Severe weather season is getting a relatively late start, especially in North Carolina. Even on the Great Plains, last week was the latest into the season a major tornado – an EF-4 – was recorded. The reason was how long into spring the wintry weather extended.
Major storms require a strong difference in air masses to develop. Here in NC, we usually see them develop along a cold front or warm front. In the Great Plains, another type of boundary often sets up in the spring and summer called a dry line. While cold and warm fronts delineate the boundary between air masses with notable differences in temperature, a dry line is the boundary between a moist air mass and drier air mass.
In the US, the moister air is usually on the eastern side of the dry line because the moisture streams up from the Gulf of Mexico. The drier air is on the western side of the boundary because it’s coming from the desert southwest. The trigger for storms popping up along the dry line is often the surface temperature warming enough to break the cap.