In Poland, summer precipitation can exceed 100 mm per day in mountain and foothill areas. Some of these extreme precipitation episodes are associated with cyclones (lows) that move along different routes over Poland from the Mediterranean Sea. With the low-pressure systems moving northwards, a large supply of atmospheric moisture arrives at higher latitudes from the intense evaporation of water over the Mediterranean basin and over areas along the migration route of the low. The accumulated water resources in the air are the necessary building blocks for the development of clouds and precipitation.
In the Mediterranean basin, lows are formed (cyclogenesis) most often in the cool half of the year but also occur in summer. With the cause of cyclogenesis in summer usually being different compared to the cooler part of the year. In the cool half of the year, cyclogenesis is caused by the contact of thermally contrasting air masses on the polar front, while in summer it is usually the result of strong heating of the Mediterranean area, which favours the upward movement of warm and moist air masses (convection) and the formation of an area of reduced pressure near the earth's surface. The development of convection may also be further encouraged by the influx of cool air masses from higher mountainous areas.
One of the most significant source areas where lows form is the Ligurian Sea area, particularly its northern part, the Gulf of Genoa. The lows that form over this area are called "Genoese lows", but only a small proportion of them reach Poland – usually only a few times a year. One of the shortest routes for the Genoa low to move over Poland is via Italy, Hungary, Slovakia or the Ukraine and is linked to the course of the undulating jet stream in the upper troposphere, which influences the routes of low pressure systems.
The Genoese low that will reach over Poland may persist for a few days (2-5), with no major changes in position. This results in high precipitation totals, which during such precipitation episodes, sometimes exceed 2-3 times the average monthly precipitation. The persistence of such a low-pressure system for an extended period of time, more or less in the same place, is favoured by the blocking situation in the atmospheric pressure field associated with the presence of a high-pressure system close to Poland.
Additional factors favouring high precipitation totals in a short period of time are the high water vapour content of the air coming from the Mediterranean regions and areas along the route of the low and the contact of tropical air masses with cooler air masses near or/and over Poland. The perpendicular direction of the inflow of air masses relative to the mountain barriers in southern Poland forces the air to rise and condense the water vapour it contains, resulting in the formation of precipitation clouds giving intense precipitation in mountain and foothill areas.
Summer precipitation associated with the Genoese low tends to be frontal in nature (precipitation associated with atmospheric fronts within the low that separate thermally contrasting air masses). Despite the relatively infrequent occurrence of Mediterranean lows over Poland, the share of precipitation associated with them accounts for around 10% of the annual total. The largest share of precipitation of Mediterranean origin in the annual precipitation total occurs in the south-western part of Poland and in the Lublin region. In addition, daily rainfall totals exceed 80 mm in 25% of the cases.
The climate scenarios do not give a clear answer as to whether we can expect more frequent lows from the Mediterranean in our country. On the one hand, the very high temperature increase observed in southern Europe in recent years may intensify evaporation and increase the potential for more water vapour in the air necessary for cloud formation and precipitation. On the other hand, the global warming model assumes that warming may cause a shift of the subtropical high pressure system towards higher latitudes, which would result in fewer cyclones in the Mediterranean.
Forecasting models at a smaller spatial scale (regional) present a variety of different scenarios. Some predict an increase in the frequency of Mediterranean cyclones, others a decrease, but with an increase in intensity. These divergent climate scenarios testify to the very high degree of complexity of processes in the climate system, which are very difficult to predict and give a clear answer as to what, for example, the model of the precipitation regime in Poland associated with the Genoese low will look like.
The rate and amount of increase in summer air temperature in the Mediterranean area, which may also be important for changes in precipitation totals in Poland during the onset of lows from this part of Europe is particularly worrying. Although the Genoese low rarely occurs in our country in July, the probability of high precipitation totals, should it occur, is very high, as evidenced by daily precipitation totals on 8-9 July 2025. The rather rapid rise in air temperature to high values in spring and early summer in 2025 in the Mediterranean undoubtedly created favourable conditions for the development of a low in the Gulf of Genoa region, which reached as far as our country bringing record daily precipitation totals in southern Poland.
About one in four lows from the Mediterranean brings summer rainfall in excess of 50 mm per day, which poses a high risk of flooding in our area, especially if high rainfall totals persist for several days in a row. Due to the flood risk associated with large amounts of precipitation falling in a short period of time, preventive measures to reduce the negative effects of extremely high rainfall totals are important.
An effective method of flood protection is the construction of reservoirs, both on a larger scale such as the Racibórz Dolny flood control reservoir commissioned in 2020, as well as small, more natural water reservoirs. Forests and wetlands also have natural retention functions. Protecting them from overuse and degradation is therefore particularly important in view of the threat of extreme precipitation and its negative effects.
Author: Dr Piotr Piotrowski, Department of Meteorology and Climatology, Faculty of Geographical Sciences, University of Lodz
