This is very odd, but every time we get a heavy rain, the hot water in our house is lukewarm at best. We have a fairly modern gas-fired hot water heater (GE GG40t06avg01, mfg 2011), and at all other times the water is piping hot, no problem. I can't seem to figure out what the connection could be - is the rain getting into the gas lines, and watering down the gas? Has anyone ever heard of this kind of problem? Any suggestions would be of help.
Some hot water heaters have a thermodisc at the bottom of the flue, to detect flue gas spillage and shut off the HWH. The rain could be causing a downdraft in the flue, thus pushing hot flue gas back. The HWH shuts off until the thermodisc resets and the heater comes back on. When you check it, the HWH has probably reset and is running normally. But it had been cycling on and off for a while, lowering the water temperature.
One service call involved a HWH shutting off each evening. The house was located on the slope of a small mountain. Cold evening air would move down the slope and into the flue. Roof angles and local terrain can make it worse. The solution was to extend the B-vent up a few more feet.