In this post we’ll take a look at the term “Zonal Flow” which is one configuration the jet stream can take on. We saw a lot of this pattern in the Winter of 2011-2012 which was mild and on the dry side with very little snow.
The image shows the jet stream in a zonal flow pattern. Notice the path is generally east to west. There are no large north or south meanders in the “river of air”.
Within the fast and west to east flow, there are ripples which travel through the fast flow. These ripples are called “shortwaves” because the wave length is much shorter than a long wave, which span parts or all of continents. Click the image below to make it bigger. These short waves represent the mid and upper level reflections of the weak weather systems which travel the Central U.S. in this type of pattern.
The graphic below (click to make it bigger) is a generic representation of the upper level temperatures which impact temperatures at the surface. The strong west to east jet stream concentrates the cold weather near an upper low over the Arctic and prevents it from slipping very far to the south. Temperatures moderate the further south you go.
This graphic illustrates the typical surface pattern across the U.S. associated with this type of pattern in the cool season. Incoming wet Pacific storms bring frequent rains to the west coast and especially the Pacific Northwest. These storms weaken and become stripped of moisture crossing the coastal ranges and the Rockies. The formerly cool and moist Pacific air becomes mild and dry due to downsloping. The Gulf of Mexico is generally cut off and unable to import moisture into the fronts as the move east, so the pattern is a dry one.
The downslope flow mentioned above is outlined in this graphic (click to make it larger). Cool, moist and unstable Pacific air masses arriving on the west coast are forced up and over up to three mountain ranges before emerging onto the plains. Moisture is wrung out on the California Coastal Range, then the Sierra Nevada range. The air mass then crossing the Great Basin is much less moist and not as cool. This air is then forced up and over the western slopes of the Rockies. The remaining low and mid level moisture falls here as rain and high elevation snow. East of the mountains, only high level moisture remains, typically as a thick layer of cirrus clouds which spread rapidly eastward through the plains. The air mass warms at a rate of 5.4 degrees per 1,000 feet of descent and because warm air can hold much more moisture than cool air, it becomes much drier. The warm air does modify as it crosses east, but generally remains mild by cool season standards all the way to Missouri and Illinois.