Monthly review July 2021

The month of July was 1.7 degrees too warm compared to the reference period 1961-1990
at the weather station Dresden-Klotzsche. With a monthly mean of 19.7 °C, however, it was
only in 16th place in the ranking of monthly means since 1961 (61 years). Incidentally, the
absolute heat record was set in July 2006 (23.5°C) and remains unsurpassed.
218 hours of sunshine were recorded, an increase of +17 hours compared to the official
reference period 1961-1990. The record is still held unsurpassed by July 2006 with 347
hours.

The sum of all single measurements of precipitation reached 109.8 mm in July at our
measuring station in Coswig. The monthly sum corresponded to 181 % of the normal value
for this month. We thus registered a far above-average second summer month in terms of
the total amount of precipitation that fell. Nevertheless the Coswig value was exceeded at
most stations in Saxony (e.g. Garsebach 144.7 mm or Dresden-Klotzsche 128.7 mm).

Weather characteristics:
A pronounced omega weather pattern (trough-wedge-trough pattern) established itself over
Central Europe as early as the end of June 2021. Omega patterns are known for being stuck
in a particular area for a very long time and, depending on the dominant influence of the
trough (low pressure area in high altitude) or alternatively of the wedge (high pressure area
in high altitude), can then also lead to catastrophic amounts of rain or to long-lasting
droughts, respectively. These pressure formations blocking the westerly flow usually lead to
a remarkable consistency of the weather situation and have gained importance in the course
of climate change in Europe (caused by changes in the properties of the jet stream).

With an overall markedly slow eastward shift of the omega weather pattern, Germany
moved more and more into the area of the western trough during July 2021, with record
precipitation leading to catastrophic flooding over western parts of Germany (Fig.1).

These warm and humid air masses continued to move slowly southeastward together with
the trough and, especially on July 17, now also triggered heavy precipitation in eastern
Saxony (centered on Saxon Switzerland), which led to local flooding (Fig.2). In Lichtenhain-
Mittelndorf in the Elbe Sandstone Mountains, for example, a precipitation total of 111 mm
in 48 hours is documented. The boundary between heavy precipitation in the east and
comparatively less noticeable rainfall in the west of Saxony was unusually sharp pronounced
over the Dresden urban area (Fig.3). The Coswig area even remained largely dry on July 17,
as the map shows. The overall dry weather finally continued here until the end of the month.

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Fig.1: Omega situation over Europe / Trough situation over Central Europe on 15.07.2021, the
day of catastrophic precipitation in NRW and RP (source: Berlin weather map).

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Fig.2: Omega situation over Europe resp. trough situation over Central and Eastern Europe on 17.07.2021, the day of widespread heavy precipitation in Eastern Saxony (source: Berlin weather map)

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Fig.3: 24-hour precipitation totals on 17 July 2021 in Saxony (source: unwetterzentrale.de)

Author: Wilfried Küchler