Heavy Precipitation Events in New Jersey: Attendant Upper-Air Conditions

Abstract

The first of an anticipated multipart study of atmospheric conditions occurring before and during heavy precipitation events in New Jersey, representative of the mid-Atlantic region, is presented. Upper-air data parameters were analyzed for 81 cases of heavy precipitation observed in the period 1958?93. These variables consisted mainly of standard level (850?100 mb) temperature, moisture, wind, equivalent potential temperature, vorticity, and height, plus selected advections, divergences, and stability indices. Means and standard deviations of variables, stratified by season, were calculated over the event location near the start time of the event. In addition, sample mean values were compared between the event sample and a climatological sample. A standard t test was used to determine which variables are significantly different between the two samples. Composite maps are presented for selected variables of interest that confirm that the large-scale environment near the beginning of events is altered significantly from the background climatology, as expected. Histograms are used to show frequency distributions for variables that had associated high levels of significance, which further illustrate the upper-air changes that occur and form a basis for selecting key values of important variables for possible operational use. A key values table is presented to facilitate the operational use of these results. The most common findings among the four seasons are higher moisture from 850 to 400 mb, moisture convergence in the lower troposphere, wind divergence in the upper troposphere and convergence in the lower troposphere, warm advection in the low to midtroposphere, positive moisture advection (except for summer), and higher temperatures at most levels (except for summer). While these results are not unexpected, the magnitude of midtropospheric moisture and wind divergence difference (obtained using radiosonde wind observations) between samples is somewhat surprising.