The determination of observed atmospheric differences between heavy and light precipitation events in New Jersey, USA

The climatology of a limited sample of heavy precipitation events occurring in New Jersey, USA, is studied via statistical averaging and frequency his tograms of environmental conditions at the event location. Also. the spatia l distribution of related circulation features is examined. In addition, st atistical differences between conditions accompanying the heavy (HEAVY) and a selected sample of light (LIGHT) precipitation events is determined. A l arge number of surface. standard level. stability and wind shear variables are employed as well as synoptic-scale circulation features. Variables that are identified as statistically significant, after a Student's t-test is a pplied to a sample mean and standard deviation are listed by season. In add ition. scatter plot and composite maps are produced to illustrate condition s concurrent with the onset of heavy precipitation. In general, there are only slight differences between values obtained for t he HEAVY sample and the LIGHT sample. However. the differences are large en ough for some variables in some seasons that the forecaster may be able to use these results to advantage. In particular: (i) a significantly warmer a nd moister atmosphere at selected levels is indicated for the HEAVY sample for Autumn and Winter but not for Spring and Summer: (ii) upper-tropospheri c divergence is significantly greater in all seasons except Summer: and (ii i) wind shear is significantly larger in all seasons except Summer. There is much similarity in the mean position of examined synoptic features such as troughs, vorticity maxima, jet axes and jet streaks for the two sa mples. However, the amplitude of circulation in the troposphere is much lar ger for the HEAVY sample. especially in the lower troposphere. In addition. the 850-hPa wind maximum axis (low-level jet indication) is much more ofte n oriented from south to north and located within 300 km of the event locat ion for the HEAVY cases. There was surprisingly no significant sample difference found for static st ability and precipitable water in any season. Based on the results reported here. the forecaster is particularly advised to examine the tropospheric circulation amplitude and the position of low a nd upper-level jet axes and jet streaks before forecasting heavy precipitat ion in the mid-Atlantic region of the USA. Copyright (C) 2001 Royal Meteoro logical Society.