During January, along the same latitude, (i. e. Charleston, SC and San Diego, CA), you notice that San Diego is 20-30 F warmer than Charleston almost every day of the month.

What is the best explanation as to why this is?

The cold front from the North can reach further on the East coast, around Charleston, then the West around San Diego. Also, there will be much more cloud cover in Charleston.

Explanation:

Even though both cities lie on similar latitudes, they experience different cloud cover and weather. The oceanic current, The Gulf Stream, passes by Charleston. This being moist and hot air inland. This caused a cloud cover. There is less incoming solar radiation, meaning less heat. Also, hotter temperatures in the West because of the Westerly Wind blowing from West to East. As it does so it cools due to the large area of land. The land has a low specific heat capacity than water so as it moves away from the Pacific Ocean it cools. The Westerly Winds also force any polar vortexes over to the East of the country.