Place the warm water in a small bowl, and sprinkle the yeast over the top.
Let stand until yeast has dissolved and is foamy, about 15 minutes.
Measure flour into a large bowl. Make a well in the center, and pour in the yeast mixture and cold water. Use a sturdy spoon to mix it together until sticky and difficult to stir, but nevertheless thoroughly combined. Cover and allow to ferment for 24 hours in the refrigerator before using.
Store in the refrigerator for up to 2 weeks. To use, rinse a measuring cup in cool water, scoop out the amount of starter needed, and bring to room temperature.
