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Other Indications of Development
Extrapolation

Aerographers Mate 1 & C
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. The disturbance is decelerating or moving at a constant  speed. .  The  system  has  a  northward  component  of motion. . A migratory anticyclone passes to the north of the storm center. .  A  strong  net  outflow  (divergence)  is  manifested by anticyclonic flow in the upper levels (200 hPa). .  Long  waves  are  slowly  progressive.  (Applicable to genesis also.) . The trade inversion is absent; convection deep. Other  considerations  to  bear  in  mind: l Intensification occurs when the cyclone passes under an upper-level trough or cyclone, provided there is  relative  motion  between  the  two.  There  is  some indication  that  intensification  does  not  take  place  when the  two  remain  superimposed. . Poleward movement of the cyclone is favorable for intensification; equatorward motion is not favorable. . Intensification occurs only in areas where the sea surface temperature is 79°F or greater, with a high moisture content at all levels. l  Other  factors  being  equal,  deepening  will  occur more rapidly in higher than in lower latitudes. (Coriolis force  is  stronger.) l When a storm moves overland, the intensity will immediately   diminish. The   expected   amount   of decrease in wind speeds can be 30 to 50 percent for storms with winds of 65 knots or more; and 15 to 30 percent for storms with less than 65 knots. If the terrain is rough, there is more decrease in each case than if the terrain is flat. Once an intense tropical cyclone has formed, there will be further changes in its intensity and in the course of its motion within the Tropics and during recurvature. The  forecaster  should  consider  these  changes  in connection with the predicted path of movement. MOVEMENT Tropical cyclones usually move with a direction and speed that closely approximates the tropospheric current that  surrounds  them.  Logically,  therefore,  charts  of  the mean flow of the troposphere should be used as a basis for predicting the movement of tropical cyclones, but lack  of  observations  generally  precludes  this  approach. Generally there is a tendency for tropical cyclones to  follow  a  curved  path  away  from  the  Equator; however, departures from this type of track are frequent and of great variety, Tropical  cyclones  move  toward  the  greatest  surface pressure  falls  and  toward  the  area  where  the  surface pressure falls increase fastest with time. Calculation is necessary for this rule to be used. Numerous theories have been advanced to explain the cyclone tracks of the past and to predict those of the future. Observational data have never been sufficient to prove or disprove most of them. Tropical  storms  move  under  the  influence  of  both external and internal forces. The external forces are a result of air currents that surround a storm and carry it along. The internal forces appear to produce a tendency for  a  northward  displacement  of  the  storm  (which probably is proportional to the intensity of the storm), a westward  displacement  that  decreases  as  latitude increases, and aperiodic oscillation about a mean track. Initial Movement of Intense Cyclones The  movement  of  cyclones  that  are  undergoing  or have just completed initial intensification (about 24 hours) will be as follows: l Storms developing in westward moving wave troughs in the easterlies move toward the west and also with  a  pole  ward  component  given  by  an  angle  of approximately 20° to the right of the axis of the trough looking  downstream.  (See  fig.  11-1.) .   The   motion   of   storms   developing   from preexisting vortices can be extrapolated from the track of these vortices. Figure 11-1.-Illustration of initial movement of tropical cyclones forming in a wave trough In the easterlies. 11-4







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