Heavy precipitation events in New Jersey: Attendant upper-air conditions

The first of an anticipated multipart study of atmospheric conditions occur ring before and during heavy precipitation events in New Jersey, representa tive of the mid-Atlantic region, is presented. Upper-air data parameters we re analyzed for 81 cases of heavy precipitation observed in the period 1958 -93. These variables consisted mainly of standard level (850-100 mb) temper ature, moisture, wind, equivalent potential temperature, vorticity, and hei ght, plus selected advections, divergences, and stability indices. Means an d standard deviations of variables, stratified by season, were calculated o ver the event location near the start time of the event. In addition, sampl e mean values were compared between the evens sample and a climatological s ample. A standard t test was used to determine which variables are signific antly different between the two samples. Composite maps are presented for selected variables of interest that confir m 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 tha t occur and form a basis fur selecting key values of important variables fo r possible operational use. A key values table is presented to facilitate t he operational use of these results. The most common findings among the four seasons are higher moisture from 85 0 to 400 mb, moisture convergence in the lower troposphere, wind divergence in the upper troposphere and convergence in the lower troposphere, warm ad vection in the low to midtroposphere, positive moisture advection (except f or summer), and higher temperatures at most levels (except for summer). Whi le these results are not unexpected, the magnitude of midtropospheric moist ure and wind divergence difference (obtained using radiosonde wind observat ions) between samples is somewhat surprising.