Heavy rainfall that fell throughout the night on Wednesday and into the early morning hours Thursday caused flows on the Big Wood River to rise slightly above official flood stage for the second time in the past month.
As of 9:30 a.m. Thursday, the Big Wood River was flowing at 6.08 feet, as measured at the Bullion Street Bridge in Hailey. Official flood stage on the Big Wood River is 6 feet.
Measured at 3 p.m. Thursday, the Big Wood had dropped to 5.92 feet and was flowing at 4,040 cubic feet per second.
The heavy rainfall combined with high temperatures and humidity led the National Weather Service to issue a flood warning at 7:30 a.m. Thursday for the Big Wood River. The flood warning will likely extend at least through Monday morning, NWS hydrologist Sherrie Hebert said.
NWS forecasts predict the area of Idaho's central mountains—which includes the Big Wood River drainage—will pick up at least half an inch of rain today, June 9, Hebert said. Certain localized areas may receive up to a full inch of rain, she added.
Due to the expectation of continued rainfall and high humidity, the National Weather Service predicts the Big Wood River will rise to moderate flood stage and will crest at 6.6 feet early today and will be at 6.5 feet Saturday.
NWS weather forecasts for Saturday indicate a slight chance of showers and thunderstorms in the afternoon and into the evening. Predictions for Sunday and Monday indicate much of the same.
Perhaps surprisingly, Hebert said the primary reason for the rise in the river Thursday was not the heavy rainfall, but, rather, a faster rate of snowmelt at higher elevations.
"In some places that rate doubled," she said.
Thursday's heat and humidity released up to 2 inches of water from upper elevation snowpacks in places such as Galena Summit and Dollarhide Summit, Hebert said.
Combined with the heavy rainfall, the Big Wood was forced to rise, she said.
"That would be like two and a half inches of rain falling throughout the day."
In Hailey, the rising river entered the low-lying Della Vista neighborhood again Thursday, City Clerk Heather Dawson said.
This led the city to place high-water warning signs near War Eagle Drive, Dawson said.
Further predictions for heavy rainfall will have city officials on their toes, she said.
"We will definitely be on standby watching for what happens."
After raging for more than a week above flood stage, the Big Wood River dropped below the 6.0 mark on May 25.
The Big Wood first overflowed its banks in Blaine County on May 16. The river reached its peak May 21, when the water reached 7.92 feet and was flowing at 7,800 cubic feet per second. This was the fifth highest measurement ever taken of the river, and the overall flow exceeded any previously recorded volume.