Fall began today at 11:44 am. During the fall we tend to see stronger mid-latitdue systems. An example is last week when a strong high pressure air mass dominated our weather to keep us dry and seasonable. We can expect the same type of weather pattern to come our way this week for the start of fall. However, if you’re looking for some rain, it’s only a matter of time before a potent low pressure system comes our way.
Why is this? This is because as we head into fall and eventually winter, the North Pole starts to face away from the sun due to the Earth's axis tilt of 23.5 degrees. This means shorter days and longer nights. In the summer, the North Pole faces the sun - so we see longer days and shorter nights. So, how does this cycle bring on the seasons? Well, the seasonal change in the number of hours of daylight is the primary effect that determines when our seasons occur. The tilt of the Earth’s axis leads to a contrast in temperature between the tropics and the poles that is large in the winter and small in the summer. With strong temperature contrasts in the winter, we see stronger weather systems. In the winter dynamics play a bigger role than thermodynamics while the opposite is true for summer. Also, as we see less solar heating, temperatures will drop about 10 degrees per month into winter.
The uneven heating of the Earth’s surface is the ultimate driver of weather systems. The atmosphere and oceans try to balance out the difference in heat by transporting heat out of the tropics and towards the poles. As we lose solar heating through the fall and into the winter months, the polar vortex becomes less constricted and tries to expand southward. This vortex separates cold air from warm air. The subtropical jet also retreats southward.
This creates a more meridional (amplified) flow rather than a zonal (flat) flow. Summer weather usually features a zonal flow while winter weather features a meridional flow. When we have a more amplified wave pattern, stronger weather systems can form. These strong disturbances then create what is called downstream development. This can simply be explained by holding one end of a jumprope and waving your arm up and down to create waves. A low builds a ridge while a ridge deepens a low. Amplified wave patterns can also often cause cut off lows, or low pressure systems that are cut off from the upper level flow. The pattern gets so amplified that the waves actually "break off." We tend to see cut off lows in the spring and fall when the distribution of heat changes. Enjoy the nice weather while it's here!