Synoptic Conditions Associated with Warm Season Precipitation Events at HBEF

Increasing trends in annual precipitation and the frequency of precipitation events have been documented across the Northeast U.S., highlighting the importance of comprehensive synoptic analyses. However, the Hubbard Brook Experimental Forest (HBEF) in the White Mountains of New Hampshire, a key site for long-term ecological research, has not been extensively studied in terms of its synoptic climatology. This study aims to delineate the synoptic conditions associated with warm season precipitation at HBEF for the periods 2000-2004 and 2015-2019. Using 15-minute precipitation data from two rain gauges (RG1 and RG14) on opposing slopes of the forest, precipitation trends from April to September 1970-2019 were analyzed. An increasing trend in annual warm season precipitation was observed, along with a rise in the frequency of precipitation readings. To understand the synoptic conditions contributing to these trends, a k-means clustering approach was applied, grouping storm events based on their duration and maximum precipitation rates into four clusters: short duration and low precipitation rate, short duration and high precipitation rate, long duration and low precipitation rate, and long duration and high precipitation rate. Events from each cluster were analyzed using the Weather Prediction Center’s surface analysis archive to determine their relation to low-pressure systems and frontal passages. Four low-pressure system types (north inland, south inland, north coastal, and south coastal) and corresponding synoptic events (warm front, cold front, stationary front, occluded front, warm sector, trough axis, post-frontal, and miscellaneous) were identified, with a 3-hour cushion applied for frontal passages. Findings revealed a shift towards longer durations and higher maximum precipitation rates for warm sector events in 2015-2019 compared to 2000-2004. Despite the increasing trend, the ratio of frontal passages remained consistent while the frequency of events rose. Analysis indicated a prevalent occurrence of north inland lows, with longer duration and low precipitation rate events showing an equal mix of north and south inland lows in 2015-2019. This shift towards more south inland lows in these events may contribute to the observed longer durations.