Flash Flood — Sullivan, New Hampshire

2023-07-10 to 2023-07-11 · near Marlow Jct, Sullivan, New Hampshire

Event narrative

The Cold River came out of banks on the evening of the 10th following heavy rains. The river had been running high due to multiple rounds of heavy rain over the previous few days. After approximately 1 to 3 inches of additional rain that afternoon, the river spiked to the second highest level on record at the Alstead USGS river gage (period of record 2010-2023). The river crested at 11.67 feet, with a discharge of 10,300 cfs. The flow recorded has an annual exceedance probability between 1% and 0.2% levels. The high flowing water came out of banks along Route-123A in South Acworth and Acworth Road in Alstead. Acworth Fire Chief rescued 12 people from their homes on Route 123A. He said several cars were swept by water but there were no injuries in those incidents. The floodwaters caused damage to the southbound lane on Acworth Road. In Lempster South Road and River Road were flooded.

Wider weather episode

Saturday, July 8th through Monday, July 10th was an active weather period with numerous rounds of heavy rainfall moving across the state. Thunderstorms developed during the afternoon of Saturday, July 8th with scattered thunderstorms from the White Mountains down through the Monadnocks. Storms came to an end during the evening hours. Localized 3-hr rainfall totals ranged from 1 to 3', but no flooding was reported.

There was a short break in rainfall activity the night of July 8th through noon on July 9th, before a cluster of showers and thunderstorms developed around noon on Sunday across Cheshire County. The storms continued to train over the same region with storms repeatedly hitting the same areas in Cheshire County. Thunderstorms and showers expanded in aerial coverage across western New Hampshire through the late evening. The rain ended in the hardest hit areas of Cheshire County by 9 PM on July 9th.

After a brief lull in the rainfall activity around midnight, rainfall activity began to increase once again during the early morning hours of July 10th. A large area of rainfall pushed into Western New Hampshire as an area of low pressure moved into Northern New England. Rainfall was light with areas of moderate rainfall rates through the morning hours of July 10th. Overall rainfall rates were not as heavy as previous days but the rain was more widespread and constant across the Connecticut River Valley region of New Hampshire. There was a short lull in rainfall across Southwest New Hampshire during the afternoon hours, before a heavy band of heavy rain showers with intense rainfall rates pushed through the region during the evening of July 10th. Rainfall rates up 1 to 1.25' per/hour were reported with this band. Additional flooding was reported as saturated soils led to rapid run-off, especially in Sullivan and Northern Cheshire County.

View location on OpenStreetMap → (43.1954, -72.2884)

Source: NOAA Storm Events Database, event_id 1124164. Narrative written by the NWS forecast office that issued the report.