To gain access to trapped water in the house line, crack the faucet at the lowest level and drain the lines. I have installed a faucet in my basement to insure I can collect the water from the lines that run under my house. The basement is where I plan to be during a weather alert. Your water heater tank holds 30 - 40 gallons. Check your water heater tank because it may have a foot or more of sediment in the tank bottom.
Sediment removal is a good reason to drain the tank every year.
In addition, the removal of sediment will improve the water heater's efficiency. The hot water tank can be drained by opening the faucet at the bottom of the tank. You may need to open the hot water faucet elsewhere in the house to allow the release of the vacuum to allow a free flow of water. The faucet at the bottom is threaded to receive a regular garden hose. The water in a water bed can also be used.