Track and Circulation Analysis of Tropical and Extratropical Cyclones that Cause Extreme Precipitation and Streamflow Events in the New York City Watershed

The Ashokan reservoir in the Catskill Mountains supplies approximately 40% of New York City’s daily drinking water. This presentation will examine the meteorological conditions associated with basin-scale extreme precipitation and streamflow events in the reservoir and identify the tropical cyclones (TCs) and extratropical cyclones (ETCs) associated with each event. Using station observations from 1950 – 2012, extreme events are defined as the top 100 events for 1-day precipitation, multi-day precipitation, and 1-day streamflow. We find that ETCs account for approximately 70 – 80% of the extreme precipitation and streamflow events. TCs account for the majority of the remaining extreme events, and 70% of the TCs associated with extreme precipitation events go through extratropical transition. The seasonality of the hydrometeorological events are shown to be distinct, with extreme precipitation events occurring largely in the warm season and extreme streamflow events occurring mainly in the cool season. The dates associated with each hydrometeorological metric are also compared with one another. Approximately 43% of the extreme events concurrently result in an extreme event among all three hydrometeorological metrics.

Analysis of the paths of storms associated with extreme events does not reveal a most-likely pathway for the ETCs or TCs. Using all tracks that pass within a fixed distance of the reservoir, we find the probability of an ETC passing within 1000 km of the region causing an extreme event is 1.4%. For TCs passing within 500 km, the probability is 26%. For 1-day extreme precipitation events, cyclone tracks associated with ETCs reveal distinct seasonal pathways and circulation patterns. ETCs that occur in winter and fall have more meridional tracks whereas ETCs that occur in spring and summer exhibit more zonal tracks. Composites of all ETCs reveal statistically significant anomalies of moisture and temperature downstream of a negatively-tilted upper-level trough present over the region on the days of the extreme precipitation events. Similarly, for TCs, composites reveal statistically significant anomalies of moisture and temperature over the Northeast. TCs that occur in summer exhibit a weak upper-level trough whereas TCs that occur in fall have a more zonal upper-level flow pattern as well as higher moisture anomalies than those in summer.