Contribution to New Jersey Precipitation from Named Tropical Cyclones

The contribution of precipitation from tropical cyclones to the hydrologic climatology of New Jersey was studied from 1971-2011 using observations from National Weather Service Cooperative Observing (Coop) stations. Among the questions explored are whether such events are critical to mid-summer to mid-fall precipitation totals, can they be relied upon to alleviate summer drought conditions, and are they primarily responsible for heavy rainfall and subsequent major flooding events during the tropical season. Answers to these and other questions will prove useful to diverse user communities, including water resource management, agriculture, emergency management, and fish and wildlife.

We first identified any hurricane, tropical storm or numbered depression that remained named or numbered within 500 miles of any NJ location during the study interval. Next, four-day precipitation totals were generated from daily observations at 34 NJ Coop stations, along with 92 stations in surrounding states. A four-day window was selected in order to account for slow moving or expansive storms, and also because of variations in the time of day at which daily observations are taken at Coop stations. While it is possible that some rainfall that is not directly associated with the storm may be included in an event total or perhaps additional rainfall outside of the window may be directly linked to the storm, this does not contribute significantly to study results. There were only 47 missing observations from NJ study stations, and when found they were substituted with observations from an appropriate nearby station.

Observations from 64 events are included in the analysis at monthly, seasonal, annual, and decadal intervals. Individual stations are examined, along with the state as whole and divided into three regions. Observations from nearby states assisted in the verification and mapping of results from the network of NJ stations.

On average, precipitation from these events contributes 3-5% of the annual precipitation across New Jersey. However this rises to a high of 15-25% of September rainfall in the northern half of the state and 10-15% in the south. The higher elevations in the north had the highest percentages. These results differ from Brotak and Shulman, who found a September maximum of 40% along the central coast, with 15-25% elsewhere. It must be noted that their manuscript did not include a well-defined definition of a tropical event, thus a direct comparison with our results is not possible. On a decadal basis, the 1970s had the highest percentage, average 5% statewide (it was also the wettest of the four decades studied), while the 1980s had a 2% contribution. These results in addition to others that examine the impact of individual events and where these storms fall within dry and wet seasons will be discussed in this presentation.