CHANGES IN INTENSITY DURING MOVEMENT OUT OF THE TROPICS

. A storm drifting slowly northward with a slight east or west component will preserve its tropical characteristics.

. Storms moving northward into a frontal area or area of strong temperature gradients usually become extratropical storms. In this case the concentrated center dies out rapidly while the area of gale winds and precipitation expands. The closed circulation aloft gives way to a wave pattern and the storm accelerates to the northeast.

. If the tropical cyclone recurves into a trough containing a deep, slowly moving surface low, it normally will overtake this low and combine with it.

. If the tropical cyclone recurves into a trough containing an upper cold core low, it appears to move into the upper low in most cases.

SHORT-RANGE PREDICTION BY OBJECTIVE TECHNIQUES

As in all so-called objective techniques, no one method will serve to produce an accurate forecast for all tropical storms or extratropical systems. In the following section several techniques that have been developed in recent years are discussed. It should be mentioned that objective techniques should not be considered as the ultimate forecast tool and that all other rules, empirical relationships, and synoptic indications should be integrated into the final forecast. Wholly inaccurate forecasts may result if objective methods are the only ones considered in preparing a forecast.

One of the most prominent and widely used of these methods was devised by Veigas-Miller to predict the 24-hour displacement of hurricanes based primarily on the latest sea-level pressure distribution. Sea-level pressures were used rather than upper air data due primarily to the longer available record of sea-level data, and also because of the advantage of denser areas and time coverage, and the more rapid availability of the data after observation time. In addition to the sea-level pressure field, this method also incorporates the past 24-hour motion and climatological aspects of the storm. Pressure values are read from predetermined points located at intersections of latitude and longitude lines with values divisible by 5. Two different sets of equations are used: one set for a northerly zone, between latitudes 27.6 and 40.0N, and the other set from longitudes from 65.0 to 100.0W. The southerly zone encompasses the same longitudes as the northerly zone, but the latitudes are for 17.5 and 27.5N.

The method described in the preceding two paragraphs was tested by Veigas and Miller on 125 independent cases, about equally distributed between the two zones, during the years 1924-27 and 1954-56. The average vector errors in 24-hour forecast position were about 150 nautical miles for the northerly zone and 95 nautical miles for the southerly zone.

30-Hour Movement of Certain Atlantic Hurricanes

R. J. Shafer has developed an objective method for determining the 30-hour movement of hurricanes by use of sea-level data over the ocean areas and upper air data over the land areas. Motion is described in two components: zonzl and meridional. Westerly motion is determined by consideration of the component of the sea-level pressure gradient surrounding the hurricane and is opposed graphically by the mean temperature field between 850 and 500 hPa. The correlation of these parameters is modified by extrapolation and the geographic locations. Meridional motion is similarly predicted by sea-level and thickness parameters modified by extrapolation. The meridional and zonal computations are then combined into the final 30-hour forecast.

In a test of dependent and independent data it was found that some 85 percent of the storms predicted in the 31 sample cases fell within 2 of the predicted position.

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