Some winters bring a fast, straight jet stream and a run of similar weeks. Others bring a buckled flow that parks cold in one region and warmth in another. The difference comes down to a temperature contrast and what it does to the wind.
What drives the jet in the first place
The jet stream exists because the tropics are warm and the polar regions are cold, and that contrast creates a pressure difference that grows with height.
Air accelerates in response to that difference, and the earth's rotation turns the resulting motion into a band of strong west-to-east wind near the top of the troposphere.
The strength of the jet is therefore tied directly to the size of the contrast: a sharp temperature gradient produces a fast jet, and a weak one produces a sluggish jet.
Why a fast jet keeps weather moving
Surface systems are steered by the flow above them, so a strong jet pushes storms along quickly and gives no pattern time to establish itself.
Waves in a fast flow are shallow and progressive, moving eastward at a steady rate, which produces changeable weather with no single condition lasting long.
Forecast confidence is generally higher in this regime, because the systems are well defined and their motion is governed by a strong and predictable steering current.
How a weak jet produces stuck patterns
When the contrast weakens, the flow slows and begins to meander north and south in large amplitude waves rather than running roughly parallel to the latitude lines.
Those waves move slowly or stall, and a location under a southward dip receives repeated cold outbreaks while another under a ridge stays dry and mild for weeks.
The same stalled configuration produces both extremes simultaneously, which is why an unusually cold region and an unusually warm one often appear in the same season.
What blocking adds to the picture
Occasionally a large high pressure area establishes itself at high latitudes and obstructs the normal eastward progression entirely.
Systems approaching it are deflected around it or halted, and whatever weather was in place beneath the block continues for as long as the block persists.
Blocks are notoriously difficult to predict more than a week ahead, and their onset and collapse are among the largest sources of error in extended forecasts.
Why the polar stratosphere matters
High above the surface, a circulation of cold air encircles the pole through winter, and its strength influences the jet stream far below it.
When that circulation is disrupted and warms abruptly, the effect propagates downward over following weeks and is often followed by a buckled jet and cold outbreaks in the mid-latitudes.
Because the disruption is detectable before its surface consequences appear, it is one of the few genuinely useful signals available at the several-week range.