Meteorological Conditions Associated with High Streamflow Events Along the Charles and Mystic River

The meteorological conditions associated with high streamflow events from 1950-present in the Charles and Mystic sub-basins of Massachusetts are explored. These rivers feed directly into Boston Harbor, through numerous population centers and high-risk assets. High streamflow events are identified using a combination of daily USGS gauge records and record crest dates from the NWS Advanced Hydrological Prediction Service. USGS gauges with minimum nearby flood-control measures are chosen to provide the strongest link between high streamflow and natural factors. Precipitation is the largest factor associated with high streamflow events, although other conditions such as snow depth, snow melt, soil moisture, and fraction of frozen earth can also be important.  When possible, the MERRA-Land reanalysis dataset is used to assess these additional conditions (available for events after 1979). Both gridded precipitation products (CPC Unified dataset) and station-based precipitation products (United States Historical Climatology Network daily data, as well as NWS Cooperative Station hourly data) are used to assess the precipitation relevant to the events. For the largest events, the preceding precipitation tends to be widespread with pulses of short-duration, high intensity rainfall over the course of several days. Some events occur after a single high-intensity pulse of precipitation, particularly for the Mystic sub-basin – in these cases the pulses tend to contribute the highest single-days totals. With the exception of these single-day precipitation pulses, preceding precipitation does not need to be extreme itself to generate a high streamflow event. For both sub-basins, the majority of high streamflow events occur in the late winter and early spring, although events can and do regularly occur in the summer or autumn. Most of the events are associated with synoptic storms (coastal lows or classic nor’easters), while several are associated with tropical storms, and very few with localized convection. For the synoptic storm-related events in late winter and early spring, heavy snowfall followed by a brief warmup contributes to significant snowmelt preceding the high streamflow events.  High moisture transport into the region precedes most events, contributing to the high precipitation totals. High antecedent streamflow and/or soil moisture does not appear to be a major factor for each of the high streamflow events.