Flash Flood — Coos, New Hampshire

2023-12-18 · near Gorham, Coos, New Hampshire

Event narrative

Significant damage occurred on the Peabody River on December 18th following a heavy rain and rapid snowmelt event. The USGS river gage in Gorham went from under 100 cfs around midnight, to over 13,000 cfs by noon. The streamflows were estimated to be around the 1% annual exceedance or 100-year flood level. Rainfall amounts where estimated around 5 inches with an additional 3 inches of rapid snowmelt runoff. The ground conditions were frozen, supporting complete runoff. One home off Route 16 was deemed unlivable and a complete loss after floodwaters eroded the foundation. Property was also damaged at Great Glen Trails Outdoor Center near the headwaters. Downstream in Gorham the damage was most prevalent along Bell Street and after the confluence with the Androscoggin River. Major damage occurred downstream to the town of Shelburne.

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.

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Source: NOAA Storm Events Database, event_id 1151038. Narrative written by the NWS forecast office that issued the report.