This was in a long-gone 2005 blog by Jon Richards but it's still relevant:
He explains why it didn't get as warm as it was supposed to earlier this week, and why it's so cold today
For the last three days, the predicted high temperatures in metro Atlanta haven’t been what they were forecast to be. As detailed here, Monday’s miss was caused by a slow moving warm front. On Tuesday, we were supposed to reach a high of 70 degrees, before retreating to a low in the lower sixties today. Instead, we had a high of 60 Tuesday, and broke 70 degrees for the first time this year on Wednesday.
Tuesday’s colder than expected temperatures were caused by a combination of heavy fog in the morning and a mini wedge, where the colder air closer to the ground was overrun by the warmer air approaching from the southwest.
Overnight Monday, the skies cleared, and due to Monday’s rain, fog developed close to the ground. As the warmer air approached Tuesday morning, it slid on top of the colder air near the surface, forming cloud cover and more fog that prevented the sun’s heat from warming us to the expected temperature.
On Wednesday, the high recorded at my weather station of just over 70 degrees occurred because a cold front approaching from the northwest took longer to arrive than predicted. The forecast called for a chance of rain midday as the pressure difference between the moist air already present and the drier air that was approaching with the front would cause precipitation.
Instead, the front didn’t get to Atlanta until late in the afternoon. Throughout the day, periods of sun warmed us up, and by the time the front arrived late in the afternoon, there wasn’t enough energy to cause storms, except in a few isolated areas in North Georgia.
The cold front does mean business though, as temperatures have dropped by about 15 degrees in the last few hours. Look for slightly cooler than normal temperatures over the next few days until the next front passes over the weekend
I'm trying to understand how weather systems work but it's hard!