This weekend, Czechia will experience an unusual temperature paradox. While morning temperatures in lowlands may drop to as low as 10°C (50°F), the lee sides of mountains could experience tropical nights with minimums above 20°C (68°F). This remarkable difference will be caused by the foehn effect, bringing warm, dry air to mountain lee areas.
The influx of warm air from Africa will deliver an exceptionally warm and sunny weekend. According to the Czech Hydrometeorological Institute, daytime temperatures in lowlands will exceed summer values of 25°C (77°F), with tropical temperatures possible. These values are quite unusual for the second half of September.
Those seeking cooler temperatures during the day might head to the highest mountains, where daytime temperatures will hover around 20°C (68°F). Surprisingly, though, nighttime temperatures in certain mountain areas won’t drop significantly.
The foehn effect is responsible for this phenomenon. It creates warm, dry downslope winds on the lee side of mountains. On the windward side, rising air becomes saturated and sometimes produces precipitation, while on the lee side, the now-dried air warms as it descends. The temperature difference between windward and lee sides can reach tens of degrees.
According to current Aladin model outputs, tropical nights (when temperatures don’t fall below 20°C) are expected in numerous locations this weekend. The warmest nights will occur on the northern sides of Czech mountains due to the prevailing southerly flow. The foehn effect will most significantly impact areas in Poland and Germany, while in Czechia, the highest nighttime minimums are expected in the Šumava foothills, Frýdlant promontory, and northern parts of the Jeseník and Krnov regions.
After the weekend, however, a cold front will bring an abrupt change as summer weather gives way to proper autumn. By mid-next week, temperatures above 20°C may not occur even in lowlands during the day.