September 2024 Review:

In September, air temperatures were well above their 1991-2000 average over eastern and northeastern Europe, but below average in much of western Europe. While ridging dominated the eastern part of the continent bringing above normal temperatures, multiple low-pressure systems over the west kept temperatures cooler in this region. The average European temperature was 1.74°C above the 1991-2020 September average, making it the second warmest September on record for Europe. The overall pattern during September was quite different than August, leading to the temperature contrast between the western and eastern parts of the continent. 

Much of Europe experienced above-average precipitation in September thanks to a series of strong storms in the central/western part of the continent. Farther north and east, over the western and southern Iberian Peninsula, Ireland, the northern UK, and over most of Eastern Europe, including the Baltic countries, persistent ridging led to drier-than-average conditions which contributed to severe wildfire activity.  

In addition to smaller storms during the early and latter parts of the month, a long-duration and powerful storm brought copious amounts of precipitation mid-month. This storm, named Boris, brought unprecedented heavy rain and snow in central Europe and significant rain in the northern UK as well.

In central and parts of eastern Europe, Boris produced an extreme precipitation event in Germany, Poland, Austria, Hungary, the Czech Republic, Slovakia, Romania, and Italy. A strong low-pressure system dropped down from the north and subsequently stalled as it was sandwiched between two areas of high pressure. With high atmospheric moisture content from the Mediterranean and a strong temperature contrast between the preexisiting warm airmass and the colder one moving in from the north, an extended period of extremely heavy precipitation ensued. 

The figure below shows that a wide area in central and eastern Europe saw between one half a month and three month's average precipitation over a five-day period. 

This led to catastrophic flooding, widespread damage, massive snow totals in the Alps, and several fatalities. Several one, two, three, four, and five-day rainfall records were broken. In the Alps, record snowfall of up to 2.5 m (8.2 ft) fell at upper elevations while impressive amounts of 1 m (3.3 ft) were observed as low as 1370 m (4500 ft). This historical snow storm, combined with several other solid snow events, allowed many ski resorts to open up early in September.

Below are a few shots of the deep snow in the Austrian and Swiss Alps as well as some of the flooding in central and eastern Europe.

The upper-level pattern anomaly reflects the long-duration storm during the middle of the month that brought the extreme rain and snow. Ridging over eastern Europe and near Iceland in northwestern Europe accounts for the dry conditions in these regions. 

October 2024 Outlook:

Above-normal temperatures are expected across most of Europe in October, with more ridging in northeastern Europe. Ridging is forecast for southern Europe as well, bringing more warm and dry conditions in this region. The strongest above-normal temperature anomalies are expected in northeastern Europe below the ridge axis. Colder than normal temperatures are forecast only for the northern part of the continent over Icaland and northern Scandinavia.

A strong above-average precipitation signal is noted for much of Europe in October, especially in western and central Europe. Conditions are expected to be drier than normal across northwestern Europe over Iceland and southern Europe in Italy and Spain.

Thanks for reading the September 2024 review & October outlook! Hopefully, temperatures stay cool in the Alps and additional rounds of snow arrive in October to extend the solid early season conditions. I will post the next monthly update in early November

Luke Stone
Forecaster, OpenSnow