SURFACE STREAMLINES ASSOCIATED WITH THE TORRENTIAL RAINS OF AUGUST 18–19, 1955, IN THE NORTHEASTERN UNITED STATES

Abstract

This report deals with a preliminary effort toward applying a technique advocated for use in the Tropics to a middle-latitude situation wherein the weather of the Tropics seemed for a time to have been lifted bodily to middle latitudes. Namias and Dunn [l] have shown how the floods, which occurred in the wake of the passage of the remnants of hurricane Diane in August 1955, occurred in a homogeneous tropical air mass with its associated high moisture content. The analysis of Chapman and Sloan [2] indicates the lack of fronts in the area. The temporary rejuvenation of the dying hurricane and the sudden eastward turn of the storm as it reached the 40th parallel of latitude must therefore be studied using techniques applicable under such circumstances, namely the techniques used successfully in tropical meteorology. C. E. Palmer [3] has proposed that the analysis of tropical situations be undertaken using the streamline analysis technique of Bjerknes and collaborators [4]. Using this technique Palmer was able to locate, on day-today low-level weather maps in the Tropics, lines of convergence which were associated with convective cloud patterns. His lines of convergence, however, were definitely not lines separating air masses of differing densities, though, like fronts, they occurred in regions of horizontal velocity convergence. However, he pointed out that such velocity convergence must also occur in a region where there is convergence in the streamlines along an asymptote. In the present instance a preliminary examination of the surface wind field on the map for 0730 EST of August 18 suggests a marked velocity convergence in a narrow zone extending northward from the dying storm center I located in western Virginia, the zone of convergence curving thence eastward through Pennsylvania into central Connecticut. One may note in particular that southerly winds were being observed at Allentown, Pa., LaGuardia Field, N. Y., New Haven, Conn., and Block Island, R. I., at the same time that easterly winds were being reported a short distance farther north at Scranton, Pa., Poughkeepsie, N, Y., and at Hartford (Bradley Field), Conn. The chart for 0730 EST in figure 1 shows a heavy line delineating this zone of convergence between the southerly and easterly winds. The heavy line is extended westward to show how it joins with a similar zone of convergence extending northward from the storm center separating easterly from northerly flow. The observation that this zone was later to be a line north of which the devastating flood rains occurred (and in fact had already begun ’) and a line which roughly outlined the subsequent path of the low pressure center, which had been identified previously as hurricane Diane, prompted this more thorough analysis of the streamline patterns shown in figure 1. The analyses were constructed at 6-hour intervals beginning with the chart for 0730 EST of August 18. The method of analysis used in the preparation of the charts shown in figure 1 was that which is customarily used in the analysis of streamlines in the Tropics; namely, the construction of isogons. Numerous short lines are then sketched on each isogon parallel to the wind direction. These lines are then used as additional “winds” in the construction of the streamlines. An example of this technique is given by Riehl [5]. Riehl’s example includes both a cyclonic indraft point and a hyperbolic point. The necessity for a hyperbolic point to accompany a cyclonic indraft point has been described recently by Sherman and LaSeur [SI. Similar reasoning applies in dealing with an anticyclonic “outdraft” wind system as sketched in the Adirondack Mountain area of New York State on the 0730 EST chart in figure 1. The accompanying hyperbolic point is located in south central Massachusetts.2