Is Precipitation in Northern New England Becoming More Extreme? Statistical Analysis of Extreme Rainfall in Massachusetts, New Hampshire, and Maine and Updated Estimates of the 100-Year Storm

The objective of this study was to investigate the presence of trends in extreme precipitation (denoted MAXP and defined as the annual maximum daily precipitation depth) time series for coastal northern New England and to assess changes in the magnitude of the so-called 100-year storm. MAXP depths from 48 stations with long, continuous records in Maine, New Hampshire, and Massachusetts were analyzed. At those same stations, the number of daily precipitation depths >= 2 in : (51 mm, denoted as GT2in) was also quantified for each year. Although the seasonally averaged MAXP was found to be fairly uniform throughout the year, the frequency of MAXP is highest during August through October, the typical hurricane season in New England. The presence of trends in MAXP and GT2in was evaluated over four time frames (1954-2005, 1954-2008, 1970-2005, and 1970-2008) using two statistical methods (linear regression and the Mann-Kendall trend test) and at two scales (at-site and regional). The trend analysis over the time period 1954-2005 indicated that MAXP was amazingly stationary; however, a trend in GT2in was found at some stations. More trends in both MAXP and GT2in were present in the time period 1954-2008. The majority of stations in southern New Hampshire and eastern Massachusetts showed evidence of trends in MAXP (but not GT2in) for the time period 1970-2008. That the number of trends in MAXP increased despite the shorter record length suggests a strong increase in the magnitude of extreme precipitation in northern coastal New England in the last few decades. The stationarity of the 1954-2005 record was confirmed by the regional trend analysis, as was the presence of stronger trends in coastal stations when the record was extended through 2008. Most stations that had trends in MAXP also had trends in GT2in. The generalized extreme value (GEV) distribution was used to estimate 100-year precipitation depth quantiles for the 1954-2005 record, which were then compared with Technical Paper No. 40 (TP-40) 100-year, 24-h precipitation depths. Estimates for stations along coastal Massachusetts, New Hampshire, and Maine all exceeded 7 in. (180 mm) and exceeded TP-40 by 1 in. (25 mm) or more. Stations in northeastern Massachusetts, southeastern New Hampshire, and southern Maine exceeded 8 in. (200 mm) and also exceeded TP-40 estimates by more than 2 in. (51 mm). These findings indicate that TP-40 underrepresents coastal storm depths. This study, as well as recent record-breaking events in northern New England, strongly suggests the need for updating of design storm estimates. Furthermore, extreme precipitation events of longer than one-day duration have caused large-scale flooding in the region over the last decade. The magnitude of longer duration storms (particularly two-day storms) may also be increasing, calling for engineered infrastructure that can accommodate increases in both storm magnitude and duration. DOI: 10.1061/(ASCE)HE.1943-5584.0000303. (C) 2011 American Society of Civil Engineers.