Flood — Carroll, New Hampshire

2023-12-18 to 2023-12-19 · near Intervale, Carroll, New Hampshire

Event narrative

Catastrophic flooding occurred across the Conway area on December 18th following heavy rain and snowmelt. The storm featured strong southeast winds measured between 30 to 55 mph in the region, which supported orographically enhanced rainfall and supported rapid snowmelt. The combined runoff from rain and snowmelt averaged 7 to 10 inches in the higher terrain, and 4 to 6 inches in the valley. The steep terrain and frozen ground supported rapid runoff which caused flash flooding along all streams and brooks in the region. The Swift River, a western tributary to the Saco River in Conway, was estimated to have had flows around 12-16 kcfs. The flows in the Saco River in Bartlett reached 24 kcfs. The annual chance occrrence of these flows are estimated around 1% to 0.2%, otherwise referred to as a 100 or 500 year flood. The combined flows in Conway resulted in flows over 60 kcfs as measured by the USGS gage. The stream height reached 17.72 on the evening of the 18th, exceeding the record held by Irene in 2011 which crested at 17.23 feet with a flow of 58.2 kcfs. The river climbed from 4 feet on the 17th to 17 feet in under 24 hours due to the rapid nature of the runoff.

Flooding and Damage was extensive with hundreds of residents impacted. The confluence of the Swift and Saco Rivers in North Conway drowned Washington St and shutting down both ends of West Side Road and preventing most traffic from getting in or out of the region. Over a dozen residents of Transvale Acres had to be rescued with 5 airlifted by helecopter from the National Guard. Homes in this community sustained severe damages with water several feet deep. Properties off Route 302 in Center Conway including Kenwood Circle, Robinwood Road, Burbank Road flooded with water several feet deep. Numerous campgrounds and recreational facilities including a golf course were completely inundated with water chest deep. The flooding continued along Route 113 River Street from Conway to Fryeburg.

Wider weather episode

Low pressure began organizing in the Gulf of Mexico on the 16th, setting record low sea level pressures as it traversed the Southeast through the 17th. Strong high pressure over the North Atlantic created a deep southeasterly flow that drew warm air into New England ahead of the approaching storm. By the time precipitation arrived on the evening of the 17th, temperatures were warm enough for rain everywhere except the highest peaks. Early on the 18th a shortwave trough was approaching from the Great Lakes and led to rapid deepening of the low pressure as it paralleled the Eastern Seaboard. Even as the center of the storm remained near Chesapeake Bay on the morning of the 18th, a strong low level jet had developed and winds began to gust in excess of 50 mph well inland from the coast. Damaging wind gusts continued into the afternoon. Thousands of customers lost power due to falling limbs and downed wires. The anomalously warm air mass also led to intense rainfall rates beginning on the evening of the 17th, which continued into the 18th. Upsloping winds also enhanced precipitation along the southeast facing slopes of the White Mountains. Widespread 3 to 4 inches of rain fell, with isolated pockets of 6 to 8 inches. This fell on top of snow that was ripe from previous rainfall about a week earlier. An additional 2 to 3 inches of snow water equivalent was likely added to rainfall runoff. Numerous road washouts and road closures occurred due to flash flooding and fast responding small stream and tributaries. Localized moderate to major flooding occurred along mainstem rivers with the majority of New Hampshire Rivers outside of the mountains only reaching minor flood stage.

View location on OpenStreetMap → (44.0923, -71.1209)

Source: NOAA Storm Events Database, event_id 1150890. Narrative written by the NWS forecast office that issued the report.