FORECASTING MAXIMUM GUSTS WITH
Maximum gusts associated with thunderstorms
occur over a very small portion of the area in which the
thunderstorm exists, and usually occur immediately
before the storms passage. Nevertheless, the
possibility of damage to aircraft and installations on the
surface is so great that every available means should be
used to make the best and most accurate forecast
possible to forewarn the agencies concerned.
Estimation of Gusts From Climatology and
The forecaster is aware that the season of the year
and the station location have a great bearing upon the
maximum winds to be expected. Certain areas of the
United States, and the world, have a history of severe
thunderstorm occurrence with associated strong winds
during the most favorable seasons of the year. For this
reason you should have the thunderstorm climatology
for your station, as well as for the general area, available
as to time of occurrence, season of occurrence, and the
The storms intensity and reports from neighboring
stations can give you a good indication of conditions to
expect at your station.
Forecasting Peak Wind Gusts Using the USAF
Refer to the AG2 TRAMAN, volume 2, unit 6, for
an explanation of two methods for forecasting
maximum wind gusts of convective origin. One
involves the use of the Dry Stability Index (T1) and the
other the downrush temperature subtracted from the
dry-bulb temperature (T2).
Hail, like the maximum wind gusts in
thunderstorms, usually takes place in a narrow shaft that
is seldom wider than a mile or 2 and usually less than a
mile wide. The occurrence of hail in thunderstorms was
discussed earlier in this chapter. However, a few
additional facts concerning the occurrence and
frequency of hail in flight should be discussed at this
l Since hail is normally associated with
thunderstorms, the season of the maximum occurrence
of hail is coincident with the season of maximum
occurrence of thunderstorms.
. When the storm is large and well developed, an
assumption should be made that it contains hail.
l Encounters of hail below 10,000 feet show the
hail distribution to the equally divided between the clear
air alongside the thunderstorm, in the rain area beneath
the storm, and within the thunderstorm itself.
. From 10,000 to 20,000 feet, the percentages
range from 40 percent in the clear air alongside the storm
to 60 percent in the storm, with 82 percent of the
encounters outside the storm beneath the overhanging
. Above 20,000 feet, the percentages reflect 80
percent of the hail is encountered in the storm with 20
percent in the clear air beneath the anvil or other cloud
extending from the storm.
Climatology is of vital importance in predicting hail
occurrence, as well as hail size. Good estimations of
the size of hail can be gleaned from reports of the storm
passage over nearby upstream stations. Here too,
modifying influences must be taken into account.
Hail Frequency as Related to Storm Intensity
Fifty percent hail frequencies may be expected in
storms exceeding 46,000 feet, based on radar reports.
With a maximum echo height of 52,000 feet, a 67
percent hail frequency can be expected; and only 33
percent at 35,00 feet, Mean echo heights are 42,000 feet
for hail and 36,000 feet for rain.
A Yes-No Hail Forecasting Technique
The technique presented in this section of the
chapter is an objective method of hail forecasting. It
uses the parameters of the ratio of cloud depth below the
freezing level and height of the freezing level. The data
used in this study were derived from 70 severe
convective storms (34 hail-producing and 36
nonhail-producing storms) over the Midwestern States.
Severe thunderstorms, as used in the development of
this technique, were defined as those thunderstorms
causing measurable property damage due to strong
winds, lightning, or heavy rain. Severe thunderstorms
with accompanying hail were defined as those
thunderstorms accompanied by hail where hail was
listed as the prime cause of property damage, even
though other phenomena may also have occurred. All