FORECASTING MAXIMUM GUSTS WITH NONFRONTAL THUNDERSTORMS 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   storm’s   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 Storm Intensity 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 associated   conditions. The  storm’s  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 Method 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). FORECASTING  HAIL 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 point. 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 cloud. .  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 and Height 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 5-11