Jet Stream Tracker — Live Position and Forecast
The map below shows today's actual jet stream — wind at the 250hPa level, roughly 34,000ft, right where the polar jet runs — over the UK and North Atlantic, updated as new model data arrives. The tightest band of the brightest-coloured, fastest-moving wind is the jet core itself; drag the timeline underneath the map forward and you can step through the forecast to see how its position and shape are expected to change over the next several days.
Live interactive map © Windy.com. Brightest colours and tightest packing = the jet core; use the altitude/level control inside the map to compare 250hPa against other levels.
Jet streams were first noticed during the Second World War, when pilots flying bombing missions across the Atlantic found their eastbound flights to Britain arriving unexpectedly early, sometimes helped along by tailwinds well over 100mph, while the same routes flown westbound took noticeably longer. Those narrow ribbons of fast-moving high-altitude wind were what came to be called jet streams. For the general explanation of what they are and how they form, see our Jet Streams Explained page — the rest of this page covers how the jet stream's shape and position drive Britain's longer, more persistent weather patterns.
Rossby Waves: The Jet Stream's Meanders
Rather than flowing straight west to east, the jet stream meanders north and south in broad, slow-moving waves called Rossby waves — the atmospheric equivalent of the ripples on a pond, except each "ripple" can be a thousand kilometres across. Where the jet bulges northward, the air beneath spins clockwise, building a high-pressure ridge associated with settled, dry weather; where it dips southward, the air spins anticlockwise into a low-pressure trough, which is the recipe for the unsettled, rain-bearing weather that crosses the UK so regularly. Because Rossby waves themselves tend to drift slowly westward relative to the jet's eastward flow, on an ordinary week they roughly cancel out and the whole wave pattern edges only slowly across the map, bringing the familiar week-to-week change in British weather.
An example five-wave Rossby wave pattern — the jet stream (and the ridges/troughs riding on it) typically holds somewhere between three and seven of these waves around the hemisphere at once. Diagram: NOAA/NWS JetStream.
Each northward bulge in the wave (a ridge) tends to sit over a clockwise-spinning area of high pressure and settled weather; each southward dip (a trough) sits over an anticlockwise-spinning area of low pressure and unsettled weather. Because these waves generally drift only slowly compared with the wind speed within them — a 100kt jet stream wind can be blowing through a wave pattern that itself is barely moving — the same ridge or trough can sit over roughly the same part of the country for days at a time, which is exactly why a spell of settled or unsettled weather in the UK so often lasts several days rather than changing overnight.
A real 500mb chart: the large red dashed lines mark longwave (Rossby wave) troughs; the shorter blue dashed lines mark faster-moving shortwaves embedded within them. Green shading shows precipitation, which mainly tracks with the shortwaves as they pass through. Diagram: NOAA/NWS JetStream.
When the Pattern Gets Stuck: Blocking
Occasionally a Rossby wave's own westward drift happens to match the jet's eastward speed almost exactly, and the whole pattern grinds to a near-standstill relative to the ground — meteorologists call this blocking. A blocking high sitting over or near the UK cuts off the normal flow of Atlantic weather systems, replacing the usual changeable pattern with days or weeks of the same conditions in a row. In winter, blocking that displaces the jet stream well south of the UK lets cold continental or Arctic air flood in from the east instead of the usual mild Atlantic westerlies — the pattern behind cold snaps like the "Beast from the East". A blocked trough instead of a ridge has the opposite effect: the wet, unsettled summer of 2012 was driven by exactly this, with the jet stream parked further south than normal and successive Atlantic low after low getting funnelled straight across the UK instead of passing to the north as usual. Blocking highs elsewhere can have knock-on effects a long way from the UK too — the same kind of stuck Rossby wave pattern sat behind both the severe 2010 Russian heatwave and, simultaneously, the catastrophic Pakistan floods that summer, as the blocked jet diverted the monsoon circulation on the other side of the same wave.
Entrance and Exit Regions
Forecasters pay particular attention to the accelerating and decelerating sections of the jet, known as its entrance and exit regions, because these are where the strongest upper-level support for developing low-pressure systems tends to occur. Whether an Atlantic disturbance fizzles out as a minor blip or deepens into a full storm capable of structural damage often comes down to exactly where it tracks relative to these jet entrance and exit points, which is why jet stream charts remain one of the core tools professional forecasters use when working out how a developing system is likely to behave over the following days.