I'd add one cup of full-strength Muriatic Acid (31.45% Hydrochloric Acid) per 10,000 gallons, let it mix thoroughly with the pump running (be sure to brush the area where you add the acid over a return flow) and after some hours, cover the pool and close it up. The acid will lower your pH to around 7.8 and as the chlorine drops slowly over time the pH might drop some, but not too much (even if all 13 ppm FC were to get consumed, the pH would not get lower than 7.2 and more likely won't get that low). If the water is cold, the chlorine should last quite a while. In my pool when covered and the water at 50ºF, my chlorine loss rate is less than 1 ppm FC per week.