Movement of Lows in Relation to the Jetstream

lows move with the mean speed of the warm air above them, they will be rapidly moving systems. l If the highest mean temperatures occur under the 700-hPa ridge (isotherms and contours in phase), the ridge itself is warm while the low is cold; therefore, the low will move slowly. . Lows move with a speed of approximately 50 percent of the thermal wind for the 1,000- to 500-hPa stratum, and approximately 75 percent of the thermal wind of the 1,000- to 700-hPa stratum. Movement of Lows in Relation to the Jetstream Some of the rules for moving lows in relation to the jetstream position were mentioned previously. One basic rule however, states that “highs and lows situated under or very near the jetstream behave regularly and follow the steering indications.” Minimum deviations occur when the upper flow does not change with time. Forecasting the Movement of Lows by Statistical Techniques Since it requires many years of experience and a photographic memory to develop a mental catalog of weather patterns, a weather type or normal path classification is a boon to the inexperienced forecaster in identifying situations from the past for application to the present. There are many normal and average conditions to regulate behavior patterns of future movement and development. However, there are also many deviations from the norm. The season of the year and topographical influences are factors to be considered. If we could catalog weather types or average types, and the systems would obey these rules, it would greatly simplify the art of forecasting. However, as a rule, this does not occur. Use these statistical techniques, but do not rely too heavily on them. Normal Tracks In 1914, Bowie and Weightman published climatological tables of the average, by month, of the 24-hour speed and direction of cyclonic centers in the United States. The storms were classified with respect to the point of origin and the current location of the centers. Although these tables appear to be antiquated, some of them resemble relatively recent classifications; therefore, they are of some value to the present-day forecaster. The Marine Climatic Atlas also contains average storm tracks for each month of the year for areas over the oceans of the world. Other publications are available that give average or normal tracks for other areas of the world. Prediction of Maritime Cyclones This method is an empirically derived method for objectively predicting the 24-hour movement and change in intensity of maritime cyclones. The technique requires only measurement of the 500-hPa height and temperature gradients above the current surface center, and determination of the type of 500-hPa flow within which the surface system is embedded. Full details of this method are described in The Prediction of Maritime Cyclones, NAVAIR 50-1P-545. The deepening prediction should be made first, as this will often give a good indication of movement. The explosive intensification of maritime cyclones is a fairly common phenomenon, but is presently among the most difficult problems to forecast. Conversely, there are many situations in which it is important to predict the rapid filling of cyclones. This technique gives an objective method for predicting the 24-hour central change in pressure of those maritime cyclones whose initial positions lie north of 30 degrees north latitude. Further, the technique applies to the winter months only (November through March), although it may be used with some degree of confidence in other months. The following factors are considered the most important: . The location of the surface cyclone center with respect to the 500-hPa pattern. . The strength of the 500-hPa flow above the cyclone center. l The 500-hPa temperature gradient to the northwest of the surface center. Of the lows that deepened, the deepening was in general greater, the stronger the 500-hPa contour and isotherm gradient. The study also indicated that the preferred location for filling cyclones is inside the closed 500-hPa contours, and that deepening cyclones favor the region under open contours in advance of the 500-hPa trough. The remaining portions of the pattern indicate areas of relatively little change, except lows located under a 500-hPa ridge line fill. 3-7