But like occupants of a small boat riding that calm surface, we may be lulled during persistent zonal flow by a stretch of nearly indistinguishable day-to-day temperature changes, often remaining near the climatological mean for that time of year. Despite the unfounded desires of those living in the mid-latitudes to believe there is a truly "normal" state of weather from which departures are abnormal, such stable zonal patterns are but a temporary state. We live not in a meteorological Camelot, where it never rains til after sundown — or the Tropics where it always rains in late afternoon — but in a region subject to very changeable-weather regimes.
Thus, when a zonal flow pattern cuts off the exchange of heat between the polar region and the tropics, great thermal contrasts develop across the polar front. And this becomes the zonal pattern's downfall. Eventually, some chink, some small perturbation, develops in the zonal pattern, and a short wave, with initially small north-south extent, arises upon the polar front. If the short wave amplifies (grows in size), it can distort the flow into a new pattern that crosses the latitude lines, a pattern termed meridional. In meridional flow, cold air rushes southward while warm air streams northward.
As the polar-front meridional pattern rises into Rossby waves of deep north-south extent, it severely rocks our "temperature boat." When the wave, and associated jet stream, plunges southward, we find ourselves in a deep upper-level trough with temperatures quite cold below it. When we rise high on the upper-level ridge, or crest of the passing Rossby wave, the surface experiences warm, even hot, temperatures.
Often, the intense temperature changes brought by the meridional flow are expressed by frontal patterns associated with a surface low-pressure system: warm air moving northward behind the warm front; cold air descending southward behind the cold front. Not only are cyclonic storms spawned in the vicinity of the peak jet stream winds (called jet streaks), but they are often pushed rapidly eastward by the jet stream above.
Jet Stream is associated with Polar Front and Cyclone Family
So when you look at a weather map with the polar jet stream position drawn on it, you can generally tell what type of weather you will be having. If north of the jet, it should be relatively cold, if south, warmer conditions should prevail. And, if the jet stream is flowing overhead, look for stormy weather to dominate.
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