Blocking Highs

A blocking high can leave neighboring parts of a continent in stubbornly different weather — one under clear, persistent heat, another under repeated rain. Both can be symptoms of a large feature in the upper-level flow that has slowed or stopped the usual west-to-east progression of weather systems.

The pattern that stops the traffic

Weather in the mid-latitudes generally moves west to east with the westerly winds aloft. Those winds are not a straight belt; they run in large wavy patterns called Rossby waves, or longwaves, described by Carl Rossby in the 1930s. A single longwave runs from about 3,700 miles to 5,000 miles crest-to-crest, and at any moment the hemisphere carries somewhere between three and seven of them, most often four or five. Surface lows and their fronts generally move within this larger-scale pattern.

The longwaves themselves drift east only slowly, and occasionally they stop or retrograde — move east to west. A block is the extreme case of that stall. Centers of high and low pressure aloft arrange themselves so that approaching systems cannot get through the usual way. They are deflected around the obstacle instead, and whatever weather is already in place under each center stays there. Blocks can hold for several days at a time.

Uneven heating and major terrain help shape the longwave pattern. Continents and oceans heat differently, while mountain ranges force air to rise or divert around them. Those influences alter the distribution of temperature and pressure and therefore the speed and direction of winds aloft. A block develops when the flow becomes amplified enough that its usual eastward progression stalls or splits around persistent high- and low-pressure centers.

Omega and Rex

Two configurations get named because they recur and because they behave differently.

An omega block is two cut-off lows with a single blocking high wedged between them. On a 500 mb chart the height contours trace out the Greek letter omega — a broad dome in the middle with a low on each flank. Cut-off lows have been separated from the main belt of westerly flow and can drift slowly. NOAA's JetStream training material notes that forecast models often open these lows and return them to the main flow sooner than observed. Omega blocks are large, and their size contributes to their persistence.

A Rex block is the simpler shape: a high sitting poleward of a low. It remains nearly stationary until one of the two height centers changes intensity and unbalances the high-over-low pattern. Until then, unsettled and stormy weather is usual near the low, while dry conditions are typical near the high.

The consequences follow the geometry. Under an omega block, cool temperatures and precipitation accompany the two lows while warm, clear conditions prevail under the high. NOAA notes that omega blocks can lead to flooding or drought depending on location, while strong, persistent Rex blocks can bring flooding near the low and short-term drought under the high.

Why the high half gets hot

High pressure aloft makes air subside. Sinking air is compressed, and compression warms it in the lower atmosphere. At the same time, the subsidence helps trap heat rising from the surface. Blocking highs are therefore responsible for major heat waves rather than merely warm afternoons. Skies are usually clear because the downward motion suppresses cloud formation, allowing strong sunshine while the block persists.

Precipitation is usually shunted around the periphery of the high-pressure area rather than through its center.

Recognising one

The chart to look at is 500 mb. That pressure surface usually lies between roughly 16,000 and nearly 20,000 feet, placing it near the middle of the atmosphere. The lines on the chart are height contours, given in meters. A ridge appears as a poleward arch in those contours and a trough as an equatorward dip; a blocking high is the large, persistent ridge or closed high interrupting the flow. Charts commonly drop the final zero, so a contour labelled 564 marks 5,640 meters, about 18,503 feet.

A static chart is not enough to read a block reliably, because the smaller waves embedded in the longwave pattern — shortwaves, anything under 3,700 miles — need a loop to be distinguished. Shortwaves travel downstream at about 23 mph in summer and 35 mph in winter, and they are the chief instigator of precipitation episodes. Watch a multi-day loop of 500 mb heights: if the large-scale shape holds still while smaller disturbances slide around its edges, that is a block.

Persistence is the essential clue. Compare several days of 500 mb analyses or forecasts rather than relying on a single snapshot. A ridge that remains in nearly the same place while smaller disturbances repeatedly travel around it is behaving like a block; the surface forecasts beneath it should show the corresponding run of similar conditions.

How it breaks, and what to do while it holds

A blocking high can weaken when a shortwave moves over its top and reduces the ridge, bringing the heat wave to an end. A Rex block breaks when one of its two centers changes intensity. The timing is difficult: the same tendency to return cut-off lows to the main flow too soon can make a forecast dismantle part of the pattern before it happens in the atmosphere.

The defining hazard is duration. Persistent heat raises risks to people and strains the power grid, while repeated rainfall near the low-pressure side can increase flood risk. When a block is in place, day-to-day conditions may change little, so the useful forecast question becomes how long the pattern will last and which official heat or flood alerts are in effect.