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TO: Subscribers: Emergency Management Officials…News Media…and Other Users

SUBJECT: River Flood Stage Change: Happy Isles

Throughout the spring and summer of 2023, the National Weather Service in Hanford California has been collaborating with Yosemite National Park Rangers and hydrologists to document observed impacts from the snowmelt. These observed changes have led to a recommendation to change the current Flood Stage from 8 feet to 7.5 feet for Happy Isles. This recommendation is due to observed flooding impacts at Housekeeping camp and Yosemite Valley backpacker campground. Flood impacts when the Merced River at Happy Isles reaches 7.5 feet affect both employees to the park and park tourists. Lowering the Flood Stage to 7.5 feet will allow both the National Weather Service in Hanford, California and Yosemite National Park employees to alert the public of flooding concerns in a more timely manner than keeping the flood stage at 8.0 feet. The change to Flood Stage being lowered to 7.5 feet will mean that NWS Hanford will begin Issuing Flood Warnings for the Merced River at Happy Isles when the forecast calls for the gage to measure a river height of 7.5 feet or higher within 24 hours instead of 8 feet or higher. Various Flood Impact statements have been changed for the Merced River at Happy Isles, and the new Flood Impact Statements proposed are included in the table below.

Updated Happy Isles River Forecast Point Flood Impacts

Stage (ft)	Impacts
7	North Pines and Lower Pines campgrounds begin flooding.
7.5	Flood Stage. Flood Stage. Housekeeping camp and Yosemite Valley backpacker campground begin to flood.
8.2	Swinging Bridge overtops. Water on road at Northside Drive, Southside Drive, El Capitan crossover, and Sentinel crossover.
10	Northside Drive at Wosky Pond, Southside Drive at Chapel Meadow, El Capitan crossover, and Sentinel crossover are impassable to vehicles.
13.27	Flood of Record: 1/2/97. $178 million in damage to Yosemite Valley.

The NWS, California Nevada River Forecast Center (CNRFC), and Local and County Officials have supported the above creation of a forecast point. If you have feedback or questions regarding these changes please email any of the contacts listed below.

The data for these sites are provided by the US Geological Survey (USGS), a partner of the NWS. Without this critical data our forecasts and warnings would not be possible. The USGS website is here: https://waterdata.usgs.gov/nwis/inventory?site_no=11264500&agency_cd=USGS.

Andy Bollenbacher and Antoinette Serrato
Hydrology Focal Points
National Weather Service Hanford
andrew.bollenbacher@noaa.gov
antoinette.serrato@noaa.gov

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