RGB

17 October 2004 0001 UTC.

Looking at the the first image of the loop above you can see a reddish area over Greenland.

The area indicates an area of low PV air and the position of the upper trough at midnight.

17 October 2004 0600 UTC.

The area sinks southwards as the upper trough over Greenland extends.

At this stage we can see the first signs of development to the north east of Iceland, although the high PV air has not yet engaged the system.

Schematic

17 October 2004 1200 UTC.

The high PV air is beginning to engage the baroclinic zone.

We can now see significant development to the northeast with an emerging cloud head evident on the imagery.

17 October 2004 1800 UTC.

The upper trough has know extended and sharpened.

18 October 2004 0001 UTC.

The sharpening upper trough and increasing vorticity has fully engaged with the baroclinic zone. A rapidly deepening depression is evident on the imagery. We can also see the pre-existing frontal band is showing signs of redevelopment.

18 October 2004 0600 UTC.

Further development has now taken place.

The depression is now rapidly deepening.

A full frontal system has now developed and surface pressure is falling rapidly at the surface.

18 October 2004 1200 UTC.

The upper trough has fully extended.

From the imagery we can see the mature depression to the east of Iceland. The frontal structure is fully developed with the jet evident. In addition to the mature depression there is a secondary low pressure developing to the south of Iceland.