Extreme Dutch Weather: Thunderstorms giving birth to waterspouts and funnels (29-08-2006)

Friday, 18 December 2009

Thunderstorms giving birth to waterspouts and funnels (29-08-2006)

In the evening a 'train' of thunderstorms came from the North Sea and had landfall around 'Hoek van Holland' and 'Den Haag'. I witnessed several small waterspouts/ funnels; one lasting several minutes.
Estofex forecast this day:
"...North Sea.......
With colder air advecting over relatively warm water surfaces, GFS indicates that surface to 500m AGL lapse rates become very strong again, to superadiabatic values of >15 degrees. Expect waterspouts to occur especially where low-level winds are relatively weak and low level buoyancy strong, e.g. near the surface trough over Denmark. Beware of convection occurring at lines of convergent 10m winds, sources of vertical vorticity to be stretched. "

Some explanation.....

When a thunderstorm has a longer living rotating updraft (mesocyclone) we call it a 'supercell'. A supercell is a relatively rare appearence in Holland; only on a few days a year one or more can be spotted. They can only exist under specific wind conditions. The top of the storm is often not that heigh as that of their bigger brothers in eg. the US. They often bring extreme weather with large hail and even tornado's being possible. A so called 'wallcloud' is a lowering under the stormcloud out of which a tornado can form. The last photo from the 1st of October 2006 shows a wall cloud.

Another type of tornado is not associated with a mesocyclone. It can appear below a stormbase when there are less windy conditions. Although relatively seldom they appear more often than the 'real' tornado. I prefer to call them 'spouts'.

Other weatherphenomena I'm interested in are 'shelfclouds' and 'mammatus' clouds.