In this article Meteomatics’ Meteorology Team takes a look back over the first few weeks of September, when many parts of Europe enjoyed warm and dry weather. Plus Meteomatics take a look at the weather forecast to see what we can expect for the remainder of the month.
At the beginning of September there was an arctic cold outbreak in Scandinavia, the Baltics and western Russia (chart from 2021-09-03 15 UTC).
Whilst a high pressure system / ridge over western and central Europe (Omega Blocking Pattern: low pressure to the left of ridge and to the right), enabled warm air masses in western and central Europe to intensify heating and created a Heat Dome Effect. Which combined with large scale subsidence of air masses, and caused air masses to become very dry and warm (similar to the Heat Dome Effects experienced in the US over the summer months), leading to most of western and central Europe enjoying an Indian Summer.
Meteomatics chart below from (2021-09-08 12 UTC) shows that the Omega Blocking Pattern which weakened and almost disappeared, but a high pressure ridge was still present over western and central Europe helping retain the warm and dry air.
Change in the second week of September:
Cyclogenesis west of the UK, brought a low pressure system that moved towards east and caused unstable conditions that lead to showers and thunderstorms over some parts of central Europe.
Although a new Omega-like pattern established itself and brought warm temperatures back (chart from 2021-09-12 15 UTC).
Change ahead for the latter half of September:
Low pressure returns and is predicted to influence weather in western Europe, bringing unstable conditions that are likely to lead to showers and thunderstorms for the remainder of September (Chart 2021-09-15 15 UTC).
Large scale flow from western and northwestern directions, are likely to bring a series of low pressure systems moving into Europe and cooler temperatures in combination with precipitation.
Example: Ensemble Forecast for temperature at 2 metres (t_2m) for Paris:
The ensemble spread shows that the forecast uncertainties grow as the low pressure influence increases, and that the Indian summer will come to an end.. The Ensemble mean for Paris t_2m (green line) shows a decline in temperature within the next few days.