A Study of the Effect of La Niña on the Weather of U.S. Cities

La Niña has been shown to shift poleward the tracks of extratropical cyclones traversing the the North American continents during winter months. An analysis of extratropical cyclones during the winter of 1998-99 corroborates this phenomenon. Given such a shift from the normal track of winter storms, one would expect that this would have an effect on weather conditions. In this study, data sets for fourteen different stations across the United States was examined to determine the effects of La Niña on the normal weather conditions of U.S. cities. This study reveals the specific effects on the weather for these stations due to the 1998-99 La Niña. An analysis was completed for the period of December 1998 through March 1999, to determine the deviations from normal due to the changes associated with the 1998-99 La Niña. Temperature and precipitation data were collected for the period of interest. Deviations from 30-year normal values for both temperature and precipitation on a monthly basis were calculated, plotted in a spreadsheet and graphed.. Cooler and drier conditions were expected for the La Niña event. For the stations selected, the weather conditions did show significant deviations from normal due to the La Niña, but in many cases the temperatures were still above normal. The precipitation had much more distinct results. Most of the southern stations were much drier than normal, as nearly all storms passed to the north. The northern stations were either at or above normal as the storms tracked across the northern United States. The greatest deviations were experienced in stations along the west coast and most closely experienced the expected weather conditions for a La Niña event. This paper will present results collected during the winter of 1998-99 to determine the effect that the La Niña during this period had on the temperature and precipitation on fourteen stations in the United States compared to normal winter conditions. In addition, it will also examine variations in temperature and precipitation to the winter of 1997-98, an El Nino year, and to the winter of two other La Niña years (1988-89 and 1995-96).