
include providing forecasts of severe weather
conditions and timely warnings to aircraft and ships to
ensure the safety of their operations, as well as the safety
of their personnel.
This chapter discusses some of these phenomena
and methods that may be used to forecast severe weather
conditions.
THUNDERSTORMS
LEARNING OBJECTIVES: Recognize
phenomena associated with thunderstorm
activity. Forecast the movement and intensity
of thunderstorms.
The thunderstorm represents one of the most
formidable weather hazards in temperate and tropical
zones.
The turbulence, high winds, heavy rain, and
occasionally hail that accompany thunderstorms are a
definite threat to the safety of flight and to the security
of naval installations. It is important that the forecaster
be acquainted with the structure of thunderstorms and
their associated weather, as well as the knowledge to
accurately predict their formation and movement.
The AG2 TRAMAN, volume 1, covered
thunderstorm formation and movement. Therefore, in
this chapter, we will discuss, in more detail, the weather
phenomena associated with thunderstorms and various
methods of forecasting their intensity and movement.
THUNDERSTORM TURBULENCE AND
WEATHER
Tlmnderstorms are characterized by turbulence,
moderate to extreme updrafts and downdrafts, hail,
icing, lightning, precipitation, and, under most severe
conditions (in certain areas), tornados.
Turbulence (Drafts and Gusts)
Downdrafts and updrafts are currents of air that may
be continuous over many thousands of feet in the
vertical, and horizontally as large as the extent of the
thunderstorm.
The velocity of the downdrafts and
updrafts is relatively constant as contrasted to gusts.
Gusts are primarily responsible for the bumpiness
(turbulence) normally encountered in cumuliform
clouds. A downdraft or updraft maybe compared to a
river flowing at a fairly constant rate, whereas a gust is
comparable to an eddy or other type of random motion
of water in a river.
Studies of the structure of the thunderstorm cell
indicate that during the cumulus stage of development,
the updrafts may cover a horizontal area as large as 6
miles. In the cumulus stage, the updraft may extend
from below the cloud base to the cloud top, a height
greater than 25,000 feet. During the mature stage, the
updrafts cease in the lower levels of the cloud, although
they continue in the upper levels where cloud tops may
exceed 60,000 feet. These drafts are of considerable
importance in aviation because of the change in altitude
that may occur when an aircraft flies through them.
In general, the maximum number of high velocity
gusts are found at altitudes of 5,000 to 10,000 feet below
the top of the thunderstorm cloud, while the least severe
turbulence is encountered near the base of the
thunderstorm. The characteristic response of an aircraft
intercepting a series of gusts is a number of sharp
accelerations or “bumps” without an accompanying
change in altitude.
The degree of bumpiness or
turbulence experienced in flight is related to both the
number of such abrupt changes encountered in a given
distance and the strength of the individual changes.
Hail
Hail is regarded as one of the worst hazards of flying
in thunderstorms. It usually occurs during the mature
stage of cells that have updrafts of more than average
intensity, and is found with the greatest frequency
between 10,000 and 15,000 feet. As a general rule, the
greater the vertical extent of the thunderstorm, the more
likely hail will occur.
Although encounters by aircraft with large hail are
not too common, hail can severely damage an aircraft
in a very few seconds.
The general conclusion
regarding hail is that most midlatitude storms contain
hail sometime during their cycle. Most hail will occur
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