What type of thunderstorm often occurs in the afternoon with a northerly flow?

The correct answer is Advection Thunderstorm over Land. This type of thunderstorm is associated with warm, moist air being transported over a cooler land surface, often resulting in unstable conditions that favor convection. In the afternoons, particularly with a northerly flow, warm moist air can be pushed into cooler air masses, creating an environment conducive to thunderstorms.

These conditions typically develop as the sun's heating increases throughout the day, causing air at the surface to rise. This effect can be amplified in coastal or flat inland areas where the temperature contrast is significant. The afternoon timing aligns well with peak heating, promoting the necessary lift for thunderstorms to occur.

In contrast, other types of thunderstorms generally have different atmospheric dynamics. Orographic thunderstorms are primarily formed when moist air is forced upwards by terrain, frontal thunderstorms are associated with colliding air masses (cold vs. warm), and single-cell thunderstorms usually occur under very localized conditions and often do not have a specific directional flow associated with them. Thus, the characteristics of an advection thunderstorm align best with the conditions described in the question.