In videos and posts, you often hear me refer to the Aleutian Low, Pacific North American Teleconnection (PNA), North Atlantic Oscillation (NAO), Cross Polar Flow, and Southeast Ridge among the many factors in our weather. While there are dozens of oscillations and teleconnections, these are the ones that most directly affect our weather; especially in the winter. The map below is for 7:00 p.m. January 17, 2021 and shows an almost perfect setup for winter storms. In it you see Low Pressure over the Aleutian Island chain, High Pressure up the West Coast of the U.S. (+PNA) and Canada (-EPO), and a Trough of Low Pressure in the Central U.S. High Pressure over Western Greenland (Greenland Block) helps slow storm systems as they approach the Eastern U.S. and helps deepen the Central U.S Trough as it get squeezed between the West Coast and Greenland highs. The Aleutian Low is the area where storms will be generated that can become Central and Eastern U.S. storms. In this set up there’s also a suggestion of a possible Cross Polar Flow which can help supply cold air to descending storms. What’s missing to make this a perfect winter storm setup is a weak Southeast Ridge (See second paragraph and image).
The second image is the GEFS model forecast for 7:00 p.m. on February 3, 2021. It is the exact opposite, and is not a good setup for Indiana snow. You’ll see a strong Southeast Ridge, which supplies warm air and steers storms to our Northwest. There’s also no Greenland Block, so storms can quickly pass through the country with little hope of encountering cold air. The West Coast Trough (-PNA) also allows for warmth by allowing warm Pacific air to flow into the U.S. without encountering any Arctic air. One last oscillation that I did not label, to avoid clutter, is the Arctic Oscillation (AO). If you look at both maps you will see a Blue Line running West to East along the 540 height line. In the first map, it is wavy and fluctuates strongly from North to South. That is a (-AO), which means the Arctic circulation is weak and displaced South. That allows for waves of Arctic air to move Southward with storms. The second map shows that line is relatively flat and shifted farther North. That is a (+AO), which means the Arctic circulation is strong and cold stays bottled to the North.
ALL IMAGES VIA: weathermodels.com