May. The winter dip in the jet stream allows polar air to surge south. This cold, dry air brings with it temperatures cold enough for snow , sleet , or freezing rain to develop. Warm tropical air filled with moisture from the Gulf of Mexico often continues to flow up from the south during the winter months. When this warm, moist air mass from the south meets the cold, dry air mass from the north, they can create winter storms. The term low pressure is used to indicate an area of air pressure that is lower than other areas of pressure around it.
Air always moves from areas of high pressure to areas of low pressure.
If not for the coriolis force , winds would blow in a straight line. The earth's rotation causes winds to turn as they move away from high pressure areas into areas of low pressure. In the Northern Hemisphere, these winds circulate counterclockwise around areas of low pressure or cyclones . As the low develops, the warmer air from the south begins to flow northward on the eastern side of the low. At the same time, colder air from the north flows southward around the low's west side.