Where did the daily temperature variation go?
There are (at least) five factors at play here:


the cold air advection from the north east has driven temperatures down
the cloud cover traps longwave radiation sort of like a blanket, keeping the night time temperatures from dropping
also, the cloud cover prevents solar radiation from reaching the surface and warming it
the relative humidity is near 100%, indicating that the dew point is nearly the same as the air temperature.  Dew point serves as a minimum possible temperature, so this also prevents the air temperature from dropping
precipitation traveling through the lower atmosphere will evaporate, having two effects: cooling the atmosphere and maintaining a high dew point


(5) is only really a factor during the day when temperatures would normally be on the rise.  The precipitation will keep the diurnal heating to a minimum.