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Cut-off low

A cut-off low, also known as a cold drop or upper-level low, is a high-altitude pocket of colder air and localised lower pressure. These systems are generally invisible on surface weather maps (pressure charts) but nevertheless influence the weather and can also lead to uncertainty in weather forecasts.

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Formation of Cold Air Drops

These isolated regions of colder air at higher altitudes are caused by the wave-like motion of the jet stream. When the jet stream dips southward, it forms a low-pressure trough containing colder air. If the wave motion is particularly pronounced, cold polar air can become detached and move southward, leading to the formation of a cold air drop or an upper-level low. This system rotates counterclockwise and is referred to in meteorology as a cut-off low.

The following animation shows the development of pressure and wind at an altitude of 9 km and illustrates the formation of two cold air drops east of Switzerland. These systems then move westward along the periphery of a high-pressure area.

Animation of geopotential and wind speed at 300 hPa (approx. 9,000 m) from Thursday, 10 November, to Sunday, 13 November 2022; IFS model.
Geopotential and wind speed at 300 hPa (approx. 9,000 m) from Thursday, 10 November, to Sunday, 13 November 2022; IFS model. (©ECMWF )

Impact of cut-off lows

The cold air at high altitude produces instability in the atmosphere. The greater the vertical temperature difference between surface and higher-altitude layers, the greater the instability and the resulting uplift. This uplift can initially lead to the formation of clouds, which then also produce precipitation. In summer, or even in early autumn in the Mediterranean region (as the Mediterranean Sea is still warm), this then leads to thunderstorms.

Cut-off lows are therefore often associated with cloudy weather and precipitation or thunderstorms. In terms of wind, the lack of a pronounced upper-level jet stream and a tight surface pressure gradient mean that strong winds are not a widespread occurrence. However, strong gusts of wind are by all means possible in the vicinity of thunderstorms.

Diagram of the uplift mechanism in a cut-off low.
Diagram of the uplift mechanism in a cut-off low. The colder air at altitude causes uplift and, in turn, the formation of convective clouds and precipitation, and can even lead to thunderstorms in summer.

Cut-off lows generally move very little or slowly, as they are isolated or blocked by strong areas of high pressure. They can remain in the same region for several days or even weeks. In these circumstances, the precipitation associated with the cut-off low accumulates in the same location and leads to flooding. One such incident occurred on the Spanish coast in October 2024, when the region of Valencia experienced exceptional flooding (German). Another, older example is the flooding experienced in Switzerland in July 2021 (German), which was caused by a cut-off low lingering north of the Alps.

Conceptual diagram of a cut-off low that was located over the Iberian Peninsula and led to flooding in Spain.
Conceptual diagram of a cut-off low that was located over the Iberian Peninsula and led to flooding in Spain (situation in October 2024).

Greater uncertainty in forecasts

The limited horizontal extension – the largest cut-off lows have a diameter of a few hundred kilometres – mean that these phenomena are generally hard to forecast. The smaller the spatial extension of a weather phenomenon, the harder it is for numerical weather forecast models to map them correctly.

21 forecast scenarios for the geopotential (black isolines) and temperature (coloured areas) at 500 hPa, i.e. at approx. 5,500 metres, from the old COSMO-2E forecast model. The position and occurrence of the cut-off low is different in every scenario.
21 forecast scenarios for the geopotential (black isolines) and temperature (coloured areas) at 500 hPa, i.e. at approx. 5,500 metres, from the old COSMO-2E forecast model. The cut-off low is represented by the circular area with a bluish centre (colder air). The position and occurrence of the cut-off low is different in every scenario, leading to considerable uncertainty in the forecast.

Cut-off lows in forecasts

A cut-off low is defined by a high-altitude pocket of cold air and lower air pressure. In some cases, the upper-level low is strong enough to also generate a surface low, but that is by no means always the case.

It is therefore very difficult to recognise these phenomena on the surface weather map (with plotted isobars and fronts) or in forecast maps. In general, cut-off or upper-level lows are mentioned in the weather reports prepared by our forecasters.

The following example, from 14 November 2022, can be analysed using the surface pressure chart (showing isobars and fronts), as well as the weather report, both of which were available in our mobile app or on our website. If you only consider the surface pressure chart, it is not clear whether the rain forecast in Bern is triggered by the distant cold front. The bulletin, which can always be found above the surface pressure chart (with its isobars and fronts), can reveal this secret: a cut-off low is predicted to pass through on 14 November 2022.

It is therefore clear that the weather will be changeable and rainy on Monday. Moreover, when a cut-off low is present, there is greater uncertainty in the weather forecast. Accordingly, automatic forecasts should be treated with a certain degree of caution.

Surface weather map showing isobars (dashed lines) and fronts (blue for a cold front, red for a warm front, and purple for an occluded front) for 14 November 2022 at 1pm local time.
Surface weather map showing isobars (dashed lines) and fronts (blue for a cold front, red for a warm front, and purple for an occluded front) for 14 November 2022 at 1pm local time. Apart from the cloud and rain symbols in Bern and Nice, you can barely tell from this map that there is a cut-off low over Switzerland.
General situation, written on Saturday, 12 November, at 5.30am: the bulletin mentions the passage of a cut-off or upper-level low in the proximity of Switzerland on Monday, 14 November.
General situation, written on Saturday, 12 November, at 5.30am: the bulletin mentions the passage of a cut-off or upper-level low in the proximity of Switzerland on Monday, 14 November.