Unlike zonal flow which usually results in benign (if not boring) weather, an amplified pattern is a recipe for active and potentially stormy weather. The graphic below (click to enlarge) is one generic example of an amplified flow pattern.
The jet stream makes large meanders north and south across the hemisphere. Within the large long wave troughs (where the jet buckles south) and ridges (where the jet buckles north) smaller shortwave troughs or “shortwaves” will ripple along the jet stream. These features are important as they can be a reflection of the surface storm system, they can act to sharpen the overall trough and they can force the entire amplified pattern to shift eastward and in some cases move westward (retrograde).
The graphic below (click to make it bigger) is a generic representation of the upper level temperatures which impact temperatures at the surface. The weaker and more north-south flowing jet stream causes large north-south variations in temperature as the polar vortex is fractured, weaker and more apt to settle in more southern latitudes. In this graphic, cold is dumping into the western and central U.S. while warmth surges north over the eastern U.S. and Canadian Maritimes.
The jet stream is weaker when it is amplified and as such is unable to restrain cold outbreaks as a zonal flow can. Where the jet buckles south, cold air dumps from the Arctic southward and where the jet buckles north in response, warm air from the subtropics flows north. Those directly under the center of the trough can be subject to bitterly cold air, while those under the northward bulging ridge can experience springlike weather. Near and under the jet stream to the east of the trough and west of the ridge, the temperature contrast can fuel strong storm systems, which import energy from the amplified jet stream. Severe thunderstorms on the warm side can transition to rain, then freezing rain, sleet and heavy snow on the cold side.
The graphic below illustrates common surface weather in the cold season when an amplified pattern is established with a trough (dip) in the west and a ridge (bulge) in the east.
Cold Arctic high pressure is pushed southward across Canada into the U.S. as cold, moist storm systems evolve out of the Desert Southwest and ride north-northeast into the Great Lakes. The tracks of these storm systems depend upon how far east or west the dip in the jet stream sets up. The pattern is not always set up this way; it can be reversed with a warm ridge out west and a cold trough in the east. This setup brings chill to the east, wild swings from warm to cold to warm on the Plains and dry warm weather out west. Click on the graphic below for an example.
These amplified patterns can last anywhere from a few days to a few weeks and can shift eastwards and even “retrograde” or shift westward. When the pattern becomes locked in for an extended period of time or it recurs frequently, it can have major implications on the character of the winter. The winters of 2009-2010, 2010-2011 and 2012-2013 all featured a frequently amplified flow and for the Midwest were cold and snowy. During the winter of 2011-2012, the flow type was the opposite. was zonal, and the weather warm and rather dry. Why do these patterns tend to recur and persist? Larger scale hemispheric patterns such as the Arctic & North Atlantic Oscillations, El Nino and La Nina play a role, but those are discussions for another post.