Q: I have heard that we still need to be careful of water pipes freezing because a thaw will push the frost farther into the ground. Is that true?
A: Yes and no. Yes, we may continue to have pipe freezing problems including new problems even after the weather begins to warm, but, no, warmer weather or thawing upper layers of the ground do not push the frost deeper.
Once the surface of the ground falls below 32 degrees F, the water in the ground will start freezing and the frozen area will get deeper and deeper. The temperature of the advancing "frost line" is what causes the unfrozen layer beneath it to freeze, not the temperature at the surface.
Even when the weather warms and the surface begins to thaw, the frost line is still cold enough to keep freezing more ground and going deeper. It will continue to advance until the depth of thawing ground catches up with it.
In other words, in the first couple of weeks of a thaw the frost will continue to go deeper despite the surface temperature, not because of it.