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During the early hours of 9 June 2004, Hamburg and its surroundings were hit by a severe, convective storm that caused local flooding, with up to 32 mm of rain falling in less than 30 minutes. There were severe traffic problems and much wind damage, with local gusts up to force 10 on the Beaufort scale (29 metres/second). The fire service in and around Hamburg received around 500 calls to pump out water from flooded house cellars and roads, or to remove uprooted trees and fallen branches. A large, 2-Megawatt wind power station in the County of Rendsburg-Eckernförde was totally destroyed by a lightning strike.
The storm over Hamburg not only occurred at an unusual time (early hours of the morning), but also with an unusual synoptic situation for Europe. The storm, which had formed during the night over the North Sea, was related to the leading edge of a warm, sub-tropical airmass, marked as a warm front on the weather chart (see below). Whilst the upper-level flow was from a northwesterly directions, at lower levels the flow was from the southeast.
The satellite images below, which were taken about one hour after the storm had reached its maximum intensity (06:00 UTC), show the typical structures of an enhanced-V storm. They also show a large overshooting top (shown in the high-resolution visible image, and also in the difference image WV6.2-IR10.8, where there are large positive values of up to +5°C), and an area of higher cloud top temperatures to the lee of the overshooting top (sinking of stratospheric flow behind the overshooting tops) and the horizontal V-structure of the coldest cloud tops (also visibile in the RGB composite images). However, due to some kind of "anvil contamination" from an earlier storm located further to the north, the V-structure is parly interrupted on the northern side of the storm.
According to US experts, it is quite common for enhanced-V type storm clouds to develop on warm frontal boundaries with moisture convergence, strong shear in low layers, a jet streak aloft with high-level divergence, and an arriving moist unstable airmass. All these ingredients were present in the Hamburg area on the 9 June 2004. The satellite signature (the enhanced-V), together with the weather radar images, helped forecasters to issue an early warning.
However, it should be noted that not all severe storms have an enhanced-V structure, so that other satellite signatures, such as cloud top growth, the size of the ice particles or storm movement have to be used to evaluate the likely severity of a storm.
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