Hydrometeorological Conditions Preceding Extreme Streamflow for the Charles and Mystic River Basins of Eastern Massachusetts

Two adjacent river basins in the Metro Boston region of Massachusetts, the Mystic and the Charles, are examined for hydrometeorological links to high streamflow events (HSFEs), 1950–2014. HSFEs are defined as one or more continuous days of streamflow above the mean annual maxima for a selected gauge in each basin. There are notable differences in the HSFEs for these two basins. HSFEs have shorter duration (1–3 days) in the Mystic basin, while HSFEs for the Charles can last from 3–9 days. The majority of Mystic HSFEs are immediately preceded by extreme precipitation (occurring within 24 hours), while only half of those for the Charles are preceded by extreme precipitation (in this case occurring 2–5 days earlier).

While extreme precipitation events are often linked to HSFEs, other factors are often necessary in generating high streamflow, particularly for the Charles, as more than 50% of HSFEs occur at times when streamflow, soil moisture, and total precipitation are statistically above average for a period of at least 2 weeks before the HSFE. The majority of HSFEs occur during late winter and spring (~52% occur during February–June for the Mystic, while ~80% occur for the same period for the Charles), and these HSFEs are frequently linked to the passage of strong coastal lows, which produce extreme precipitation in the form of both rain and snow. For these coastal lows, Mystic HSFEs are linked to strong moisture feed along the Massachusetts coastline and intense precipitation, while Charles HSFEs are linked to strong cyclones located off the Mid-Atlantic and longer-duration precipitation. Envirotranspiration is additionally examined as a factor in the generation of late spring HSFEs.

Summertime HSFEs are strongly linked to extreme precipitation alone, as soil moisture and snow effects are seasonally low during this period. As with winter HSFEs, differences in precipitation mechanisms (shortwaves, warm conveyor belts, extratropical and tropical cyclones, and convection) for summertime HSFEs along the two basins are examined.