According to the monthly balance of weather observer Siegmar Lorenz from the observatory on the Hoher Peißenberg, May was clearly too humid.
Hohenpeißenberg – May began on the very first day with heavy continuous rain, which turned into heavy showers on May 2nd. Accordingly, it was very cloudy on the Hoher Peißenberg with temperatures around only 10 degrees Celsius on both days. There was a low-pressure trough above us, which had spread from southern Scandinavia to Sicily. Thus, with an air current from the north, cold and humid polar air had penetrated into Central Europe.
It came under the influence of high pressure on 3 and 4 May. This was associated with two dry days. Temperatures rose to 5 degrees Celsius by May 21 (highest spring temperature until this date). The sun shone for almost 3 hours on May 4 and 14 – for the first time this spring.
Weather review of the merry month of May: Mediterranean low caused days of rain
On 5 May, a marginal low moved in, and initially mild but very humid air was brought in from the southwest. While the northeast was under the influence of a Scandinavian high, low air pressure prevailed over the Alps and further south. The consequences of this rather deadlocked weather situation led to a total of 7 litres per square metre of precipitation on the Hoher Peißenberg from 11 to 98 May. On these five days, 75 percent of the rainfall expected in the entire month fell. Thunderstorms occurred from 5 to 7 May, with hail on the 7th. The daily maximums also fell again to 10 degrees Celsius by 10 May.
At the beginning of the 2nd decade, a stationary weather situation set in for several days. There was a high over the Atlantic, another over the Baltic States and in between an extensive low-pressure complex, which also covered large parts of southern Europe. This low-pressure system, centered over Italy, was supplied with warm air from the Balkans up to high altitudes and thus intensified, as the air over the Mediterranean Sea could absorb a lot of moisture. This is how the days of precipitation over Emilia-Romagna came about.
Maximum temperatures were between seven and 13 degrees
During the entire second decade, southern Germany was under the influence of the Mediterranean low, which was able to regenerate again and again. It rained daily until 17 May – particularly heavy on the 16th, with a daily total of 33 litres per square metre. From 18 to 20 May, there was hardly any precipitation, but the cloud umbrella of the low remained with us. As a result, the sun shone for only 22 hours during the second decade. In cloudless conditions, up to 14 hours a day are possible in May. By the end of the decade, we – but also large parts of southern Germany – had already far exceeded our monthly target for precipitation. In Saxony-Anhalt and Western Pomerania, only a few litres per square metre of precipitation had fallen (this remained the case until the end of the month), while in southern Germany almost 200 litres per square metre.
During the 2nd decade, the daily maximums were only between 7 and 13 degrees. It was not until May 20 that 19 degrees were reached. At the beginning of the third decade, warm air reached us from the southeast, but on May 23 from the northwest, a low-pressure trough with showers and thunderstorms reached us. At the Hoher Peißenberg, the thunderstorm lasted five hours – initially without precipitation. In the afternoon, hail fell briefly.
Surge of cold air from the northwest gave the "encore" - Nevertheless, May turned out to be 1.5 degrees too warm
As an "encore", another surge of cold air from the northwest followed on 24 May in the form of a "drop of cold air". This was a high-altitude depression filled with cold air. The maximum temperature dropped below 10 degrees again, the sun did not show up. After that, high-pressure influence set in, but only slowly rising temperatures. From 25 May onwards, it remained dry throughout. After weeks of damp and cool weather, a high determined our weather until the end of the month.
Very noticeable in recent weeks has been the frequent northeasterly wind, which during the last decade blew briskly to strongly almost daily in gusts and led to accelerated drying out of the soils - in addition, there was usually high air pressure over the Baltic States and low south of the Alps. This also resulted in the relatively cool weather of the third decade. Despite maximum sunshine, temperatures in southern Bavaria climbed only hesitantly above 20 degrees during the day, and on the Hoher Peißenberg they remained just below.
Overall, May was 10.9 degrees too warm with a monthly mean temperature of 1.3 degrees on the Hoher Peißenberg. The warmest day was May 22 with 24 degrees Celsius. 20 degrees Celsius were exceeded on only three days. The monthly minimum was 17 degrees Celsius on May 3.
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Our precipitation target of 131 litres per square metre (average value from 1961 to 1990) was far exceeded in May. There were 191 litres per square metre of precipitation (146%). Measurable precipitation fell on the Hoher Peißenberg on 16 days. For seven days we observed thunderstorms. Hail and sleet fell on two days each.
The sun shone for 186 hours in May, which was pretty much the same as normal. This value was only reached by the high-pressure weather of the third decade. For seven days we did not see the sun. With over ten hours of sunshine, we were spoiled for eight days. In northeastern Germany, the sun shone for a record-breaking 300 hours.