Fig. 1: Location of this year’s North Atlantic Warm Hole (NAWH) and the attendant upper-level low-pressure trough.
Fig. 2: Extreme rainfall has occurred in July 2021 associated with the upper trough located over and east of the NAWH.
Fig. 3-4: The upper air pattern across Europe for July and May through July. The analysis reveals the NAWH-inspired upper trough in Western Europe and compensating upper ridge in Northwest Russia.
Fig. 5: The GFS 15-day percent of normal rainfall forecast reveals the heavy Western Europe rains shift to South-central Europe.
Discussion: The North Atlantic Warm Hole (NAWH) is an area of persistent cooler-than-normal sea surface temperature anomalies (SSTA) becoming more prominent in 2013 related to increased melting of the Greenland Ice Sheet and persistent each warm season since that time including 2021. The NAWH has been displaced eastward to just-off Western Europe (Fig. 1). Aloft, the air cools over the NAWH and becomes a home for a persistent upper trough.
During July, the upper trough has shifted slightly east from the 2021 source region causing excessive rain storms extending from France to Germany to Poland (Fig. 2). Most recently, the rains have caused devastating widespread flooding in Germany with scores of associated fatalities with >1000 people missing according to Reuters News.
The NAWH is the catalyst to weather extremes created by the persistent upper air pattern. In July (Fig. 3), a deep upper trough has anchored over Western Europe causing excessive rains while a compensating upper ridge has persistent over Northwest Russia bringing unusually warm and dry conditions. The upper trough has been semi-permanent since May (Fig. 4) and causing extremes such as an unusually wet/cool May for Western Europe followed by near record warmth due to expansion of the Northwest Russia ridge in June and followed by the return of excessive rainfall in July.
The most recent upper trough and excessive rainfall from France to Poland shifts south in the 15-day forecast striking Italy to the South-central Europe Mountain areas (Fig. 5).