3. WEATHER FORECASTING 



Short-, Medium-, and Long-Term Forecasting 



The effects of weather on human 

 activities and the importance of ac- 

 curate weather predictions and timely 

 weather warnings for human safety 

 and comfort hardly need stating. The 

 farmer, the seafarer, the aviator, the 

 man on the street all share a com- 

 mon concern for the weather. Hurri- 

 canes, tornadoes, floods, heavy snows, 

 and other severe weather phenomena 

 take a heavy toll of lives and cause 

 billions of dollars loss in damage and 

 disruption each year. The cost would 

 be even greater were it not for 

 weather warnings and forecasts. 



As the science of weather predic- 

 tion grows, it touches an ever wider 

 range of human problems. Today 

 there is much concern about air pol- 

 lution and the possible effects of pol- 

 lutants on weather and climate. The 

 mathematical models used in numeri- 

 cal weather prediction provide the 

 best known means of determining 

 how pollutants are spread over large 

 distances and how they might affect 

 weather patterns. Furthermore, math- 

 ematical modeling has reached the 

 stage where interactions of the at- 

 mosphere with the ocean can be 

 taken into account. This development 

 opens up the possibility of predicting 

 changes in the physical state of the 

 upper layers of the ocean, which 

 might prove useful to the fishing in- 

 dustry and other marine activities. 



Although of great economic value, 

 present-day forecasts fall well short 

 of perfection. Even modest improve- 

 ments in accuracy would result in 

 substantial additional benefits. With 

 the new tools now available, espe- 

 cially the meteorological satellite, op- 

 portunities exist for increasing the 

 accuracy of forecasts at all ranges — 

 short, medium, and long. 



The Nature of Weather Prediction 



It is customary, and for some pur- 

 poses useful, to divide the subject of 

 weather prediction into three cate- 

 gories: short, medium, and long 

 term. These categories are generally 

 understood to refer to time ranges of 

 0-24 hours, 1-5 days, and beyond 

 5 days (e.g., monthly and seasonal 

 forecasts), respectively. While it is 

 often convenient to discuss the fore- 

 cast problem under these headings, 

 it is important to realize that they 

 do not necessarily represent logical 

 divisions of the subject in terms of 

 methodology employed, concepts in- 

 volved, or phenomena treated. 



Weather prediction, as presently 

 practiced, is actually a highly com- 

 plex subject. It deals with such 

 diverse phenomena as thunderstorms, 

 tornadoes, hurricanes, and cyclonic 

 storms, and with a wide variety of 

 weather elements — wind, tempera- 

 ture, and precipitation, to name a few 

 of the more important. Moreover, it 

 involves the use of an assortment of 

 techniques, some based on human 

 judgment, others founded on physical 

 law and numerical computation. 

 Weather forecasting is still a mixture 

 of art and science, but a mixture in 

 which the scientific ingredient is be- 

 coming increasingly dominant as fun- 

 damental understanding of the at- 

 mosphere grows and more and more 

 application is found for numerical 

 methods. 



In the following sections we will 

 review the principal elements in- 

 volved in prediction at different 

 ranges, dividing the subject according 

 to the pertinent phenomena. Figure 

 IV-8 shows the geographical range, 

 both latitudinally and in height, of 

 data needed for forecasting. To aid 



in the discussion, it is desirable first 

 to summarize briefly the methods 

 employed in weather prediction. 



Prediction Methods 



Numerical Weather Prediction — 

 This is the term applied to forecast 

 methods in which high-speed digital 

 computers are used to solve the 

 physical equations governing atmos- 

 pheric motions. In order to compute 

 the future state of the atmosphere 

 accurately, the initial or present state 

 must be specified by observation. 

 Numerical methods are most success- 

 fully applied in predicting the be- 

 havior of the synoptic-scale disturb- 

 ances (cyclones, anticyclones, jet 

 streams) of middle and high latitudes. 



Extrapolation — In this method, 

 successive positions of the feature 

 being forecast, for instance a low- 

 pressure center, are mapped, and the 

 future position is estimated by con- 

 tinuing past displacements or trends. 

 Since the advent of numerical weather 

 prediction, this method has fallen into 

 disuse in predicting motions of syn- 

 optic systems, but it is still useful in 

 other connections, for example, in 

 predicting movements of individual 

 thunderstorms seen on a radarscope. 



Steering — In the steering method, 

 a smaller-scale weather system or 

 feature is assumed to move with the 

 direction and speed of a larger-scale 

 current in which it is embedded. 

 Thus, a hurricane may be displaced 

 according to the broad-scale trade- 

 wind current in its vicinity. The ac- 

 curacy of the method depends on how 

 well the basic steering assumption 

 is satisfied and how accurately the 

 steering current is known or pre- 

 dicted. 



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