There was a relatively weak trough located over Germany on the 9th of April 2014, as can be seen in the map below:

Trough.gif - Bild entfernt (keine Rechte)
(c) - www.wetter3.de

The ground analysis confirmes that the thunderstorm was not related directly to any front.

Ground analysis.gif - Bild entfernt (keine Rechte)
(c) - DWD and wetter3.de

The temperature values were also not too great...

Tempm.gif - Bild entfernt (keine Rechte)
(c) - wetter3.de

And there was a relatively strong wind blowing from the W.

wind.gif - Bild entfernt (keine Rechte)
(c) - wetter3.de

In Generall, with a positive lifted index and a CAPE of about 300, the conditions didn't look that great. But what came out, was quite strong (in Berlin) regarding the time of year.

Here the lightnings tracked on the 9th of April 2014. Note that they were all recorded in the region favored by the forecasts.
Lightnings.gif - Bild entfernt (keine Rechte)
(c) - Lightningmaps.org

The shower was already a thunderstorm at 12UTC.
12UTC rad.gif - Bild entfernt (keine Rechte)
(c) - Lightningmaps.org

13UTC rad.gif - Bild entfernt (keine Rechte)

14UTC rad.gif - Bild entfernt (keine Rechte)